Mortar Types N, O, S and M: Which One Belongs on Your Chicago Wall

By Affordable Tuckpointing Pros, a Chicago tuckpointing and brick repair company in business since 1996. License number MC6337.

Direct answer

Mortar types N, O, S and M are the four grades of masonry mortar defined by ASTM C270, and they differ mainly by how much lime goes in against the cement. More cement makes a harder, stronger mortar; more lime makes a softer, more permeable, more forgiving one. [1] The Brick Industry Association recommends Type N for normal use, including most veneer work, and tells specifiers to pick the mortar with the lowest compressive strength that still meets the requirements of the job. [3] For repairing an existing wall the rule is tighter still: the repointing mortar must be equal to or lower in compressive strength than the mortar already in the joint, because a stronger mix concentrates stress at the brick-and-mortar interface and can spall the face off the brick. [4] That is why Type O and the older Type K show up on historic buildings, and why Type S and Type M almost never belong in a repointing joint on a house. [1] [3] [4]

TL;DR

• The four ASTM C270 types are M, S, N and O, in decreasing order of strength. BIA lists minimum 28-day compressive strengths of 2,500 psi for Type M, 1,800 for Type S, 750 for Type N and 350 for Type O. [2]
• The National Park Service adds a fifth, Type K, at 75 psi, which it says has the highest lime content of the mixes containing portland cement and is now used mainly on preservation work. [1]
• The letters come from the words MASON WORK, taking every other letter. [1]
• What actually changes between the types is the lime. In cement-lime mortar BIA puts the lime at a quarter part for Type M, over a quarter to a half for Type S, over a half to one and a quarter for Type N, and over one and a quarter to two and a half for Type O, against one part cement. [2]
• Stronger is not better. BIA states plainly that stronger is not necessarily better when specifying mortar and that, in fact, the opposite is often true. [3]
• On repair work the ceiling is set by what is already there. BIA says the compressive strength of the repointing mortar must be equal to or lower than that of the original mortar, and that a stronger mix can lead to spalling of the brick face. [4]
• NPS puts the same rule in permeability terms: new mortar must be softer and more vapor permeable than the masonry units, and no harder or less permeable than the historic mortar. [1]
• The bag at the hardware store is usually masonry cement. NPS says masonry cements generally do not contain lime, produce high strength mortars that can damage historic masonry, and are generally not recommended on historic masonry buildings. [1]

One liner: The mortar type is not a quality grade you upgrade. It is a compatibility setting, and on an existing wall the wall has already chosen it for you.

What do the letters N, O, S and M actually mean?

In short: They are strength grades from ASTM C270, and the letters are not initials. They are every other letter of the words MASON WORK.

There is no word behind the M in Type M. The National Park Service explains the naming directly: the letters identifying the types are taken from the words MASON WORK, using every other letter. [1] Write out M-A-S-O-N W-O-R-K and read the first, third, fifth, seventh and ninth letters and you get M, S, N, O, K. That is the whole system. It is a mnemonic, not a classification, and it is the reason the letters seem to run in no sensible order.

The grades themselves come from ASTM C270, the Standard Specification for Mortar for Unit Masonry. BIA says mortar specified for use in masonry must comply with the requirements of ASTM C270, and that the standard classifies masonry mortars into four types: M, S, N and O. [2] Each one is made of aggregate, water, and one or more of four cementitious materials, which BIA lists as portland or hydraulic cement, mortar cement, masonry cement and lime. [2]

NPS works from the same standard but counts five. It lists them in decreasing order of approximate general strength as Type M at 2,500 psi, Type S at 1,800 psi, Type N at 750 psi, Type O at 350 psi and Type K at 75 psi, and notes that Type K has the highest lime content of the mixes that contain portland cement, although it is seldom used today except on some historic preservation projects. [1] NPS also records a sixth designation, L, which it says identifies a straight lime and sand mix with no cement at all. [1]

So the honest version is that there are four current types and two legacy ones. If a contractor quotes you Type K on a nineteenth-century building, that is not an error. BIA still gives proportions for it, describing Type K as the weakest and generally reserved for historic masonry applications, at one part portland cement, four parts hydrated lime and fifteen parts fine sand. [4]

The order of numbers in a mortar specification is fixed and worth knowing before you read a quote. NPS says that unless specified otherwise, proportions for mortar mixes are always given in the order cement, then lime, then sand. [1] A mix written 1:2:9 is one part cement, two parts lime and nine parts sand, and nothing else.

What is the real difference between Type N, O, S and M?

In short: Lime. The cement stays at one part and the lime climbs as you move down the strength ladder, which is what makes the softer types softer.

Line the four types up as cement-lime mortars and the pattern is obvious. BIA gives the proportion requirements against one part cement: a quarter part hydrated lime or lime putty for Type M, over a quarter to a half for Type S, over a half to one and a quarter for Type N, and over one and a quarter to two and a half for Type O. [2] The cement does not move. The lime is the dial.

That is the whole mechanism behind the strength numbers. NPS puts it in one sentence: if mixed with higher amounts of portland cement, a harder mortar is obtained, and the more lime that is added, the softer and more plastic the mortar becomes, increasing its workability. [1] BIA says the same thing from the other end, that compressive strength increases with an increase in cement content and decreases with an increase in water content, lime content or over-sanding. [2]

The sand does not change much across the types. Under the ASTM C270 property specification BIA gives the aggregate ratio as not less than two and a quarter and not more than three and a half times the sum of the separate volumes of cementitious materials, for every type. [2] Mortar is mostly sand in all four grades. NPS arrives at the same place from first principles: well graded sand generally has a thirty per cent void ratio by volume, so around thirty per cent binder by volume should generally be used, which is where the familiar one-to-three binder-to-sand ratio in mortar specifications comes from. [1]

Check the arithmetic against a real historic mix and it holds. NPS gives Type O as 1:2:9, one part cement to two parts lime to nine parts sand. [1] Two parts lime against one part cement sits inside BIA’s Type O band of over one and a quarter to two and a half, and nine parts sand against three parts of combined cementitious material is exactly three times. [1] [2] The preservation mix and the modern standard are describing the same material.

Strength is only the headline property. BIA’s property specification also fixes a minimum water retention of seventy-five per cent for every type, and caps air content at twelve per cent for cement-lime Types M and S and fourteen per cent for cement-lime Types N and O. [2] Those two numbers matter more to how a joint behaves in a wall than the compressive strength does, which is the point of the next section.

There is also a property most homeowners have never heard of that explains why soft mortar survives on old buildings. BIA calls it extensibility, another term for maximum tensile strain at failure, and says high-lime mortars exhibit greater plastic flow than low-lime mortars, which acting together impart some flexibility to the masonry and permit slight movement. [2] Where greater resiliency for movement is desirable, BIA says the lime content may be increased while still satisfying the other requirements. [2] A soft joint moves with the building. A hard one makes the building move somewhere else.

Lime does one more thing cement cannot. BIA describes autogenous healing: because lime hardens on contact with carbon dioxide in the air, if small hairline cracks develop, water and carbon dioxide penetrating the joint react with calcium hydroxide from the mortar to form calcium carbonate, which seals the cracks and limits further water penetration. [2] A high-lime joint is, to a limited degree, self-repairing.

Which mortar type does my wall need?

In short: For most house walls above grade, Type N, with Type S as the alternate. Below grade and in retaining walls the recommendation moves to Type M.

BIA publishes a table of mortar recommendations based on use, and for a normal building it is short. Above grade, it recommends Type S for reinforced or loadbearing walls with Type N as the alternate; Type N for veneer or non-loadbearing walls with Type S as the alternate; and Type N for parapets and chimneys, again with Type S as the alternate. [3] At or below grade the recommendation changes: Type M for foundation walls and retaining walls, and Type M for sewers and manholes, with Type S as the alternate in both cases. [3] Inside, it is Type N for loadbearing walls with Type S as the alternate, and Type N for partitions with Type O or Type S as the alternate. [3]

BIA offers an even shorter version for people who do not want the table, calling it the simplistic selection method: Type N for normal brickwork applications and Type S for stronger brickwork applications, where normal includes most anchored brick veneer and stronger is typically needed in high-seismic and high-wind areas. [3] Its own summary of recommendations leads with the same line, that Type N mortar is recommended for normal use, including most veneer applications. [2] [3]

Notice what is not in that table. Type M is recommended for foundations, retaining walls, sewers and manholes, and BIA separately recommends it for most exterior paving. [3] It is not recommended anywhere on the visible face of a house wall. If a quote for work on your front elevation specifies Type M, the specification and the standard have parted company, and it is a fair question to ask why.

Type S has a legitimate case in one direction. BIA says mortar for each project should be selected to balance construction requirements against the performance of the completed masonry, and that high lateral loads from wind or seismic activity may require a mortar that develops high flexural tensile strength. [3] It also notes that building code requirements may limit the use of some mortar types under certain conditions, giving the example that TMS 402 does not permit Type N or masonry cement mortars in ungrouted or partially grouted portions of the lateral force-resisting system for structures in Seismic Design Categories D, E or F. [3] That is a code question for the designer of a new structure, not a reason to harden the mix on an existing house.

BIA also treats the two middle grades as close cousins in practice. A footnote to its selection table says Type S and Type N mortars are typically used interchangeably depending on the preference of the mason, and that provided any compressive strength requirements for the application are met, Type S is an acceptable alternative to Type N and vice versa. [3] The gap that matters is not between N and S. It is between the modern pair and the soft repair grades below them.

Why is the strongest mortar the wrong answer?

In short: Because the joint is designed to be the weak part, and a mortar stronger than the wall moves the failure into the brick, where it is not repairable.

This is the part that runs against instinct, so it is worth quoting the sources rather than paraphrasing them. BIA: one property of mortar that is often overemphasized is compressive strength, stronger is not necessarily better when specifying mortar, and in fact the opposite is often true. [3] Its first fundamental guideline of mortar selection is to select a mortar type with the lowest compressive strength meeting the project requirements. [3]

BIA gives the reason in its technical note on mortar materials. Unlike concrete, it says, compressive strength is not the primary consideration for brick mortar selection, especially in brick veneer, and bond strength, workability and water retention are more important than compressive strength in most applications and should be given principal consideration during mortar selection. [2] Compressive strength is the number people quote because it is the number that is easy to measure, not because it is the number that decides whether your wall stays dry.

On an existing wall the argument stops being theoretical. BIA is blunt about it: to avoid irreparable brick damage, the compressive strength of the repointing mortar must be equal to or lower than the compressive strength of the original mortar, and using a mortar with higher compressive strength may significantly impair the surrounding brickwork. [4] The mechanism it describes is that stronger repointing mortar increases the stress concentration at the brick and mortar interface and can lead to spalling of the brick face. [4] Its companion note repeats the warning, saying stronger repointing mortar will create stress concentrations on existing historic masonry and result in spalling. [3]

NPS reaches the same conclusion by a different route, through moisture rather than stress. It says mortars for repointing should be softer or more permeable than the masonry units and no harder or more impermeable than the historic mortar, in order to prevent damage to the masonry units, and that it is a common error to assume that hardness or high strength is a measure of appropriateness, particularly for lime-based historic mortars. [1] Stresses within a wall caused by expansion, contraction, moisture migration or settlement have to be accommodated somewhere, and NPS describes the historic role of mortar as a bedding material, not unlike an expansion joint, rather than a glue holding the units together. [1]

Permeability is the other half of it. NPS says high lime mortars are more permeable than denser cement mortars, and that moisture was historically able to migrate through the mortar joints rather than through the units. [1] Where the masonry unit itself has a relatively high permeability, it says a soft, high lime mortar is necessary to retain sufficient permeability, and that masonry deterioration caused by salt deposition results when the mortar is less permeable than the masonry unit. [1] Seal the escape route with a hard joint and the water leaves through the brick face instead, which is the failure our post on why Chicago brick spalls covers in detail.

NPS does add one careful caveat that stops this from being a simple soft-is-always-better rule. Softness or hardness, it notes, is not necessarily an indication of permeability, and old, hard lime mortars can still retain high permeability. [1] The two properties travel together most of the time, but they are not the same property, and the test that matters is compatibility with what is already in the wall.

Does the age of the building change the answer?

In short: It changes it completely. A wall built before portland cement was in common use was built on lime, and it needs a mortar in the Type O or Type K range.

NPS dates the transition precisely enough to use. Mortar formulations prior to the late nineteenth century used lime as the primary binding material, it says, and traditional mortar was lime putty combined with local sand, generally at one part lime putty to three parts sand by volume. [1] Portland cement was patented in Great Britain in 1824 and first manufactured in the United States in 1872, but it was not in common use throughout the country until the early twentieth century, and up until the turn of the century it was considered primarily an additive. [1] Masonry cement, the premixed bagged product, arrived later still: NPS puts its introduction in the 1930s. [1]

BIA tells the same story in the context of choosing a repair mortar. Prior to the early 1900s, it says, masonry mortars consisted primarily of lime, portland cement was not commonly added to mortar mixes until after that period, and mortars with no or little cement content have lower compressive strength values. Hence it is likely necessary to use a weaker mortar for repairing older masonry than would be used for new construction. [3]

That gives you a usable rule of thumb for the age of your building, and the sources name the grades. BIA says Type O mortar is often used for repointing older brickwork and Type N may be suitable for repointing newer brickwork. [3] Its maintenance note says Type N is generally recommended for modern applications, Type O is appropriate where mortars with higher cement contents may be too strong for proper performance, and Type K is the weakest and generally reserved for historic masonry. [4] And its maintenance checklist says simply: for repointing, use prehydrated Type N, O or K mortar. [4]

Type S does not appear on that list, and neither does Type M. That is not an oversight. Those are the grades for building new walls and for holding back earth, not for putting a soft historic wall back together.

There is a separate standard for the oldest buildings. BIA notes that ASTM C1713, the specification for mortars for the repair of historic masonry, can be used in lieu of ASTM C270 for structures that were not constructed with modern mortars. [3] If your building is in that category, the four-type ladder is not the right framework at all.

When the answer is genuinely unclear, both sources point to testing rather than guessing. BIA says that if necessary the existing mortar can be tested to determine the proportions of ingredients for the repointing mortar, and that samples should be extracted from the wall and sent to a laboratory. [3] NPS agrees but with a warning attached: a mortar analysis by a qualified laboratory can be useful by providing information on the original ingredients, but there are limitations, replacement mortar specifications should not be based solely on laboratory analysis, and analysis requires interpretation. [1] It notes that factors such as the original water content, the rate of curing and weathering cannot be established through laboratory analysis at all. [1]

NPS is also candid about why the analysis is harder than it sounds. Historic mortars, it says, were not prepared to narrowly defined specifications from materials of uniform quality; they contain a wide array of locally derived materials combined at the discretion of the mason. [1] A number from a lab is an input to the decision, not the decision.

What does the brick itself have to do with the mortar type?

In short: How thirsty the brick is decides how much water the mortar has to hold on to, and BIA publishes a selection table based on exactly that.

Brick drinks. The property that measures it is initial rate of absorption, or IRA, and BIA says it indicates the brick’s suction and whether it should be considered for wetting before use, with the IRA at the time of laying being what influences bond strength. [2] In practically all cases, it says, mortar bonds best to brick with an IRA of 30 g/min/30 in.² or less when laid, and if the brick exceeds that value it should be wetted three to twenty-four hours before laying and be surface dry when laid. [2]

The mortar side of that relationship is water retention, which BIA defines as the ability of a mortar to hold water when placed in contact with absorbent masonry units. [2] Its rule is symmetrical: a mortar with low water retention will lose moisture more rapidly and should be used with a low-IRA brick, while a mortar with high water retention will keep moisture and should be used with a high-IRA brick. [2]

BIA turns that into a second selection table, based on brick IRA rather than building segment. For brick up to 10 g/min/30 in.² it recommends Type S across cement-lime, mortar cement and masonry cement mortars. For brick between 10 and 30 it recommends Type N or S in all three. For brick above 30 laid dry, it recommends Type N cement-lime mortar. For brick above 30 wetted prior to laying, it recommends Type N cement-lime, Type N or S mortar cement, and Type S masonry cement. [3] Those recommendations, it notes, apply to construction in temperatures from 40 to 100 degrees Fahrenheit. [3]

The most practical thing in that section is a detail about how a mason tunes the mix without leaving the type. BIA points out that using a cement-lime mortar under the ASTM C270 proportion specification allows the mason to adjust the amount of lime to better match the IRA of the brick: Type N cement-lime mortar always includes one volume of cement, but the lime is permitted to range between a half and one and a quarter, a common proportion is 1:1, and if the brick has a low IRA the lime could be reduced to three quarters or a half to lower water retention while still qualifying as Type N. [3] The type is a band, not a recipe.

Texture matters too, though less. BIA says mortar bond is greater to roughened surfaces such as wire-cut surfaces than to smooth ones such as die-skin surfaces, and that sanded and coated surfaces can reduce bond strength. [2] It rates bed joint surface texture as influencing bond to a lesser degree than IRA. [3] And it reports that several researchers have shown IRA appears to have little influence on bond strength when the appropriate mortar is used, which is the entire argument for matching the two in the first place. [2]

Bond has two halves that are easy to confuse. BIA separates them: extent of bond refers to the amount of intimate contact between mortar and brick, which is enhanced by good workability and which provides durability and resistance to water penetration, while bond strength refers to the force required to separate the mortar from the brick and provides resistance to cracking. [3] A strong mortar that does not wet out the brick properly gives you the second without the first, and it is the first that keeps rain out of a tuckpointed joint.

What is actually in the bag at the hardware store?

In short: Usually masonry cement, which is a different product from cement-lime mortar and which NPS specifically does not recommend on historic masonry.

The type letter on a bag tells you the strength grade. It does not tell you which cementitious system produced it, and there are three. BIA describes cement-lime mortar, made from portland or other hydraulic cement combined with hydrated lime or lime putty; mortar cement, a proprietary hydraulic cement defined by ASTM C1329; and masonry cement, a proprietary product defined by ASTM C91. [2] All three come in Types M, S, N and O, and BIA’s property specification assigns them the same minimum compressive strengths. [2]

They are not equivalent in the properties that matter. BIA says masonry cements are widely used because of their convenience and good workability, but that in the model building codes the allowable flexural tensile and modulus of rupture stress values for masonry built with masonry cement mortar are lower than those for masonry built with non-air-entrained portland cement-lime or mortar cement mortar. [2] It therefore says the use of masonry cement should be based on the requirements of the specific application. [2] Elsewhere it reports that mortars made with cement-lime and mortar cement materials typically have higher flexural bond strengths than masonry cement mortars. [2]

Mortar cement was created to close that gap. BIA says ASTM C1329 includes requirements for minimum flexural bond strength that are not included in other cement specifications, with the intent of providing performance similar to a portland cement-lime mortar with the ease of use and workability of a masonry cement. [2] The minimums it gives are 115 psi for Type M, 100 psi for Type S and 70 psi for Type N. [2] Because of the strict controls on air content and the minimum strength requirement, it says, mortar cement and portland cement-lime mortars are treated similarly in the building code. [2]

On an old building NPS goes further than a caution. It describes masonry cement as a preblended mortar mix commonly found at hardware and home repair stores, designed to produce mortars with a compressive strength of 750 psi or higher, which may contain hydrated lime but always contains a large amount of portland cement along with ground limestone and other workability agents, including air-entraining agents. [1] Because masonry cements are not required to contain hydrated lime and generally do not contain it, NPS says, they produce high strength mortars that can damage historic masonry, and for this reason they are generally not recommended for use on historic masonry buildings. [1]

Air is the hidden variable in all of this. BIA says ASTM C270 limits air content because research has historically shown that high air entrainment can significantly reduce the bond between mortar and brick or reinforcement, and that as air content increases, compressive strength and bond strength are reduced while workability and resistance to freeze-thaw deterioration are increased. [2] A cement-lime mortar with no air-entraining components typically contains about eight per cent air and is permitted a maximum of only twelve, which is why BIA does not recommend air-entrained materials in that formulation. [2] Masonry cement under ASTM C91 requires a minimum of eight per cent air and permits up to twenty per cent for Type N and eighteen for Types S and M. [2] And BIA warns against stacking them, saying two air-entraining materials shall not be combined in mortar. [2]

For a freeze-thaw climate that trade-off is genuinely two-sided, and the sources do not pretend otherwise. BIA says that with respect to freeze-thaw damage, mortar generally contains sufficient entrapped and entrained air to resist it in unsaturated applications, and that although increasing air content may theoretically increase durability, a decrease in bond strength, compressive strength and other properties can result, so air-entraining admixtures should be used with caution. [2] NPS is more open to it in hard exposures, saying no definitive study has determined whether air-entraining additives should be used to resist frost action and enhance plasticity, but that in areas of extreme exposure requiring high-strength mortars with lower permeability, air entrainment of ten to sixteen per cent may be desirable. [1] Two primary sources, one unresolved question, and no number either of them will stand behind for a normal house wall.

How is mortar actually specified, and why does that matter to you?

In short: By proportion or by property, and the default is proportion, which means your quote should name a recipe rather than a test result.

ASTM C270 offers two ways to get to a mortar type and they are not interchangeable. BIA describes the proportion specification as requiring that mortar materials be mixed according to given volumetric proportions, with no laboratory testing required either before or during construction. [2] The property specification instead requires a mix of the actual materials to meet specified properties under laboratory testing conditions, with compressive strength, water retention and air content tests performed before construction on mortar mixed in the laboratory with a controlled amount of water. [2]

If nobody says which, the answer is already decided. BIA states that per ASTM C270 the requirements of the proportion specification are the default if neither is specified, and repeats it in its summary of recommendations, adding that for typical project requirements the proportion specification should be used. [2] There is one reversal worth knowing: where the mortar uses a preblended dry mortar mix under ASTM C1714, the property specification requirements become the default instead. [2]

The two routes do not produce the same material even at the same type. BIA warns that mortar prepared by the proportion specification is not to be compared with mortar of the same type prepared by the property specification, and that a mortar mixed to the proportion specification will have a higher laboratory compressive strength than the corresponding mortar of the same type made from the same materials mixed to the property specification. [2]

This is also where most arguments about test results come from. BIA says properties of field-mixed mortar cannot be compared with the requirements of the property specification, because of the different amounts of water used, the different mixers and the different curing conditions, and that field mortar will generally be wetter and have a lower compressive strength than laboratory mortar. [2] Field sampling, where specified, is for tracking project consistency from beginning to end, not for compliance. [2] Its quality assurance note adds that compressive strength results obtained from field-sampled mortar under ASTM C780 cannot be compared with the minimum requirements of the ASTM C270 property specification, and can be expected to be lower and more variable. [3]

There is a neat physical reason for that gap. BIA explains that the compressive strength of the mortar laid in the wall will be much higher than that measured in a field-sampled specimen, because the brick adjacent to the wall mortar absorbs the mix water and effectively lowers the water-to-cement ratio of the wall mortar. [3] The mortar in your joint is stronger than the mortar in the bucket it came from.

For a homeowner the practical translation is short. BIA says field testing of mortar is not necessary on most projects, and that for mortar specified by ASTM C270 the key to quality assurance is adherence to the material proportions added to the mixer, which makes observation during measuring and mixing an essential component. [3] It says material measuring and batching should be by volume. [3] What you want on a residential job is a named mix and a mason who measures it the same way every batch, not a laboratory report.

How should repointing mortar be mixed and placed?

In short: Prehydrated, drier than laying mortar, packed in quarter-inch lifts, and tooled when it is thumbprint hard.

Repointing mortar is not mixed like laying mortar, and the difference has a name. BIA calls it prehydration and says the repointing mortar should be prehydrated to reduce excessive shrinkage. The process it gives is to mix all dry ingredients thoroughly, add only enough clean water to produce a damp consistency that will retain its shape when formed into a ball, and hold the mix in that dampened condition for one to one and a half hours before adding water for placement. [4] Its selection note describes the same sequence and says the resulting consistency for placement is still significantly drier than the consistency used to lay brick. [3]

NPS describes the same practice from the historic side, saying repointing mortar is typically prehydrated by adding water so it will just hold together, allowing it to stand for a period before the final water is added, with half the water added first and mixed for approximately five minutes and the remainder added in small portions until the desired consistency is reached. [1] It notes the total water may vary from batch to batch depending on weather. [1]

The drier mix is not fussiness. BIA says the drier consistency makes it easier to place the mortar into the joint and compact it, and helps minimise shrinkage over time. [3] A wet mix slumps out of the joint and shrinks back from the brick as it cures, which opens exactly the crack the repair was supposed to close.

Placement is done in layers. BIA says the mortar should be packed tightly into the joints in layers no more than a quarter inch thick and tooled when thumbprint hard, with the last layer tooled to match the original profile. [4] Its maintenance checklist repeats it: install repointing mortar in multiple quarter-inch lifts, tooling each when thumbprint hard. [4] NPS gives the same quarter-inch figure and adds that where existing mortar has been removed to a depth greater than one inch, those deeper areas should be filled first, compacting the new mortar in several layers and packing it well into the back corners. [1]

Wetting the joint first is a balance rather than a step. BIA says the joints to be repointed should be dampened, but that to ensure a good bond the brickwork must absorb all surface water before the repointing mortar is placed. [4] Damp, not wet.

High-lime mixes then need to be kept damp while they carbonate, which is the one part of the job that happens after the crew has packed up. NPS says the preliminary hardening of high-lime mortars takes place fairly rapidly as water is lost to the porous masonry and to evaporation, and that a high lime mortar left to dry out too rapidly can result in chalking, poor adhesion and poor durability. [1] Its remedy is misting with a fine nozzle for a day or two after repointing, initially as often as every hour and gradually reduced to every three or four hours, with walls covered in burlap for the first three days to keep them damp and out of direct sunlight. [1] Once carbonation begins, it says, it continues for many years and the lime gains strength as it reverts to calcium carbonate within the wall. [1]

Two more constraints apply to any mortar. BIA says ASTM C270 requires all mortar to be used within two and a half hours after mixing, or two hours in hot-weather construction, and that mortar should not be used once it has begun to set. [2] And NPS warns that once portland cement has been added to a lime putty and sand mortar the mortar can no longer be stored, giving a window of thirty minutes to an hour and a half with no retempering for Type O and Type K mixes made that way. [1]

Mortar type questions Chicago homeowners ask

Is Type S mortar stronger than Type N, and should I ask for it?

Yes to the first, usually no to the second. BIA puts the minimum 28-day compressive strength of Type S at 1,800 psi against 750 psi for Type N. [2] But it recommends Type N for normal use including most veneer applications, reserving Type S for stronger applications such as high-wind and high-seismic areas, and it says to select the type with the lowest compressive strength that meets the requirements. [3] On repair work the existing mortar sets the ceiling, and BIA says the repointing mortar must be equal to or lower in strength than what is already there. [4]

What is Type K mortar and why is it not in ASTM C270?

BIA says ASTM C270 classifies mortars into four types, M, S, N and O. [2] NPS lists five, adding Type K at 75 psi, and says it has the highest lime content of the mixes containing portland cement and is seldom used today except on some historic preservation projects. [1] BIA still gives proportions for it as a repointing mortar, calling it the weakest and generally reserved for historic masonry, at one part portland cement to four parts hydrated lime to fifteen parts fine sand. [4] For structures not built with modern mortars, BIA points to ASTM C1713 as an alternative to C270 entirely. [3]

Can I just buy a bag of mortar mix at the hardware store?

You can, but check what it is. NPS describes masonry cement as the preblended mix commonly found at hardware and home repair stores, designed to produce mortars of 750 psi or higher, and says that because masonry cements generally do not contain lime they produce high strength mortars that can damage historic masonry and are generally not recommended for use on historic masonry buildings. [1] BIA adds that code allowable flexural tensile values for masonry cement mortar are lower than for cement-lime or mortar cement mortar. [2]

How do I find out what mortar is already in my wall?

Have it tested, then treat the result as evidence rather than a verdict. BIA says the existing mortar can be tested to determine the proportions of ingredients, with samples extracted from the wall and sent to a laboratory. [3] NPS says a laboratory analysis can be useful for identifying original ingredients but that replacement mortar specifications should not be based solely on it, that analysis requires interpretation, and that factors including the original water content, rate of curing and weathering cannot be established that way. [1]

Does more cement make the joint last longer?

No, and BIA says the opposite is often true. It calls compressive strength an often overemphasized property, says stronger is not necessarily better, and puts bond strength, workability and water retention ahead of compressive strength for most applications. [2] [3] A mortar stronger than the wall around it concentrates stress at the brick and mortar interface and can lead to spalling of the brick face. [4]

Is mortar type the same question as tuckpointing versus repointing?

No. Those two words describe the same physical operation, which is covered in our post on tuckpointing vs repointing. The mortar type is the separate and more consequential question of what goes back into the joint. BIA’s repointing set is prehydrated Type N, O or K. [4]

Can the mason adjust the mix without changing the mortar type?

Yes, and it is normal practice. BIA notes that Type N cement-lime mortar always includes one volume of cement while the lime is permitted to range from a half to one and a quarter, that a common proportion is 1:1, and that the lime can be reduced to three quarters or a half to lower water retention for low-IRA brick while still qualifying as Type N. [3] The type is a band, not a single recipe.

Stats box

Definition bank

Entity cards

Type N mortar

Type O mortar

Type S mortar

Type M mortar

ASTM C270

Repointing mortar

Getting the right mix into your joints

In short: Ask the contractor which mortar type is going into the wall and why that one, and you have asked the question that decides whether the repair protects the brick or damages it.

Everything above collapses into one question you can ask on a walkthrough: what type is going back in, and what is in the wall now? A contractor who answers with a type and a reason is working from the same standards quoted on this page. One who says stronger is better is quoting the opposite of what BIA and NPS both say. [1] [3]

The consequences are not evenly matched. Mortar too soft for its exposure wears and gets repointed again, which is what BIA describes as ordinary maintenance. Mortar too hard for the brick around it moves the failure into the units, and BIA is explicit that this can lead to spalling of the brick face and irreparable brick damage. [4] One mistake costs you another repointing cycle. The other costs you brick.

On our own tuckpointing work in Chicago the mix is decided before the grinder comes out, by looking at what is in the joint and how the brick behaves, not by defaulting to whatever is on the truck. If you want that written down for your building, get a free quote and we will tell you which type your wall is asking for.

Sources

1. National Park Service, Preservation Brief 2, “Repointing Mortar Joints in Historic Masonry Buildings”, Mack and Speweik, rev. 1998. https://www.nps.gov/orgs/1739/upload/preservation-brief-02-repointing.pdf
2. Brick Industry Association, Technical Note 8, “Mortars for Brickwork”, March 2020. https://www.gobrick.com/media/file/8-tn-8-2020-03.pdf
3. Brick Industry Association, Technical Note 8B, “Mortars for Brickwork — Selection and Quality Assurance”, March 2020. https://www.gobrick.com/media/file/8b-tn-8b-2020-03.pdf
4. Brick Industry Association, Technical Note 46, “Maintenance of Brick Masonry”, December 2017. https://www.gobrick.com/media/file/46-maintenance-of-brick-masonry.pdf