Stone Sills That Drain the Wrong Way: Negative Slope on a Chicago Wall

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

Direct answer

A stone sill is the unit under a window whose prime function is to channel water away from the building. [1] When its top surface has gone flat, or worse has started to pitch back toward the wall, it stops shedding water and begins delivering it into the masonry under the opening. The Brick Industry Association does not file that under cosmetics: neutral or negative slope on masonry sills and water tables appears on its list of observed conditions that are structural, sit outside the scope of routine maintenance, and warrant an investigation by a professional engineer. [2] That is the same list that contains cracks through multiple brick units and out-of-plane movement of the wall. [2] The practical consequence is that repointing the joints under a back-pitched sill treats the symptom. The National Park Service is blunt about this: the root cause has to be dealt with before the work begins, or mortar deterioration continues and the repointing has been a waste of time and money. [3]

TL;DR

• The prime function of a sill is to channel water away from the building. Everything else about it is secondary to that. [1]
• BIA wants the top surface of a concrete or stone sill sloped away from the building at least 15 degrees from horizontal, and also sloped from the ends toward the center at 1/8 in. (3 mm) to 12 in. (300 mm). [1]
• Neutral or negative slope on masonry sills and water tables is on BIA’s list of conditions that are structural and warrant investigation by a professional engineer. [2]
• A sill is supposed to project past the wall face by at least 1 in. (25 mm), and to carry a drip whose inner lip sits at least 1 in. (25 mm) back from that face, so water cannot track back to the wall. [1]
• Under the stone there should be flashing, turned down at the face to form a drip and turned up and outward at least 1 in. (25 mm) at each end. Without those end dams, BIA says collected water has a path into the adjacent wall and there is no way to predict where it goes. [1]
• Weepholes belong on top of the flashing, not one course up, spaced at most 16 in. (400 mm) with wicks or hidden flashing and at most 24 in. (600 mm) for open weeps. [1]
• Inadequate slope and drips needed are both line items under caps, copings and sills in BIA’s inspection checklist, which is to say this is a thing you are meant to be looking for. [2]
• If you repoint under a sill without fixing the slope, NPS says the deterioration continues regardless. Correct the cause first. [3]

In one line: a stone sill exists to throw water clear of the wall, and when it stops doing that the Brick Industry Association stops calling it maintenance. [2]

What is a stone sill actually supposed to do?

In short: It is a water-management detail, not a decorative ledge. BIA says the prime function of a sill is to channel water away from the building.

Ask most people what the stone under their window is for and you will get an answer about appearance. That is not what it is for. BIA states the purpose in one line: the prime function of a sill is to channel water away from the building. [1] Every other property of the unit, its material, its colour, its profile, is subordinate to that job. In the same document BIA frames the whole exercise of detailing around the question of whether an element should channel and direct the flow of moisture, and notes that it is possible to produce a detail with excellent esthetic value while compromising performance, but that a detail which compromises performance is doomed to fail. [1]

That failure has a predictable shape. BIA lists the ways it shows up: cracking, structural failure, moisture penetration to the interior, or efflorescence. [1] Those are the things a homeowner eventually calls about. They are downstream of a detail that stopped moving water in the right direction.

Sills are usually brick, concrete, stone or metal. [1] Chicago’s older housing stock leans heavily on limestone, and there is a real advantage in that: BIA notes that metal, concrete and stone sills normally require fewer joints than brick sills do, and therefore present fewer potential avenues for water penetration. [1] A one-piece stone sill under a standard opening is a genuinely good detail. It is also a detail with a single point of failure, because when that one unit stops draining correctly there is no redundancy anywhere in it.

The other thing worth saying early is that the sill is not an isolated part. BIA groups caps, copings, sills, lintels and sealant joints together as components incorporated in the brickwork that may require periodic inspection and repair, and warns that neglecting maintenance of these components may lead to deterioration of other elements in the wall. [2] The sill is the component that fails first and takes other things with it.

How much slope should a stone sill have?

In short: At least 15 degrees from horizontal away from the building, plus a second slope from the ends toward the center of 1/8 in. per 12 in.

This is where the published guidance is more specific than most people expect. BIA says that since the primary function of sills is to divert water away from the building, the top surface should slope downward and away from the building. [1] For brick sills, that slope should be at least 15 degrees from horizontal, allowing that it may vary somewhat according to the sill configuration of the window unit, particularly with wood windows. [1]

For concrete or stone sills, BIA asks for two slopes rather than one. They should be sloped away from the building, and also sloped from the ends toward the center. The slope away from the building should be at least 15 degrees from horizontal; the slope from the ends should be 1/8 in. (3 mm) to 12 in. (300 mm) toward the center of the sill. [1] On sills longer than 4 ft (1.2 m), that end slope should extend for at least a distance of 2 ft (600 mm) from the ends. [1]

The second slope is the one almost nobody knows about, and it is the one that explains a specific pattern of damage. Water that reaches a sill does not distribute itself evenly. Draining the ends toward the center keeps it away from the jambs, which is exactly where the sill meets the surrounding brickwork and where a path into the wall would be shortest. A sill with adequate slope away from the building but no end-to-center slope will concentrate water at its two ends and produce deterioration at the jambs while the middle of the sill looks fine.

There is also a projection requirement that works with the slope rather than independently of it. BIA says the sill should extend a minimum of 1 in. (25 mm) beyond the face of the wall at its closest point to the wall. [1] A sill flush with the brick has nowhere to discharge to. It will run water straight down the face of the masonry below the opening, and the vertical streak of soiling and saturation below a window is the visible record of that.

None of these numbers require special equipment to check. A torpedo level set across the top of the sill, front to back, will tell you in a second whether the surface still falls away from the wall or has gone flat or backward. That single measurement is the whole diagnosis for a large share of sill problems.

What does negative slope mean, and why is it structural?

In short: It means the sill now drains toward the building instead of away from it, and BIA puts it on the same list as cracks through multiple units and out-of-plane wall movement.

Neutral slope means the top surface has gone flat. Negative slope means it has gone past flat and now falls back toward the wall. In either case the detail has stopped doing the one thing it exists to do, and in the negative case it is actively feeding water to the masonry it was installed to protect.

BIA’s treatment of this is the part worth sitting with. Its maintenance document draws a line between conditions that are maintenance and conditions that are structural, and says structural issues are more involved than maintenance and outside the scope of that document, generally warranting an investigation performed by a professional engineer to determine the cause and recommend the appropriate repair method. [2] It then gives examples of observed conditions that are structural: cracks in brick masonry exceeding 0.075 in. (2 mm) in width, cracks through multiple brick units, cracks following a stepped or diagonal pattern, widespread spalled brick, accumulated rust on lintels or shelf angles, out-of-plane movement of brick masonry or other wall elements, and neutral or negative slope on masonry sills and water tables. [2]

Read that list again and notice the company negative slope is keeping. A bulging wall is on it. A crack running through multiple brick units is on it. And so is a sill that no longer drains the right way. BIA is not saying the sill itself is about to collapse. It is saying that a sill which has changed its pitch is evidence that something moved, and that what moved is a question for an engineer rather than for a mason with a bag of mortar.

That is the logic of it. Stone sills are not installed with a back pitch. If one now has a back pitch, either the unit rotated, or what it bears on settled, or the masonry under it lost enough material to let it tip. BIA makes the same point in the other direction elsewhere in the document by warning that indications of structural issues can sometimes appear similar to conditions that require maintenance. [2] A tipped sill looks like a maintenance item. It reads like one to a contractor pricing the job by the linear foot. The document exists partly to stop that misread.

BIA also puts the condition on its inspection checklist directly. Under the heading for caps, copings and sills, the items to look for are inadequate slope, cracked units, hairline cracks in mortar, loose units, open joints, out-of-plumb and drips needed. [2] Inadequate slope is the first line. Drips needed is the last. Between them they describe most of what goes wrong with a sill.

The practical implication for a building owner is narrow and useful. If a sill has gone flat and nothing else on the wall has moved, you are likely looking at a repair. If a sill has gone negative and there are also stepped cracks, displaced units or a wall that is out of plumb, the sill is a symptom and pricing sill work alone is pricing the wrong thing. [2]

Where does the water go when the sill drains backward?

In short: Into the joints under the opening, then into whatever the wall has behind them, and it surfaces as stains, efflorescence and failing mortar well away from the sill.

Water arriving on a back-pitched sill runs to the low point, which is now the junction between the stone and the wall. That junction is a joint, and BIA is clear that the joint where the sill and window make contact should be sealed with a high-quality sealant. [1] Sealant is not a permanent material. BIA’s own table of estimated time before repairs may be necessary gives sealant joints 5 to 20 years, against 100 to 150 plus years for brick walls and 50 plus for mortar in walls. [2] The sealant is the shortest-lived thing in the assembly, and a negative slope puts standing water directly on it for its entire service life.

Once past that line, the water is inside the wall, and where it emerges is not necessarily near the sill. BIA’s table of possible causes of masonry distress connects a set of observed conditions to a set of causes, and the causes include incompletely filled mortar joints, missing or clogged weeps, deteriorated or torn sealant, capillary rise, and missing or damaged flashing. The observed conditions those produce include cracked units, spalled units, deteriorated mortar, efflorescence, moisture-related stains, corrosion of concealed materials and damaged interior finishes. [2]

Efflorescence is the most legible of those. BIA describes it as white deposits on the brick surface left when moisture carrying dissolved salts evaporates, and makes the point that matters here: potential water ingress issues in the area should be investigated and resolved before cleaning the efflorescence, otherwise it will return. [2] A band of white bloom on the brick below a window is not a cleaning problem. It is a report that water is moving through that masonry and evaporating out of its face, and the sill above it is the first thing to check.

Corrosion of concealed materials is the one that gets expensive. Anything steel in the wall below or beside the opening is being wetted repeatedly by a detail that is supposed to keep it dry. Damaged interior finishes are the version of this the occupant notices, and BIA specifically advises that interior surveys should note stains or damage to finishes that may indicate potential water ingress. [2] Plaster lifting under a window is exterior evidence that happens to be visible from inside.

The sequence matters more than any individual symptom. BIA’s summary of its own maintenance guidance ends on it: if the problem is moisture related, then the source of moisture should be determined and corrected before other repairs are initiated. [2] NPS says the same thing at greater length, listing leaking roofs or gutters, differential settlement, capillary action causing rising damp and extreme weather exposure as root causes that should always be dealt with prior to beginning work, and warning that without appropriate repairs to eliminate the source of the problem, mortar deterioration will continue and any repointing will have been a waste of time and money. [3]

Why the flashing under the sill matters more than the stone

In short: Flashing is the layer that collects whatever gets past the stone, and BIA requires it wherever a cavity or air space is interrupted, which includes every sill.

The stone is the first line. The flashing is the one that decides what happens when the first line is imperfect, which over a long enough period it always is. BIA puts the requirement plainly: in general, when a collar joint, cavity or air space is interrupted, such as at sills, at the base of the walls, at lintels over openings and at shelf angle supports, flashing should be provided in the wall. Its function is to serve as a collector for any moisture penetrating the wall or the sill. [1]

Collecting water is only half the job; the flashing also has to discharge it. BIA says the flashing should extend through the brick to the exterior face of the wall at its lower end and be turned down at least 6 mm to form a drip. [1] Flashing that stops short of the face does not drain to the outside. It drains to somewhere inside the wall, which is the opposite of the intent.

Then there are the ends, and this is the detail most often missing in older work. BIA says the flashing at the sill should extend beyond the ends of the sill to the first head joint outside the jamb of the opening, and should be turned up and outward for a distance of at least 1 in. (25 mm) at each end. The reasoning is given directly: if the ends are not turned up and out, the moisture collected on the flashing will have a path into the adjacent wall and there is no way to predict where it may go. [1] A flat piece of flashing with open ends is a tray with two sides missing. It collects water efficiently and then hands it to the wall.

Material choice is constrained too. BIA lists copper, lead and plastics as flashing materials for sills, and explicitly does not recommend aluminium or asphaltic-impregnated felt: aluminium because alkalies in the cement of the mortar may attack it and cause corrosion, and asphaltic-impregnated felt because it is easily punctured during construction. For the same reason plastic films of less than 20 mil thickness should also be avoided, and some plastics are subject to continued degradation after extended exposure to sunlight. [1] BIA adds that once flashing has been punctured it ceases to fulfil its function, so in-place flashing should be inspected for punctures and tears and appropriately repaired before brickwork is laid on it. [1]

Service life is worth knowing before anyone specifies a material. BIA’s estimated times before repair give metal coping and flashing 20 to 75 years and plastic flashing 5 to 25 years. [2] On a building where the sills are being rebuilt anyway, that spread is the difference between doing this once and doing it twice.

Weepholes finish the path. BIA says that once moisture has been collected on the flashing it must be removed from the wall, and that this is the function of weepholes. They should be placed on top of the flashing, not one course up. [1] Spacing is specified: where wick-type materials are employed or hidden flashing is used, weepholes should have a maximum horizontal spacing of 16 in. (400 mm); with open weepholes and no wicks the horizontal spacing may be increased to 24 in. (600 mm) maximum. [1] Weeps a course too high hold a reservoir of water permanently on the flashing below them, and weeps that have been pointed shut during a previous repointing job do the same thing. Missing or clogged weeps appears on BIA’s inspection checklist and again in its table of probable causes of distress. [2]

What is a drip, and why does every sill need one?

In short: It is the feature that stops water running back along the underside of the stone to the wall. BIA says every sill should have one.

Water running off the edge of a sill does not reliably fall. Surface tension will carry it around the edge and back along the underside toward the building unless something interrupts it. The interrupting feature is the drip, and BIA’s requirement is unqualified: every sill should be provided with a drip, whose function is to prevent water from returning to the exterior face of the wall. [1]

On a brick sill the drip is simply the lower corner of the brickwork, which a properly sloped sill produces for free. [1] On a concrete or stone sill it has to be made. BIA says it is usually formed, or cut into the bottom face of the sill, and can be cut in several shapes, vee-shaped, rectangular, semi-circular, or a combination of these. The shape is not important; its presence and location are. [1]

Location is specified as a dimension. The inner lip of the drip should be located a minimum of 1 in. (25 mm) from the exterior face of the wall. [1] That distance is what guarantees the water releases into open air rather than somewhere it can find its way back to the masonry. Combined with the requirement that the sill project at least 1 in. (25 mm) past the wall face, [1] it defines a specific geometry, and a sill that fails either half of it will wet the wall below regardless of how good its top slope is.

This is why BIA’s inspection checklist for caps, copings and sills carries drips needed as a discrete line item alongside inadequate slope. [2] Missing drips and back-pitched tops are different failures with the same outcome, and a sill can have one problem without the other. On older Chicago limestone the common finding is a drip that was cut correctly and has since been filled, usually by a previous coating or by mortar during a repointing job that treated the underside of the sill as a joint to be packed.

Why do the mortar joints under a sill fail first?

In short: Because that is where the water is being delivered, and because the repair is usually made with mortar that is too strong for what it is going into.

The joints immediately under and beside a sill are the ones taking the water a failed sill sheds. BIA lists the conditions that call for repointing: mortar erosion exceeding 1/4 in. (6.4 mm), crumbling mortar, mortar with voids, hairline cracks in the mortar, and cracks between the brick and mortar. [2] Under a back-pitched sill you will usually find several of those at once, and they will be concentrated in a band two or three courses deep that stops abruptly where the wetting stops.

Repointing that band is the right repair, and doing it with the wrong mortar is one of the more expensive mistakes available in masonry. BIA is direct: 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, because stronger repointing mortar increases the stress concentration on the brick and mortar interface and can lead to spalling of the brick face. [2] Type N is generally recommended for modern applications, Type O is appropriate where mortars with higher cement content would be too strong, and Type K, the weakest, is generally reserved for historic masonry. [2]

NPS frames the same rule in terms of the relationship between the three materials rather than in terms of a mortar type. 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. [3] It adds the correction that most people need to hear: it is a common error to assume that hardness or high strength is a measure of appropriateness, particularly for lime-based historic mortars. [3] On a nineteenth-century Chicago wall with soft brick and a lime mortar, a modern high-cement mix packed into the joints under a sill will not fail. The brick around it will.

Preparation is specified in both documents and the numbers are close enough to act on. BIA says deteriorated mortar should be removed by a toothing chisel or a special pointer’s grinder to a uniform depth that is the minimum of twice the joint width, generally 3/4 in. (19 mm), or until sound mortar is reached. [2] NPS asks for a minimum depth of 2 to 2-1/2 times the width of the joint to ensure an adequate bond and prevent mortar popouts, which for most brick joints means roughly 1/2 to 1 inch, and adds that any loose or disintegrated mortar beyond that minimum depth should also be removed. [3]

Both are wary about power tools. BIA flags the potential for power tools to damage the brick surrounding the mortar being removed, and notes that using a grinder on head joints will not remove the full depth of material without damaging adjacent brick, so the remaining mortar has to be taken out with a chisel. [2] NPS says the traditional method of hand chisels and mash hammers, though labour-intensive, poses the least threat of damage and produces the best final product, and that the use of power tools by unskilled masons can be disastrous for historic masonry, particularly soft brick. [3]

Filling is done in thin lifts, not in one pass. BIA says the mortar should be packed tightly into the joints in layers no more than 1/4 in. (6.4 mm) thick and tooled when thumbprint hard, with the last layer tooled to match the original profile. [2] NPS describes the same process, filling the back of the joint first by applying approximately 1/4 inch of mortar and packing it well into the back corners, then adding another layer of about the same thickness as soon as the previous one has reached thumb-print hardness, and stresses that each layer needs time to harden before the next is applied. [3]

BIA also recommends prehydrating repointing mortar to reduce excessive shrinkage: 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 it in that condition for 1 to 1-1/2 hours before adding water for placement. [2] Since it is difficult to determine which joints are letting moisture through, BIA advises repointing all the mortar joints in the affected wall area rather than spot-patching. [2]

Can a stone sill be re-sloped, or does it have to be replaced?

In short: That question belongs to the engineer BIA says should look at negative slope, because the answer depends on why the sill moved.

There is no honest general answer to this, and the reason is worth stating rather than papering over. BIA classifies neutral or negative slope on masonry sills as structural and says such conditions generally warrant an investigation performed by a professional engineer to determine the cause of the issue and to recommend the appropriate repair method. [2] The scope of the repair is an output of that investigation, not an input to it. Anyone who quotes a fixed remedy for a back-pitched sill before establishing why it is back-pitched is guessing.

What can be said is what the finished detail has to achieve, because that is specified. Whatever is done, the result needs the slope away from the building of at least 15 degrees from horizontal, the end-to-center slope of 1/8 in. (3 mm) to 12 in. (300 mm) where the sill is concrete or stone, the projection of at least 1 in. (25 mm) past the wall face, a drip with its inner lip at least 1 in. (25 mm) back from that face, flashing turned down at the face and turned up and outward at least 1 in. (25 mm) at the ends, and weeps on top of the flashing at the specified spacing. [1] Any proposal can be measured against that list.

Replacement brings its own requirements that are easy to overlook. BIA says that brick masonry sills of short length, 4 ft (1.2 m) or less, need no special anchorage, but sills of brick, concrete, metal and stone having long runs should be anchored to the masonry below or behind the sill. That anchorage requires penetration of the flashing, and BIA warns that care must be taken to ensure these penetrations are adequately sealed so that the flashing functions as intended. [1] A long replacement sill that is anchored without sealing the anchor penetrations has a new leak designed into it.

Long runs also need movement accommodated. BIA says prefabricated brick, precast concrete or stone sills should have section lengths as long as is practical, that the joints between long sill sections should be constructed using a soft joint, and that in very long runs it may be necessary to provide expansion joints at the ends where the sill abuts the jamb. [1] Where expansion joints are aligned with window jamb lines, the expansion joint should also be installed through the sill, and if the sill extends beyond the jamb the expansion joint should be continuous around the entire sill extension, as should the flashing. [1]

One thing that is not a repair for this: sealing the stone. BIA allows that water repellents may be used to reduce the amount of water absorbed by masonry subject to extreme exposures, and names sills among those, but attaches conditions. Their use should be considered only after completing repair and replacement of brick, mortar joints and other building elements, and they should not replace or be considered equivalent to essential, code-required details that resist water penetration, such as flashing and weeps. [2] Only water repellents that permit evaporation and the passage of water vapour, such as siloxanes and silanes, should be used, and film-forming coatings should not be applied to exterior brickwork. [2] BIA’s estimated time before repair for water repellents on walls is 5 to 10 years. [2] A coating over a sill that drains the wrong way buys a few seasons and changes nothing about where the water goes.

How do you catch a failing sill before the wall pays for it?

In short: Seasonal inspections, binoculars for the upper floors, and a level on the sills you can reach.

BIA’s maintenance programme has three components: conducting general periodic inspections to document existing condition and identify potential performance issues, performing known regularly scheduled maintenance tasks, and executing specific repairs to correct issues identified during the inspection. [2] Sills sit in the first component, because the condition that matters is visible and measurable long before it produces damage worth repairing.

Timing is part of the method. BIA says inspections can be performed monthly, yearly, biennially or on any schedule deemed appropriate, and recommends seasonal inspection periods so that the behaviour of building materials in various weather conditions can be observed. [2] For a sill, the informative moment is during or just after rain, when you can see whether water is leaving the stone at its outer edge or pooling on it and tracking back to the wall. A dry sill tells you less than a wet one.

Getting eyes on the upper floors is the part most owners skip. BIA recommends that exterior surveys be performed with binoculars to permit close-range observation of conditions on upper floors, and that adjacent balconies or roof areas be used to observe portions of the facade that are difficult to see from the ground. [2] Sills above the second floor are the ones that go unexamined longest, and they are also the ones getting the most exposure.

Record what you find in a form you can compare against later. BIA says documentation of each inspection should include comments, photographs and sketches to identify changes in materials, potential performance issues and subsequent maintenance tasks, that documenting conditions on floor plans or elevation drawings can help identify patterns of damage, and that repairs should be documented with before-and-after photographs, with both inspection and repair records kept and referenced during future inspections. [2] A pattern across an elevation is the thing that distinguishes a sill problem from a building problem, and you cannot see a pattern in a single visit.

One caution on intervals. BIA’s estimated times to repair are based on brickwork in vertical applications exposed to normal weathering conditions, and it notes that sills, parapets, chimneys and copings which experience more severe exposures may require repairs at shorter intervals. [2] Sills are horizontal surfaces on a vertical building. They hold water, snow and ice in a way the wall does not, and the published intervals are the optimistic end of what to expect from them.

Stone sill questions Chicago building owners ask

What does negative slope on a stone sill mean?

It means the top surface of the sill now falls back toward the building instead of away from it, so water collects against the wall rather than draining off the front edge. Neutral slope is the flat version of the same failure. BIA lists neutral or negative slope on masonry sills and water tables among observed conditions that are structural. [2]

Is a back-pitched sill a structural problem?

BIA treats it as one. It appears on the list of conditions that are structural, outside the scope of routine maintenance, and that generally warrant an investigation performed by a professional engineer to determine the cause and recommend the repair method, alongside cracks through multiple brick units and out-of-plane movement of the wall. [2]

How much slope should a stone sill have?

For concrete or stone sills, BIA asks for a slope away from the building of at least 15 degrees from horizontal, plus a slope from the ends toward the center of 1/8 in. (3 mm) to 12 in. (300 mm). On sills longer than 4 ft (1.2 m) the end slope should extend at least 2 ft (600 mm) from each end. [1]

How far should a sill stick out past the wall?

At least 1 in. (25 mm) beyond the face of the wall at its closest point to the wall. [1] The drip on the underside should have its inner lip at least 1 in. (25 mm) back from the exterior face. [1]

What is a drip on a sill and do I need one?

It is a formed or cut recess in the underside of the sill that stops water travelling back along the bottom face to the wall. BIA says every sill should be provided with one, and that while the shape can be vee, rectangular, semi-circular or a combination, its presence and location are what matter. [1] On a brick sill the drip is the lower corner of the brickwork itself. [1]

Should there be flashing under a stone sill?

Yes. BIA says flashing should be provided wherever a collar joint, cavity or air space is interrupted, and names sills specifically. It should extend through the brick to the exterior face and be turned down at least 6 mm to form a drip, and it should extend beyond the ends of the sill to the first head joint outside the jamb, turned up and outward at least 1 in. (25 mm) at each end. [1]

Why do the ends of sill flashing have to be turned up?

Because otherwise the water the flashing collects runs off its ends into the wall. BIA puts it directly: if the ends are not turned up and out, the moisture collected on the flashing will have a path into the adjacent wall and there is no way to predict where it may go. [1]

Where should the weepholes be?

On top of the flashing, not one course up. [1] Maximum horizontal spacing is 16 in. (400 mm) where wick-type materials or hidden flashing are used, and up to 24 in. (600 mm) for open weepholes with no wicks. [1] Missing or clogged weeps is on BIA’s inspection checklist and on its list of probable causes of masonry distress. [2]

Can I just repoint the joints under the sill?

Not on its own. NPS says the root cause of deterioration should always be dealt with before work begins, and that without repairs to eliminate the source, mortar deterioration will continue and any repointing will have been a waste of time and money. [3] BIA’s summary reaches the same conclusion: if the problem is moisture related, the source should be determined and corrected before other repairs are initiated. [2]

What mortar should be used under a sill on an old building?

One no stronger than what is already there. BIA says the compressive strength of the repointing mortar must be equal to or lower than that of the original, because a stronger mortar increases stress concentration at the brick and mortar interface and can lead to spalling of the brick face. [2] NPS states it as a relationship: softer or more permeable than the masonry units, and no harder or more impermeable than the historic mortar. [3]

Will sealing the sill fix it?

No. BIA allows water repellents on masonry subject to extreme exposures including sills, but only after repair and replacement of brick, mortar joints and other elements is complete, and says they should not replace or be considered equivalent to essential code-required details that resist water penetration such as flashing and weeps. [2] Its estimated time before repair for water repellents on walls is 5 to 10 years. [2]

How often should sills be inspected?

BIA recommends periodic inspections on a schedule deemed appropriate, with seasonal inspection periods so material behaviour can be observed in different weather, and advises binoculars for upper floors. [2] It also notes that sills, parapets, chimneys and copings experiencing more severe exposures may need repairs at shorter intervals than its published estimates. [2]

Stats box

Definition bank

Entity cards

Stone sill

Negative slope

Sill flashing

Drip

Weepholes

Reading a sill before anyone prices the repair

In short: Put a level on it, look underneath it, and ask where the water is meant to go. Those three checks decide what kind of job this is.

A sill that drains the wrong way is not a cosmetic complaint and it is not a repointing job. BIA has put neutral and negative slope on masonry sills into the structural column, next to cracking through multiple units and out-of-plane wall movement, and recommends a professional engineer establish the cause before anyone recommends a repair method. [2] That is the first thing a serious quote will acknowledge.

The second thing is whether the proposed work reaches the parts that actually move water. A rebuilt or re-set sill that recovers its 15 degree slope away from the building and its 1/8 in. per 12 in. slope toward the center, projects 1 in. past the wall face, carries a drip set 1 in. back from that face, sits on flashing turned down at the front and turned up at both ends, and drains through weeps sitting directly on that flashing, is a sill that will outlive the people who paid for it. [1] A sill that gets a bead of sealant and a coat of repellent is a sill you will be discussing again in a few seasons. [2]

The third is sequence. Both of the documents behind this article end up in the same place: correct the source of the water before repairing what the water damaged. [2] [3] If a quote proposes repointing the courses under the opening without saying a word about the slope of the stone above them, it has priced the damage and left the cause in place.

If you want a straight read on what your sills are doing, we can look at them and tell you which category they fall into. See our stone sill repair service for what the work involves, or get a free quote and we will come out and put a level on them.

Sources

1. Brick Industry Association, Technical Notes 36, “Brick Masonry Details, Sills and Soffits”, revised July/August 1981, reissued January 1988. https://www.gobrick.com/media/file/36-brick-masonry-details-sills-and-soffits.pdf
2. Brick Industry Association, Technical Note 46, “Maintenance of Brick Masonry”, December 2017. https://www.gobrick.com/media/file/46-maintenance-of-brick-masonry.pdf
3. 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