Three Years Old, and the Bottom Metre Had Gone
The stone came off the front elevation in an afternoon, a bar and a skip, and the wall behind it read like a diagram of its own failure. A band of oriented strand board about knee height, black at the edges and soft enough to take a thumbnail, running the full width of the elevation and stopping cleanly where the panel above it was still dry and bright. Then four separate patches higher up, one around each front window, each one widest at the head and tapering down past the jambs in a fan — the shape water makes when it lands on top of something and spreads before it soaks in. Two bottom plates were spongy at the ends. One rim joist had a dark stripe that only showed once the barrier came off.
The stone itself was not the problem, which is the part that catches people out. Not a cracked joint on the elevation, no spalling, no piece drummy enough to sound hollow under a knuckle, no failure of bond anywhere the units were prised. What had failed were two terminations: the line where the wall stopped at the bottom, and the line where it stopped over each opening. Everything between those two lines had done its job for three years, and everything above and below them had rotted, because water arriving in the wall had nowhere to go once it was in.
That is the shape of nearly every adhered stone failure worth investigating, and it is why the detailing hours on the next one belong almost entirely at the base and at the heads. What follows is that job: what the assembly is actually doing with water, what the bottom edge has to be made of, and what has to be wrapped around each opening before a single sheet of lath is stapled up.
A Wall Designed to Be Wet
Adhered manufactured stone is thin units set in a mortar bed over a scratch coat over lath — cement, sand and lightweight aggregate from front to back, with a stone face on it. Every one of those materials takes on water and holds it. A wind-driven shower does not run off a stone elevation the way it runs off a coated metal panel; it soaks in, travels sideways through the joints, and sits in the setting bed for hours after the rain has stopped and for days after a long wet spell in cold weather. The trade term for this is a reservoir cladding, and the description is literal: the wall is a sponge with a decorative face, and it is supposed to be.
That is only a problem if the water has no route out and nothing to dry into. Two things have to be true behind the stone. There must be a continuous water-resistive barrier that the wet mortar cannot bond to, so that water reaching it runs down a surface rather than wicking across into sheathing; and there must be a drainage path from the back of that barrier down to daylight, uninterrupted from the top of the wall to the bottom. ASTM C1780, the standard practice for installing adhered manufactured stone masonry veneer, is the document that sets out both, and ASTM C1670 covers the units themselves. The reason two layers of asphalt-saturated building paper behind the lath became the traditional detail over wood-based sheathing — Grade D paper to Federal Specification UU-B-790a — is precisely this bond question: the scratch coat grips the outer layer hard, so the inner layer stays as a plane the mortar has never touched.
The Layer That Is Actually Doing the Draining
Read the wall outward from the studs and it is seven separate purchases, of which one is the drainage and the rest are structure, bond and appearance. That distinction matters on a takeoff, because the drainage layer is usually the cheapest item in the stack and it is the only one whose absence produces the strip-out described above.
The older two-layer paper detail relies on the sacrificial outer sheet to keep a drainage plane behind it, and on a good day that is what it does. What it cannot do is stay open once the scratch coat has squeezed through the lath and pressed the two sheets flat together against the sheathing, which is what a firmly buttered scratch coat on a cold morning does over a fair proportion of the wall. Current manufacturer literature and the detailing options published by the Masonry Veneer Manufacturers Association have moved steadily toward a deliberate drainage space behind the lath for exactly this reason — an entangled-mesh drainage mat, a dimpled sheet, or a purpose-made drainage board that also carries the lath fixings. The thickness is a property of the product, not a rule of thumb, and it is the veneer manufacturer's written instruction that the warranty is actually enforced against.
Whatever provides the gap, two rules govern how it is installed and they are both about the ends of it. The drainage layer has to run down to and onto the base termination, not stop tidily fifty millimetres above it, because a gap that discharges onto the back of a barrier is only moving the wet patch down. And it has to be cut around every opening so that it lands on the head flashing rather than behind it. On the quantity side it is bought against wall area with a real waste allowance, since it is cut around openings, penetrations and every internal corner, and the offcuts from a cut-up elevation are not small.
What sits between the stone and the studs
- Stone units — the weather face and a water store rather than a seal; flats are bought by area and corners by the linear foot of arris Manufactured Stone Veneer Corner Unit Calculator
- Scratch coat and setting bed — two separate mortar operations bought against the same elevation area, and the pair of them stay damp long after the face has dried Manufactured Stone Veneer Mortar Coverage Calculator
- Metal lath — the reinforcement the scratch coat keys into, fastened back through everything outboard of the barrier into framing Stucco Lath Calculator
- Drained gap — the only part of the wall whose job is to stay empty, and the part that has to arrive at the base termination rather than near it Cavity Wall Drainage Mat Area Calculator
- Water-resistive barrier — laid in courses lapped downward, in the number of layers the substrate and the veneer instruction between them call for House Wrap Calculator
- Weep screed — one bent profile doing three jobs at once: barrier termination, lath and render stop, and the slotted lip water leaves by Stone Veneer Weep Screed Linear Footage Calculator
- Sheathed frame — carries every lath fixing and, in the failures worth investigating, is the only component that was ever actually damaged Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof)
Take the gap off the same gross elevation areas you priced the stone from, and set the waste high — this is a roll product cut around every opening, every corner and every penetration on the wall.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The total veneer wall area needing cavity drainage.
Extra material for roll overlap, cuts around ties, and openings.
Drainage mat needed
352 ft²
They open the calculator with your figures already in it
Cavity Wall Drainage Mat Area Calculator: 352 ft² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The waste factor is applied to whatever area you type in, so openings are either counted twice or not at all depending on how you measured. Enter the gross elevation of a wall carrying glazing and the 10% allowance is added to the window openings as well, returning more than the netted-out wall needs; enter the net area instead and that same 10% has to absorb every offcut around the reveals.
- Wall area is the right basis only for a full-height drainage mat or mesh. Products that are mortar-collection devices sitting in the bottom few courses of the cavity are bought by the linear metre of wall, so a 10 m long wall needs 10 m of that product, not the 33 m² (355 ft²) this page returns for a 30 m² (323 ft²) elevation.
- The waste factor does not know your roll width or your lap width. Side laps consume a fixed width at every vertical seam rather than a percentage of the area, so a 1 m wide roll lapped 100 mm loses roughly 11% to overlap alone, which is more than the 10% default and more than twice the 5% floor this field accepts, before a single cut around a tie, a cavity tray or a reveal.
- Sizing the mat says nothing about whether it can drain. It only works if it runs down onto a cavity tray or DPC and terminates above open weep holes, and the mat itself consumes cavity width, so the residual clear cavity, the wall tie length and the tie embedment into each leaf have to be checked against the specification separately. None of those follow from an area.
Everything Above Lands on One Piece of Bent Metal
A weep screed is a formed profile, usually galvanised steel or a rigid plastic, and it looks trivial in a bundle on the ground. It is doing three jobs simultaneously, and it is the only component on the wall that is doing more than one. Its vertical attachment flange is fastened to the framing through the sheathing and gives the water-resistive barrier a hard edge to terminate against. Its ground sets the thickness of the scratch coat and stops the lath at a consistent line. And its lower lip is slotted or perforated and turned outward, so that water arriving down the drained gap is discharged clear of the foundation instead of pooling on the flange.
One rule decides whether it works, and it is a lap rule rather than a fixing rule: the barrier goes over the screed's attachment flange, never behind it. ASTM C1063, which governs the installation of lathing and furring for cement plaster, sets this out and it is the same detail whether the finish above is render or adhered stone. Behind the flange, every millimetre of water running down the barrier is delivered into the top edge of the sheathing at the exact height where the framing is least able to dry. Over the flange, the same water is delivered onto metal that is already pointing away from the building. That single decision, taken by whoever fixed the paper on a Thursday morning, is the difference between the two walls in the introduction.
The second thing that ends a screed's usefulness is nothing to do with how it was installed. It gets buried. The clearances below are the figures the United States model code sets for adhered masonry veneer, and they are repeated in ASTM C1780; the ones that fail are almost never wrong on the day of the final inspection. They are wrong two summers later, when a bed has been topped up with mulch, a drive has been resurfaced over the old one, or a patio has been laid to the wall. Photograph the screed line at practical completion, and tell the client in writing what the number is and what will happen if it is covered.
The quantity itself is the easy part of the job: the screed runs continuously along the base of every wall face receiving stone, so the take is a base perimeter rather than an elevation area. What the perimeter figure does not include is the splice laps where lengths meet, the offcuts at every external and internal corner, and the short returns at porch piers, bay faces and wing wall ends. A run with a dozen corners in it generates a great deal more scrap than a straight elevation of the same length, which is what the waste allowance is for. Lap splices in the direction water travels along the run, and where a base sits in permanently damp ground or in a coastal exposure, pay the difference for a stainless or plastic profile rather than watching a galvanised one rust from the lip inward.
- Fix the screed through its attachment flange into framing, level and continuous, before any barrier or lath goes near the wall.
- Lap the water-resistive barrier down over the flange so its lower edge finishes on the metal, and tape or seal the barrier laps to the flange only where the barrier manufacturer says to.
- Bring the drainage layer down onto the screed so it discharges over the lip rather than terminating above it.
- Stop the lath at the screed's ground; lath run down past it holds mortar in the drainage lip and blocks the weeps.
- Keep the scratch coat and setting bed off the lip, and clean any mortar that finds its way there before it sets.
- Check the finished clearance again after landscaping, paving and any deck framing is complete, not before.
| What is below the wall | Model-code clearance | How it gets lost |
|---|---|---|
| Earth, planting bed or gravel margin | 102 mm (4 in.) from finished grade to the bottom edge of the veneer | Mulch topped up annually, a bed raised by a landscaper, or spoil left against the wall by a later trade |
| Paving, path or driveway | 51 mm (2 in.) from the finished paved surface | A drive resurfaced over the existing one, or a path relaid on a thicker bed without anyone looking up |
| A walking surface on the same foundation | 12.7 mm (½ in.) where the slab and the wall are supported by the same footing and move together | Tile or a topping laid over the original slab, closing a clearance that was tight to begin with |
| Anything outside the US model code | Not these numbers | UK work is checked against a damp-proof course above finished ground level under NHBC Standards and Approved Document C; Australian work against AS 3700 and AS/NZS 2904. Read the adopted document before quoting a figure from this table |
Measure the base perimeter face by face rather than taking the building's overall dimensions, then let the waste allowance carry the splice laps and the corner offcuts — a base with a porch and a bay in it is not a straight run.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The total perimeter of the wall base receiving stone veneer.
Extra material to allow for cut waste and overlaps at corners and splices.
Weep screed needed
157.5 ft
They open the calculator with your figures already in it
Stone Veneer Weep Screed Linear Footage Calculator: 157 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Length is all this settles; where the screed sits is a code rule it never sees. The IRC puts the weep screed at or below the foundation plate line and a minimum of 100 mm (4 in) above earth or 50 mm (2 in) above paving, with the water-resistive barrier lapped OVER its nailing flange. Set it lower than that, or run the barrier behind the flange, and the drainage path this component exists to create runs water into the wall rather than out of it.
- The perimeter is a plan measurement and the screed follows the ground. On a sloping site or a stepped foundation the base line steps down with the footings, and every step is an extra cut, a lap and a closure; the same applies where the run rises over a garage door head or crosses a porch roof. A length taken off the floor plan is short on any elevation that does not sit on one level.
Where the Run Stops, and What a Roof Does to It
A base perimeter is not the only place a stone wall ends horizontally. It ends over a porch roof, over a canopy, at a band course, where stone gives way to boarding on the gable above, and at the top of any wall that finishes short of a soffit. Each one of those is the same problem as the base and needs the same three functions performed by whatever profile is chosen: the barrier is terminated, the lath and mortar are stopped at a clean line, and the water in the drained gap is discharged outward. A casing bead with no drainage function, run across the bottom of a stone panel above a canopy, is a shelf inside the wall.
The roof-to-wall intersection deserves naming on its own, because it is where the largest single volume of water on the elevation is delivered. Where a sloping roof edge runs into a wall face, step flashing carries water down the junction and the last piece at the eave has to be a kickout — a formed diverter that throws the flow out into the gutter rather than letting it run on down behind the cladding. The International Residential Code requires one at that intersection in Section R703, and its absence is the single most reliable predictor of concentrated rot in a stone or rendered wall, because it delivers a roof's worth of runoff into one stud bay. The stone has to be held clear above the roofing, too, with the vertical leg of the flashing running up behind the barrier so the lap order at that junction reads the same as everywhere else.
Every Opening Is a Second Base Course
An opening is a hole cut through every layer of the wall at once, and the assembly has to be reconstituted around it in a way that keeps water travelling downward and outward. The fan-shaped stains around the four front windows in the introduction were not window leaks in the sense that the units failed; the units were fine. Water travelling down the face of the barrier reached the head of each opening, found the top edge of the window's own flange, and was held there long enough to find the joint. There was no metal at the head and no membrane running past the corners, so the water did the only thing left to it.
The order in which self-adhered flashing goes on is the whole detail, and it is the order water travels, not the order that is convenient from a ladder. Sill first, so that anything reaching the bottom of the opening is already lying on membrane that runs out onto the face of the sheathing. Then the two jambs, lapping over the ends of the sill piece. Then the head, lapping over the tops of both jambs. Every piece runs past the corner it turns by a stated distance so that the piece placed over it can cover the whole overlap. ASTM E2112, the practice for installing exterior windows, doors and skylights, is the source of that shingled sequence, and AAMA 711 is the specification the membrane itself is bought against. A single reversed lap anywhere in that sequence builds a funnel that points into the wall, and it will not be found by any amount of sealant applied afterwards.
Self-adhered membrane on a real site is less forgiving than the detail drawings suggest. Oriented strand board usually needs a primer for the adhesive to develop its bond, cold weather application has a stated minimum temperature that is routinely ignored on a February morning, and every piece needs to be rolled rather than pressed with a gloved hand. A piece that touches down crooked cannot be lifted and repositioned without tearing both itself and the face of the board, so the offcut allowance on a first stone job with a new crew is genuinely different from the allowance on the fourth.
The take-off follows the same shape as the fixing sequence. Group the schedule by opening type rather than averaging it, because the run-past allowance is charged per opening and does not scale with size — twelve small openings consume far more membrane per square metre of glass than three large ones. Add the corner patches at the two bottom corners of every sill, and add the end laps wherever a run has to be jointed. Then work out how much of a roll that is, remembering that a sill piece which has to turn up both jambs and out over the face often needs a wider roll than the jambs do, so a single schedule may be two products.
- Cut the barrier at the opening so a head flap can be lifted clear and laid back down over the head flashing at the end of the sequence.
- Form the sill: pan or membrane sloped outward, turned up at both ends, with a corner patch at each bottom corner and a run-past onto the face beyond the jambs.
- Run each jamb piece the full height of the opening plus a run-past top and bottom, lapping over the ends of the sill.
- Set and fix the unit into the prepared opening, then flash the head over the tops of both jamb pieces.
- Lay the barrier head flap back down over the head flashing and tape the diagonal cuts, so the plane above sheds onto the metal.
- Photograph each opening finished before the drainage layer covers it — this is the last moment the sequence can be read.
Run it once per opening type on the schedule and add the results; the run-past at each corner is what makes the total, and averaging a mixed schedule into one opening size will understate it every time.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
Width of the framed opening the flashing wraps.
Height of the framed opening from sill to head.
How many openings on the schedule share these dimensions.
How far each piece runs past the corner it turns.
Width of the membrane roll you are buying.
Length of membrane on one roll.
Allowance per opening for corner patches and end laps.
Extra for misapplied pieces, release-liner losses and offcuts.
Flashing membrane required
259.6 ft
- Sill pieces across the schedule
- 51.7 ft
- Jamb pieces across the schedule
- 129.8 ft
- Head pieces across the schedule
- 51.7 ft
- Corner patch and lap allowance
- 26.4 ft
- Membrane per opening
- 19.67 ft
- Membrane area consumed
- 194.7 ft²
- Rolls to order
- 4 rolls
They open the calculator with your figures already in it
Window Opening Flashing Membrane Calculator: 260 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- One opening size per run. Group a mixed schedule by type and add the results rather than averaging the dimensions.
- Head flashing over a drainage plane, back dams, and the water-resistive barrier laps that go over the jambs are separate items.
The Head Is Where the Last Wall Failed
What was found above the four windows on the strip-out was a bead of sealant across the top of each frame, stone butted hard down onto the frame, and a piece of membrane that stopped exactly at the corners of the opening. No metal anywhere. That detail is watertight for as long as the sealant is, which is a few seasons of ultraviolet and thermal movement, and it has nothing behind it when the sealant goes.
A head over adhered stone wants a metal flashing with a vertical leg run up the sheathing behind the barrier, ends turned up into dams so water cannot run off sideways into the jamb, a horizontal leg over the head of the unit, and a drip that projects clear of the finished face. The projection is where stone differs from most claddings: the finished face is the thickness of a unit plus its setting bed out from the sheathing, which is a great deal further out than a board or a panel, and a drip sized for lap siding will die inside the stone and drain onto the setting bed. Take the projection from the actual assembled thickness of the veneer you are hanging, not from a standard trim detail.
The other half of the head detail is an exit. A flashing behind a reservoir cladding collects water on its upper surface, and if the stone is bedded tight down onto it, that water sits inside the wall against the frame indefinitely. The joint above the head flashing wants to be left open or weeped rather than bedded solid and pointed — an unsealed joint above the metal is the second exit this wall has, and along with the base it is the whole of the drainage design. It looks like an unfinished joint to anyone who does not know what it is for, which is a good reason to say so on the drawing.
| Edge | What it has to do | How it typically fails |
|---|---|---|
| Sill | Collect anything that gets in and carry it outward onto the face of the barrier, with the ends dammed so it cannot run sideways | A flat pan with no slope, or a membrane that stops at the corners so water runs off the end into the jamb stud |
| Jambs | Bridge the barrier to the frame and lap over the sill piece, so water on either plane keeps travelling down | Fitted after the head, producing a reverse lap that no amount of sealant afterwards will correct |
| Head | Divert the whole plane above the opening outward, past the jambs, and past the thickness of the stone | Sealant instead of metal, no end dams, a drip that stops short of the veneer face, or a mortar joint bedded solid on top of the flashing |
Sealant Helps at Three Joints and Dams Everywhere Else
Sealant has a real place on this wall. It seals the interior perimeter of the window to the frame as part of the air barrier, it closes joints in metal flashing where the manufacturer's detail calls for it, and it finishes the junction where stone meets a different material at a vertical line. ASTM C1193, the guide for the use of joint sealants, covers how those joints are shaped and sized so the material can actually move. What sealant must never be asked to do is close one of the two exits: a bead across the underside of a head, along the lip of a weep screed, or filling the joint above head flashing turns a drained wall into a sealed one that was never designed to be sealed.
The argument to have with whoever wants that bead is a maintenance argument rather than a technical one. Every sealant joint on the building has a service life shorter than the wall, and the whole point of a drained assembly is that it keeps working through the failure of its outermost line. A wall that depends on sealant has moved its entire water management onto a component the owner has to replace on a cycle nobody will diarise, and that is what turns a maintenance item into rotted sheathing.
Proving It Before a Single Stone Is Buttered
Every decision described above becomes invisible the moment the lath goes on, and unrecoverable the moment the scratch coat does. That makes the pre-lath inspection the highest-value half hour on the whole job, and it is worth writing into the programme as a hold point rather than leaving it to whoever happens to be on site.
Two kinds of evidence are worth having. Photographs are the cheap kind: every opening finished, both ends of the head flashing showing the turned-up dams, the base screed with the barrier lapping over its flange, and a wide shot of each elevation with a tape in it for scale. The expensive kind is a water test, and it needs to be honest about what it proves — a spray rack test to ASTM E1105 measures water penetration through an installed window and its immediate perimeter, not through the wall as a whole, so it answers the fenestration question rather than the cladding one. A hose run over the head of an opening from above, watched from inside with the barrier still exposed, proves less formally but proves it on the day it can still be fixed.
The last item on the list is calendar work rather than site work. Come back after the landscaping and the paving are finished and check the base clearance again, because that is the number this trade loses most often and it is lost by someone else's crew weeks after the scaffold has gone. Note it in the handover with the photographs attached. It is the one piece of maintenance advice on a stone elevation that will decide whether the sheathing behind it is dry in ten years.
- Walk the barrier before lath: continuous, lapped downward everywhere, and turning over the screed flange rather than behind it.
- Check every head flashing for turned-up end dams and for a drip that projects past the assembled thickness of the stone.
- Confirm the drainage layer reaches the screed and is not stopped short above it at any point along the base.
- Look for reverse laps at the four corners of every opening, which is where the sequence goes wrong most often.
- Photograph each elevation, then hose-test one opening of each type from above with the inside face still visible.
- Re-measure the base clearance after landscaping and paving, and record it in the handover with a written warning about burying it.
The drainage take-off, base upward
The stone quantities are a separate exercise and they can be got wrong by a pallet without hurting anybody. These are the lines that decide whether the sheathing is dry in a decade, and they are all cheap relative to what they protect.
- Weep screed against the base perimeter, face by face — Measured along the wall base, not around the building. Waste carries the splice laps and the offcuts at every external and internal corner, and a base with a porch, a bay and a chimney in it produces far more scrap than its length suggests.
- Drainage layer against gross elevation area, with a real cutting allowance — Cut around openings, corners and every penetration. Ordered to reach the screed at the bottom and to land on the head flashing at every opening, which is a detailing instruction rather than a quantity one.
- Opening flashing membrane, run once per opening type — The run-past at each corner is charged per opening and does not scale with size, so a schedule averaged into one nominal opening always comes in short. Sills may need a wider roll than jambs.
- Head flashing in metal, by the linear metre plus two end dams per opening — The drip projection comes from the assembled thickness of the veneer, so it is a different section from the one a boarded elevation uses. Count the dams as fabrication, not as length.
- Water-resistive barrier in the number of layers the substrate actually calls for — Over wood-based sheathing behind adhered stone this has traditionally been two, for bond-break reasons rather than for redundancy. Where a drainage product replaces the sacrificial layer, the veneer manufacturer's instruction is the document that decides.
- Kickout and step flashing wherever a roof edge dies into a stone face — One kickout per intersection, non-negotiable, plus the step flashing up the run. This is the smallest item on the list and the one whose absence produces the most concentrated damage.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.