The Rafters Are Cut and the Wall Is Not Coming Down
The wall plate is bolted, the rafters for the side return are pitched, and the inspector has been and gone leaving one item behind: stepped cavity trays where the new roof rakes down the flank of the outrigger. The brickwork he is pointing at was laid in 1908, or in 1996, and either way it went up without anybody knowing a roof would one day land halfway up it. Nothing in the request is unreasonable. Nothing about it is convenient either, because the tray has to end up in a bed joint that currently has two and a half metres of standing wall sitting on top of it.
Two things make this job different from putting a tray into a wall you are building. The first is that the line the tray needs is already occupied, so brickwork has to be taken out and put back, and every hour of that is hours the panel above is being held up by less than it was designed to be held up by. The second is that a new-build tray is bedded into both leaves as the mason climbs, and a retrofit tray cannot be — the inner leaf is plastered and lived against on the far side. The retrofit product answers that with a fixed and sealed upstand instead of a built-in one, and how well that upstand is made is the entire difference between a tray and a shelf.
What follows runs in the order the wall gets opened and closed: find out what is inside it, set the line out in courses rather than in millimetres, decide between cutting in and taking down, deal with what the cut removes, give the water somewhere to go, hand the outside over to the roofer, and get it looked at before it disappears. Prices for the two routes differ by a factor rather than a percentage, which is why the survey comes first and the quote comes second.
What sits between the tile and the plaster at an abutment
- Stepped cover flashing — the roofer's piece, dressed over the covering and let into a raked joint, sized as pieces of limited length rather than as one run Exterior Flashing Layout Trim Sheet Calculator
- Outer leaf, opened and rebuilt — the band of brickwork cut out to admit the tray and laid back over it, with open perpends left in the course immediately above Brick Calculator
- Stepped cavity tray — closes the cavity along the abutment and climbs one course at a time, so its count is the rise of the abutment divided by the gauge Cavity Wall Weep Hole Spacing Calculator
- Wall ties across the cut — severed by the slot and reinstated mechanically, since nothing can be built in once both leaves are standing Brick Veneer Wall Tie Density Calculator
- Cavity and whatever fills it — clear, partially filled or blown full years ago, and the survey answers which before anybody prices the tray Cavity Wall Drainage Mat Area Calculator
- Inner leaf and the new ceiling line — the face the retrofit tray upstand is fixed and sealed against, and the surface a failure eventually shows on
Why the Inspector Will Not Take a Bead of Mastic Instead
Above the new roof, that wall is still an external wall. It takes driven rain on its face, the outer leaf lets a share of it through as every masonry outer leaf does, and the water runs down the back of that leaf looking for the bottom. Before the extension existed, the bottom was somewhere far below, at a lintel or at the base DPC, and whatever happened down there happened outdoors. Now the extension has moved the indoors up to meet it. Somewhere around the line of the new roof, the inner leaf stops being a garden wall and starts being the wall of a kitchen, and water running down the cavity crosses that line without noticing.
The tray is the interruption. It closes the cavity along the abutment, catches everything descending from above, and turns it out through openings in the outer leaf so it lands on the roof covering, on the outside of the flashing, and runs away down the tiles. Approved Document C is what building control is applying when it asks for one — walls have to resist the passage of moisture to the inside of the building, and this junction is the single most reliable place on a domestic extension for them not to. A sealant joint at the top of a flashing does not do this job at all: it is on the wrong side of the leaf, it is trying to stop water entering rather than routing water that has already entered, and it has a service life measured against a wall measured in decades.
Find Out What the Cavity Is Full Of Before You Price It
A boroscope, a 12 mm masonry bit and half an hour will change the number on the quote more than any other half hour on this job. Drill through a perpend well above the abutment line, look, and drill again lower down. You are answering four questions and only one of them is on the drawing.
How wide is the cavity, and is it there at all. Plenty of what looks like a cavity wall from the street is a solid nine-inch wall with a snapped-header bond, and there is no tray to fit into it — that job becomes internal tanking or a lining, and it is a different conversation with a different inspector. Where there is a cavity, the width sets which retrofit tray fits: the manufacturers make them to cavity ranges and a tray that has to be squeezed will not sit flat, and a tray that does not sit flat holds water in the middle instead of moving it out.
What is in it. A clear cavity is the easy case. Partial fill boards leave a residual gap you can still work in. A cavity blown full of bead or fibre years ago is the awkward one: the fill comes out of the band you open and it does not go back the same way, the tray now has to sit over insulation that will hold water against it, and somebody has to decide whether the fill above the tray is going to stay wet. That decision belongs to whoever specified the fill and to the tray manufacturer's own instruction, not to the bricklayer standing on the scaffold with a bolster.
What is already in the wall at that height. Old lintels, a former opening bricked up, a flue, or the bearing of a joist all interrupt the line, and each turns one run of tray into two runs with a detail between them. And if the flank belongs to two houses, the Party Wall etc. Act 1996 covers cutting into it, which means notice served and time allowed for it — found on a Tuesday that is a fortnight, found on the day the scaffold arrives it is a fortnight of standing scaffold.
- Drill and scope three points along the abutment line: one at each end and one in the middle, always through a perpend rather than a brick.
- Measure the cavity width at each, and record whether the drill broke through into fill or into air.
- Scope upward as well as down — mortar sitting on ties above the line will land on your new tray the moment you disturb the wall.
- Find the ties: a metal detector across the elevation gives their grid faster than guessing, and tells you what the cut is about to sever.
- Photograph every scope view with a tape against the wall, because the quote and the argument later both rest on it.
- Check whether the flank is a party wall before anything else in this list turns into a programme.
Setting the Line Out in Courses, Not in Millimetres
A stepped tray does not follow the pitch of the roof. It follows the gauge of the brickwork, because it can only sit in a bed joint, and it climbs one course at a time in a staircase whose treads get shorter as the roof gets steeper. On standard British brickwork the riser is fixed at 75 mm — a 65 mm brick and a 10 mm joint — so the only variable is how far along the wall each step travels, and that is the course height divided by the tangent of the pitch. At a shallow lean-to pitch of 15 degrees each step runs 280 mm, comfortably longer than a brick. At 30 degrees it runs 130 mm, which is well under a brick length, so the tray steps twice within the same stretcher and the cut stops being a neat band and becomes a saw-tooth.
Take the pitch off the roof rather than off the drawing. Rafters get adjusted at the wall plate, wall plates get packed, and a lean-to that was drawn at 17.5 degrees is very often built at 19 or 20 by the time the last rafter is fixed. Two degrees changes the horizontal step by about 25 mm, and over five metres of abutment it is two or three extra courses of rise, which is two or three more trays in the order.
Then set the height of the line. The tray has to discharge above the top of the flashing upstand, and the flashing upstand has to clear the roof covering by enough that driven rain and lying snow never reach the top of it — the lead codes of practice put a minimum of 75 mm on that upstand and the tray sits above it, which in practice puts the tray line two or three courses above the tile surface rather than one. Mark it with a laser along the whole run and check it against the courses before a disc goes near the wall, because you cannot split the difference: the tray goes in the bed joint nearest the line, and if that puts it a course higher, the whole staircase moves with it.
| Roof pitch | Horizontal run per course step | Steps across 1 m of horizontal run | Raking length per 1 m of horizontal run |
|---|---|---|---|
| 15 degrees | 280 mm | 3.6 | 1.035 m |
| 20 degrees | 206 mm | 4.9 | 1.064 m |
| 22.5 degrees | 181 mm | 5.5 | 1.082 m |
| 25 degrees | 161 mm | 6.2 | 1.103 m |
| 30 degrees | 130 mm | 7.7 | 1.155 m |
| 35 degrees | 107 mm | 9.3 | 1.221 m |
| 40 degrees | 89 mm | 11.2 | 1.305 m |
| 45 degrees | 75 mm | 13.3 | 1.414 m |
Cut It In, or Take It Down
There are two honest ways to get a tray into a standing wall and one dishonest one. The dishonest one — running a continuous slot along the whole abutment with a wall saw, dropping the tray in and pointing up — is quick, and it is also a metre and a half of unsupported outer leaf resting on a strip of polyethylene. Do not price it and do not let anybody talk you into it because the mortar is soft and the wall is only single skin.
Cutting in is done in bays. Open a short length of outer leaf, no more than about half a metre at a time on ordinary brickwork and less where the mortar is lime and tired, rake out the bed joint across that bay, slide in one tray section, bed it, lay the bricks back over it, and only then move along to the next bay leaving a stretch of untouched wall between the two. The proprietary retrofit trays are made in short handed lengths with a lap built into the end for exactly this reason, so each new section slides under the one already fitted and the joint sheds downhill. It is slow, it is entirely reversible if a bay looks wrong, and it leaves the wall never carrying more than one small hole at a time. Come back to the intermediate bays once the first set has taken.
Taking the panel down is the other route, and above a raking abutment the panel is a triangle: everything outboard of the tray line up to the eaves. It costs more bricks and more scaffold and it buys three things the cut-in route cannot. You get a tray that sits perfectly and is bedded rather than wedged. You get the cavity swept out and, where the specification wants one, a strip of drainage mat dropped down the rebuilt panel to land on the tray instead of a course above it. And you get to see the inner leaf, which nobody has looked at since it was built. On a wall of soft handmade stocks with a poor lime bond, the choice is often made for you: those do not survive being cut and they do not survive being taken down and relaid either, so the bricks come from a reclamation yard and the estimate needs to say so.
Either way, match the brick before the scaffold goes up. A new panel that reads as a patch is a defect the client will see from the garden every day, and the fix — blending in reclaimed bricks and taking the replacements from an inconspicuous elevation — has to be planned when the order is placed, not when the scaffold comes down.
Put the rebuilt panel through it as a length and a height with your actual brick and joint dimensions, because a Victorian imperial stock at a 12 mm joint gives a different rate per square metre from a modern metric brick and the difference across a gable triangle is a pallet.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The length of the brick wall or veneer.
The height of the brick wall or veneer.
Extra bricks for cuts, breakage, and corners.
The actual (not nominal) length of your brick face, before adding the mortar joint.
The actual (not nominal) height of your brick face, before adding the mortar joint.
The thickness of the mortar joint between bricks, both horizontally and vertically.
The brick's depth, which becomes the wythe thickness for a single-wythe veneer wall.
Estimated brick needed
1,177 bricks
- Wall area
- 156 sq ft
- Coverage rate (from your dimensions)
- 6.86 bricks/sq ft
- Base brick count (no waste)
- 1,070 bricks
- Mortar mix needed
- 14 80 lb bags
They open the calculator with your figures already in it
Brick Calculator: 1,177 bricks — 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
- Openings and returns are not in the geometry. The count treats the wall as one plain rectangle of face area, with nothing deducted for doors, windows, vents or reveals and nothing added for corners, returns or piers. Take openings out of the length and height you enter before you read the answer.
- It counts one wythe of brick laid flat, showing its long face. A second wythe or cavity leaf, header courses and rowlock bands, and any bond that turns bricks to show their end all put more units in the same area than this returns. Brick depth changes the mortar figure only, never the brick count.
- The mortar figure is joint geometry, not a mix design. It is the volume of the bed and head joints implied by your joint width and brick depth, converted at one premixed bag's published yield; it excludes the collar joint between wythes, droppings and board waste, and it assumes every joint is solidly filled. It does not proportion cement, sand, lime or water for a site-batched mix, and it does not pick a mortar type for your exposure.
- Nothing but brick and bagged mortar is counted. No wall ties, weep holes or vents, lintels, DPC, flashing, movement joints or reinforcement, and no bedding for sills and coping.
- This is a quantity take-off, not a structural design. It says nothing about wall thickness for the height, lateral restraint, wind or retained load, foundations, or the mortar strength the exposure demands. A freestanding, retaining or loadbearing wall needs those from the building code or an engineer.
The Ties Go With the Bricks
A slot along the outer leaf takes out every wall tie in the band it crosses, and on a raking abutment that band is five or six metres long. The count itself is small — a British cavity wall carries roughly two and a half ties per square metre, so a 225 mm band along five metres of rake holds three of them — but the arithmetic is not the point. The panel above the cut is now restrained only by what is left at its edges, and the ties nearest the cut are the ones doing that work. The rule that follows is the one that governs the whole bay method: never have more of the outer leaf disconnected than the ties above and beside it can hold, and never open two bays that share the same run of ties.
Reinstating them is mechanical work, not masonry. Nothing can be built into two leaves that are both already standing, so the replacement is a remedial tie driven or resin-fixed through the outer leaf into the inner one, to the tie manufacturer's own instruction and its Agrément certificate. Fix them in the courses above and below the tray, not through it. A tie passing through a cavity tray is a hole in the tray and a wick across it, and every one of them will find the plaster eventually.
Where the panel comes down rather than being cut, the whole rebuilt area needs a full grid, set out on the storey rod before the first brick goes back. It is also the moment to look at the ties you are not replacing: a wall of this age may be carrying the flat galvanised strip ties that corrode, expand and lift the courses, and a rebuilt panel bolted to a failing grid either side of it is a repair with a date on it. That inspection costs one scope view and belongs in the same report as the cavity survey.
- Mark the existing tie grid on the elevation from the detector sweep before any line is cut.
- Set the bay lengths so no bay removes more than one tie, and stagger the bays so their tie losses are not adjacent.
- Prop or needle wherever the line passes under a lintel, a bearing, or an opening head — the tray line and a structural line at the same height is an engineer's question, not a mason's.
- Fit remedial ties in the course above and the course below the tray, never through it.
- Let each bay take before opening its neighbour, and keep a note of which bay was closed on which day.
- Scope one tie in the untouched wall while the scaffold is up, to see what the rest of the grid is made of.
Enter the area of the panel you are rebuilding rather than the band you are cutting. Read the answer as the North American coverage limit — the British density of about 2.5 ties per square metre lands a little over half as high — and take the tighter of whichever one your specification is written to.
The total brick veneer or cavity wall area.
Whether the site is in Seismic Design Category D or higher, or subject to wind pressure over 30 psf.
Wall ties needed
83 ties
This gives total tie count based on area coverage — additional ties are required within 12 in (305 mm) of openings and other discontinuities, and around every 3 ft (0.91 m) along their perimeter, beyond the uniform-field count calculated here.
- Wall area
- 220 sq ft
They open the calculator with your figures already in it
Brick Veneer Wall Tie Density Calculator: 83 ties — 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
- This is a uniform-field count taken from the gross area you type. It does not deduct window and door openings, and it does not add the extra ties required around opening perimeters or at other discontinuities — those are additional to the figure shown.
- The 2.67 sq ft (0.25 m²) and 2 sq ft (0.19 m²) coverage limits are US IRC/IBC figures. UK, European and Australian masonry standards set tie density on a different basis, so this count does not stand in for a design to those documents.
- It counts ties by area coverage only. It is not a structural design and does not check tie type, embedment, cavity width, the backup wall, or the fixings holding the tie to it. Where an engineer has specified a tie schedule, that schedule governs, not this number.
- The seismic/wind switch is a two-state choice you make. The calculation does not read your site's seismic design category or design wind pressure, and there is no band between the two coverage limits.
- It returns a total, not a layout. Setting the ties out on a grid that satisfies the coverage limit and the separate maximum horizontal and vertical spacings is a further step, and the figure carries no allowance for spares or ties lost during the lift.
Everything the Tray Collects Has to Leave the Wall
A tray with nowhere to discharge is a gutter with the outlet blocked, and it is worse than no tray at all, because it holds a standing head of water against the inner leaf instead of letting it pass. The openings go in the perpends of the course immediately above the tray, formed as the bricks go back and never drilled in afterwards. British practice puts them at not more than 450 mm centres with never fewer than two to any one tray; the North American codes work coarser than that, around 24 in for open head joints and closer for wick or tube weeps, which move far less water. Which figure applies here is a question for the specification, the NHBC Standards chapter on external masonry walls, and the tray maker's own instruction, in that order.
On a stepped run the spacing rule and the geometry argue with each other. At a 20 degree pitch the tray steps every 206 mm, so a weep in every tray is more than twice the openings the spacing calls for, and one in every second tray still lands inside 450 mm. The bottom tray of the run is the exception that overrides the arithmetic: it is the one every tray above it drains into, it always gets an opening, and it always gets a stop end turned up at its low end. A stepped run without a stop end at the foot delivers the entire collected flow of the abutment sideways into the cavity beyond the roof, which is a stain in a room nobody associates with the extension.
The top of the run needs its own component. Water arriving down the cavity from the wall above has to be caught and turned into the staircase rather than being allowed to run past the top tray, which is what a catchment or ridge tray at the head of the run does. The same applies where the stepped run turns the corner into the straight abutment across the top of the lean-to — that straight run is its own tray with its own stop ends at both ends and its own weeps, and the junction between the two is a made corner, not two trays pushed together and pointed.
| Component | Where it goes | What happens without it |
|---|---|---|
| Catchment tray at the head | At the top of the stepped run, taking the cavity above | Water from the whole wall above bypasses the staircase and carries on down past it |
| Stepped trays | One per course of rise, each lapping under the one above | The cavity is open across the abutment and the inner leaf is wetted along its whole length |
| Stop end at the foot | Turned up at the low end of the bottom tray | The collected flow leaves sideways into the cavity beyond the roof and appears in an unrelated room |
| Weep openings | Perpends in the course directly above the tray | The tray fills and holds a standing head against the inner leaf upstand |
| Straight tray at the head abutment | Across the top edge of the lean-to, stop-ended both ends | The horizontal run of the abutment leaks even where the raking run is perfect |
| Cover flashing | Outside the leaf, over the covering, lapping below the weeps | The tray discharges behind the flashing and puts its own water under the roof |
Feed it the raking length of the abutment rather than its plan length — at 30 degrees those differ by about fifteen per cent — and set the spacing to whichever of the two conventions your specification names, then add the openings for the head run and the bottom tray by hand.
The total length of cavity/veneer wall base needing weep holes.
The on-center spacing between weep holes.
Weep holes needed
18 weep holes
The plus-one term already places a weep at each end of this run, so do not add two more by hand — and where elevations meet, counting each run separately books the shared corner position twice. What is NOT counted: weeps above every door and window head, at shelf angles, and at any other flashing discontinuity, each of which needs its own row at this spacing.
They open the calculator with your figures already in it
Cavity Wall Weep Hole Spacing Calculator: 18 weep holes — 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 straight run along one flashing line, and nothing above it. Every cavity tray higher up the wall — over each lintel, at each cill, under each shelf angle, at every roof abutment — is a separate drainage compartment needing its own weeps in the course directly above its own flashing, and none of those openings are in this figure.
- Nothing here checks the spacing you typed against the code you build under. The field accepts 406 to 838 mm (16 to 33 in), which is the North American band; British practice is tighter, commonly around 450 mm centres, and other markets differ again. This is arithmetic on the figure you enter, not a verdict that the figure is permitted.
- The weep itself is outside the model — opening size, free area, and whether the type you have matches the spacing you chose. Wick and tube weeps move far less water than an open perpend, and cellular inserts and insect screening cut free area below the raw size of the hole, so the same count of openings is not the same drainage capacity.
- Position decides whether any of them work and position is not counted. Weeps belong in the perpends of the course immediately above the flashing, formed as the bricks go back; one course too high and water sits permanently below its own outlet, and an opening with a mortar dam behind it drains nothing while still counting here.
- Vents are not in this number. Openings at the head of a drainage compartment — under soffits, under shelf angles, immediately below each cavity tray — are what let the cavity dry rather than merely drain, and they are a second set of openings on top of the ones counted here.
The Strip of Wall Above the Roof Dries Slowly Now
The tray has given that cavity a floor, and the panel of old outer leaf standing above the new roof is in a worse position than it was a month ago. It sits in a sheltered pocket that the sun reaches for fewer hours, it takes splash off the tiles in every heavy shower, and snow lies against the bottom of it instead of blowing off a clear elevation. The leaf gets wetter and dries slower, and the cavity behind it now drains through half a dozen small holes rather than down a wall. Where the specification asks for a ventilated compartment — or simply where the panel is being rebuilt anyway and the openings cost nothing to form — the head of that compartment gets openings too, under the soffit or immediately below the next interruption up, counted along the same run as the weeps below them.
Two things at this junction get confused with each other and are not related. Ventilating the cavity is a masonry detail about drying a leaf. Ventilating the roof void of a cold lean-to is a roofing detail about condensation under a deck, governed by the ventilation guidance in Approved Document F and by BS 5250, and it is solved with abutment ventilators or a ventilated ridge, not with perpends. Where a roof void terminates against a wall it is easy to close off the path the void needed and pass building control on the tray while quietly failing on the roof, so settle both at the same visit and mark both on the same drawing.
Run the same abutment length through it at the spacing your specification sets for head openings, so the vent count arrives with the weep count and the inserts turn up on one delivery rather than sending somebody off site on the last day.
The total length of the brick veneer wall run.
The on-center spacing between drainage cavity vents.
Drainage vents needed
50 vents
They open the calculator with your figures already in it
Brick Veneer Cavity Drainage Vent Spacing Calculator: 50 vents — 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 result is a count of openings, not a ventilated free area. A vent fitted with a grille, a baffle or an insect screen passes only a fraction of the free area of the hole it occupies, so a run can meet this count and still fall short of the free area per unit length of elevation the wall system calls for; that free-area figure comes from the vent manufacturer and the jurisdiction, not from the spacing used here.
- This sizes one horizontal run of openings. Air only moves through the cavity when there is a path at the head as well as the base, so a drying design normally needs a second row at the top of the veneer or under the eaves, and the calculator has to be run again for that row rather than the base count being read as the whole elevation.
- The wall is treated as one uninterrupted straight run. The count does not deduct window and door openings, and it does not add the extra vents needed immediately above each cavity tray, lintel and flashing, where the cavity is closed off and becomes a separate drainage and ventilation zone with its own openings.
- The plus-one term places a vent at each end of the run. Counting elevations separately and adding the results therefore counts the shared position at every corner twice, so one vent should be deducted for each corner where two runs meet end to end.
Two Trades, One Overlap, and Nothing That Sticks
The tray belongs to the mason and lives inside the wall. The cover flashing belongs to the roofer and lives outside it. They meet at exactly one place — the weep course — and the only thing that makes the junction work is which of them is on top. Water leaves the weeps, lands on the outside face of the leaf, and must arrive on the outside of the flashing. Fit the flashing so its upstand finishes below the weep openings and the detail works by overlap alone. Fit it so the upstand covers the weeps and the tray now discharges behind the lead and straight back into the roof, which is the failure that produces a call-out on a job where everything visible was done properly.
Some retrofit trays arrive with the flashing already bonded to them as one component, which removes the argument and removes the choice: the tray's own apron is the flashing, it is dressed over the covering as the bricks go back, and the roofer is working to the mason's sequence rather than after him. Where they are separate pieces, agree the order before either trade starts. The lead goes in as pieces rather than as a run — the codes of practice cap the length of any one flashing piece at about 1.5 m because lead moves along its length with temperature and tears its own fixings if it is asked not to — so a five-metre raking abutment is four pieces and three laps, and each lap sheds downhill.
What the raking side actually gets depends on the roof covering, and this is settled by the tile rather than by preference. Flat slates and plain tiles take a soaker with each course under a stepped cover flashing. Deep-profile pantiles and interlocking concrete tiles cannot host a flat soaker at all, so the flashing itself is cut and dressed down into each pan, which takes noticeably more girth and considerably more time — and several tile makers supply a proprietary abutment system that their warranty recognises and a hand-cut lead detail may not. Ask that question before ordering the lead, not after.
One thing to keep off the whole assembly: sealant. The junction works by overlap, and a bead run along the bottom edge of the lead turns a lapped joint that could drain into a closed one that traps. Lead has its own care list on the day it goes on, and it belongs on the order rather than on the snagging sheet.
Add the laps to the run before you divide: a 5.1 m raking abutment with three 150 mm laps is 5.55 m of material, and entering the piece length as the 1.5 m the lead manuals allow rather than the stock length of the roll is what turns it into a piece count a merchant can pick.
The combined length of all flashing runs needed on the job.
The length of a single flashing sheet or cut coil piece as supplied.
Flashing sheets needed
5 sheets
They open the calculator with your figures already in it
Exterior Flashing Layout Trim Sheet Calculator: 5 sheets — 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
- Rounds up on the total when each run has to come out of one sheet. Five separate 3.5 m (11.5 ft) runs from 3.05 m (10 ft) stock take ten pieces, not the six that dividing the combined 17.5 m (57 ft) suggests, because offcuts do not join end to end. Run this per length of flashing rather than per job total.
- Says nothing about coil width. Flashing is brake-formed from flat stock, so the stock has to cover the developed girth — every leg, plus hems and the metal taken up in each bend — and coil comes in fixed widths. The right number of pieces in stock too narrow to fold the profile is not usable material.
- Blind to the metal. Aluminum against pressure-treated lumber or in contact with a copper gutter corrodes at that contact, gauge decides whether a long run stays flat or oil-cans, and pre-finished coil carries a coating that will not survive being folded the wrong way. A sheet count chooses none of that.
Signing It Off, and the Photographs Nobody Takes
Building control wants to see this open. The tray, the stop end, the weep openings and the flashing lap are all invisible within a day of being finished, and an inspector who arrives after the roof is tiled has no way to confirm any of it except by taking your word or taking a brick out. Book the visit against the stage rather than the week, tell the roofer the covering does not go on until it has happened, and expect the inspector to want to see the foot of the run and the head of the run specifically, because those are the two details that get left out.
Test it yourself first, and test it in the only direction that means anything: water on the wall above the abutment, not on the flashing. Give it fifteen minutes with somebody watching the weep course from a ladder. Water arriving at the openings and running out onto the tiles is the tray working. Nothing arriving at the openings is not good news — it means the water is going somewhere else, and somewhere else is inside.
Then write down what the wall swallowed. Which tray product and which cavity width it was made for, where each bay started and finished, which courses took remedial ties and how many, where the stop ends are, what code of lead is on the rake, and where the laps fall. Add the photographs from the survey and the ones taken into each open bay. Nobody at this house will ever see any of it again, and the next person asked to explain a damp patch above the kitchen door will be working from that file or from a drill and a boroscope. The file is a great deal cheaper, and it is the only part of this job that survives the plaster.
What to settle before a disc touches the wall
This order is a survey, a pitch measured off the rafters, and a run taken on the rake. Nothing on it comes off the plan view, and the two routes through it price differently by a factor.
- Cavity width and cavity contents, from three scope views — It settles which tray product fits, whether there is a cavity at all, and whether blown fill turns a masonry job into a specification question for somebody else.
- Pitch off the rafters, and the rise of the abutment in courses — Rise divided by the 75 mm gauge is the tray count. Two degrees of difference between drawing and roof is a whole extra course over five metres.
- Raking length of the abutment, plus the straight run at the head — Plan length divided by the cosine of the pitch. Weeps, vents and lead are all bought against the raking figure; only the head run is bought flat.
- Bay lengths, and the tie loss each one causes — No bay long enough to take out more than one tie, no two open bays sharing a run of ties, and remedial ties above and below the tray rather than through it.
- Bricks for the panel, matched before the scaffold goes up — Cut-in needs replacements for what breaks; taking the panel down needs the triangle up to the eaves. Reclaimed matching is an order lead time, not a site decision.
- Stop ends, catchment tray and corner listed as items — The three components that get value-engineered off a materials list are the three whose absence produces a stain in an unrelated room.
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.