Takeoff & Estimating — Roofing, Envelope & Finishes

Thru-Wall Flashing

A barrier through a masonry wall that directs trapped water back outside.

Quick Answer

Thru-wall flashing is an impervious sheet installed through a masonry or veneer wall, usually at the base, shelf angles, window heads and sills, to catch water that gets behind the exterior face and send it out through weeps. Estimators measure it in linear feet, adding laps, end dams and the weeps.

The Full Picture

In a drainage wall, water that gets past the brick is expected, and the flashing's job is to catch it and send it back out. The Brick Industry Association (BIA) describes through-wall flashing as an impervious material installed in a drainage wall system to contain and redirect moisture that has penetrated back to the exterior. It goes at the base of the wall, above and below openings, at shelf angles, at the tops of walls and beneath copings, and anywhere the air space is interrupted. Flashing materials commonly include sheet metal such as stainless steel or copper, and flexible membranes, including self-adhered products. Which one applies is a spec choice.

Weeps and end dams complete the detail. BIA says weeps are placed immediately above the flashing at all locations, and that weeps one or two courses higher will not do their job. The IRC and TMS code set a maximum spacing of 33 in. on center, and BIA recommends open head joint weeps no more than 24 in. on center. Where flashing is not continuous, the ends should be turned up at least 1 in. into a head joint to form an end dam, so water goes out the weeps instead of the end. BIA also notes the flashing should extend to the face of the brick at least, with a drip beyond the face preferred.

In the documents, thru-wall flashing appears in the wall sections and masonry details, in exterior elevations only as weep locations if at all, and in Division 04 (Masonry) or Division 07 when it is specified as a sheet metal or membrane flashing. The architect's details give the flashing material, the height up the backing (BIA suggests at least 8 in. for concrete masonry backing), lap requirements (at least 6 in. overlap and sealed for BIA's guidance) and terminations. Because the item sits between masonry, waterproofing and sheet metal, the scope is often split or missed in bid packages.

The unit is linear feet along each run, with laps and extra length at corners and terminations added, plus a count of end dams and a count or spacing of weeps, and separate lines for termination bar, sealant and any drip edge. Some bids carry it per opening (a head or sill flashing is an EA with a length). The pitfalls are straightforward: quoting only the sheet and not the fabricated end dams and laps, missing flashing at window heads and shelf angles because the base flashing was counted, and not allowing for the mason to leave out the open head joints that the weeps depend on.

Real Examples

→Worked takeoff: base and head flashing: A brick veneer building is 80 ft by 50 ft, so its perimeter is 2 x (80 + 50) = 260 ft. Two 3 ft doors interrupt the base flashing, so base flashing is 260 - 6 = 254 LF. Assuming 20 ft pieces, 254 / 20 = 12.7, so 13 pieces and 12 laps at 6 in. add 12 x 0.5 = 6 LF. Ten windows 4 ft wide get head flashing extending 8 in. past each jamb: 4 + 2 x (8 / 12) = 5.33 ft each, or 53.3 LF total, with 20 end dams. Total flashing is 254 + 6 + 53.3 = 313.3 LF. At 24 in. on center, 254 ft of base needs 254 / 2 = 127 spaces, so about 128 weeps.

→Shelf angle flashing: On a three-story brick veneer, the details show a relieving shelf angle at each floor line with flashing above it. The estimator counts flashing for the full length of each floor line, not just the base, and adds laps and end dams at expansion joints where the flashing is not continuous.

→Scope gap between trades: A bid package lists brick and a waterproofing allowance, but the through-wall flashing is shown only in a masonry detail. The estimator asks whether it belongs to the mason or the sheet metal sub and carries it once, rather than leaving it in neither scope.

Common Misconceptions

People assume: Weep holes are optional if the wall looks dry.

Actually: In a drainage wall, flashing without weeps traps water. BIA's Technical Note says that weeps are placed immediately above flashing so that collected moisture can leave the wall.

People assume: Flashing is measured only along the base of the wall.

Actually: Flashing is also required at window heads and sills, shelf angles, and other places where the cavity is interrupted. Each of these is its own run and often has end dams and laps.

People assume: The flashing ends can simply stop at the edge of an opening.

Actually: Where flashing is discontinuous, BIA says to turn the ends up at least 1 in. into a head joint to form end dams. Without them, water can run off the end and back into the wall.

Does MeltPlan Solve This?

Partially — adjacent

Masonry wall perimeters, opening widths and floor-line lengths that drive flashing runs are measured from the plans. MeltPlan's takeoff service has AI pull quantities from PDF plans and US-based experienced estimators review and adjust them into a verified takeoff. Reading flashing material, lap and termination requirements from the details and deciding which trade carries them stays with your estimating team.

Get masonry wall lengths and openings measured from your plans →

Frequently Asked Questions

What is thru-wall flashing?

It is an impervious sheet installed through a masonry or veneer wall to catch water that gets behind the exterior face and direct it back out through weeps. It is typically placed at the base of the wall, at shelf angles, above and below openings and at wall tops.

How is thru-wall flashing measured?

In linear feet along each run, with laps and extra length at corners and terminations added. End dams and weeps are counted separately, and termination bar, sealant and drip edges are separate lines.

How far apart should weeps be?

The IRC and TMS code limit weeps to 33 in. on center, and BIA recommends open head joint weeps be spaced no more than 24 in. on center. Check the project specification, which may be stricter.

What is an end dam?

It is the upturned end of discontinuous flashing, turned at least 1 in. into a head joint in BIA's guidance, so water is directed to the weeps instead of running off the end.

Which spec division covers thru-wall flashing?

It is commonly found in Division 04 for masonry or Division 07 for sheet metal or membrane flashing. Check the project's specs, because the scope can be split between trades.