Why do end dams matter so much?

Because without them the flashing is a channel open at both ends, and water that collects in it runs straight off the ends into the wall. An end dam turns the flashing into a tray. It is a small piece of metal and it is the difference between a sill that drains and a sill that irrigates the framing beside it.

Where should the weeps discharge?

To the outside face, through the cladding, above the finished level below. A weep that discharges into the cavity moves water from one part of the wall to another; a weep blocked by mortar droppings or sealant does not discharge at all. Both are common and both are found only when the wall starts staining.

Should the sill flashing be sealed at the front edge?

No — that is the drainage path. Sealing the leading edge is an intuitive-looking mistake that turns the flashing into a bathtub. The seal belongs at the back and ends, where water must not pass; the front is deliberately open so water can leave.

Does the flashing go under or over the water-resistive barrier?

Shingled so water always runs outward: the barrier above laps over the flashing's upstand, and the flashing laps over the cladding or barrier below. Reversing that lap at any point creates a path into the wall regardless of how much sealant is applied, and sealant is not a substitute for the correct lap order.

Materials & Quantities

Storefront Sill Flashing Calculator

Calculate the sill flashing needed for a storefront system's base, with waste allowance.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.

Total Sill Length

The total length of storefront sill needing flashing.

Measure along the base of the storefront run, including all sections.

Waste Allowance (%)

Extra material to allow for cuts, laps, and end dams.

5% is typical for a straight run; increase it for runs with many corners, offsets, or transitions.

Sill flashing needed

86.1 ft

High confidence

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Total sill flashing = sill length × (1 + waste allowance)

Inputs used

Total Sill Length: 82 ft
Waste Allowance (%): 5

Final result86.1 ft

What this calculation does not cover

The figure is running length of flashing only; nothing in the calculation derives the developed width of the metal from the sill depth, the upstand height or the drip hem, so the coil or sheet girth still has to be worked out before anything can be ordered.
End dams are folded into the waste percentage rather than counted, so a run broken into several sections gets no separate tally of the dam pieces each break needs.
Laps between lengths are absorbed as a flat proportion of the whole run, which under-allows where the flashing is made up of many short pieces and over-allows on one long uninterrupted length, because the multiplier has no idea how many joints there are.
Waste is capped at 15 percent and the sill at 200 m (656 ft), so a heavily articulated shopfront or a run longer than that has to be split across more than one calculation.
The answer is returned as a continuous metre figure and is never rounded up to whole stock lengths, so what actually gets cut and delivered will exceed it once each piece comes off a fixed size.
Only the flashing metal is measured; the weep components, the sealant at the back and ends, the fixings and the barrier that laps over the upstand are all outside this total.

Estimated cost — your price

Price per ft ($)

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.

Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Total sill flashing = sill length × (1 + waste allowance)

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Laying Out a Storefrontuses this calculator
A shopfront opening becomes a lite count, four runs of track and a glass order, and the smallest of four ceilings — not the elevation — sets the division.
Installing Windows and Exterior Doors
Planning a Revolving Door Entrance

Called something else where you work? Cill, sill and sill plate — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate storefront sill flashing in 3 steps

Total Sill LengthThe total length of storefront sill needing flashing.
Waste Allowance (%)Extra material to allow for cuts, laps, and end dams.
Sill flashing neededThe tool computes the sill flashing needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Sill flashing needed by total sill length

Page defaults, not your figures above.

Total Sill Length	Sill flashing needed (ft)
40 ft	42
60 ft	63
80 ft	84
100 ft	105
120 ft	126
140 ft	147
160 ft	168

Frequently asked questions

Why do end dams matter so much?: Because without them the flashing is a channel open at both ends, and water that collects in it runs straight off the ends into the wall. An end dam turns the flashing into a tray. It is a small piece of metal and it is the difference between a sill that drains and a sill that irrigates the framing beside it.
Where should the weeps discharge?: To the outside face, through the cladding, above the finished level below. A weep that discharges into the cavity moves water from one part of the wall to another; a weep blocked by mortar droppings or sealant does not discharge at all. Both are common and both are found only when the wall starts staining.
Should the sill flashing be sealed at the front edge?: No — that is the drainage path. Sealing the leading edge is an intuitive-looking mistake that turns the flashing into a bathtub. The seal belongs at the back and ends, where water must not pass; the front is deliberately open so water can leave.
Does the flashing go under or over the water-resistive barrier?: Shingled so water always runs outward: the barrier above laps over the flashing's upstand, and the flashing laps over the cladding or barrier below. Reversing that lap at any point creates a path into the wall regardless of how much sealant is applied, and sealant is not a substitute for the correct lap order.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.