Drip edge calculator

Count the rakes as well as the eaves

The most common way to under-order drip edge is to measure the gutter line and stop. IRC R905.2.8.5 requires drip edge at eaves and rakes on an asphalt shingle roof, and on most houses the rakes add more length than people expect, because a rake is measured up the slope, not across the ground.

Take a 40 ft × 28 ft house with a plain 6/12 gable roof. The eaves are 2 × 40 = 80 ft. Each rake is half the width times the pitch multiplier, 14 × 1.118 = 15.7 ft, and there are four of them — another 62 ft. Order for the eaves alone and you are 44% short.

Hips, valleys and dormers add more. Every dormer has two small rakes and its own short eave; a shed dormer adds an eave the width of the dormer.

Ten-foot pieces, lapped two inches

Drip edge is stocked in 10 ft pieces — that is what Amerimax and Gibraltar sell in every profile — and the code requires adjacent segments to overlap not less than 2 in, upper over lower. So each extra piece only gains you 9 ft 10 in, not 10 ft:

pieces = ⌈ (run − lap) ÷ (piece length − lap) ⌉

An 80 ft eave run is 8.13 pieces, so 9. Divide 80 by 10 and you get 8, and eight pieces lapped 2 in only reach 78 ft 10 in — you finish 14 in short at the last corner, which is exactly where a joint should not be. Count each run separately, too — a piece left over from the eaves is not automatically long enough to be useful on a rake.

The install sequence is the part that leaks

This catches experienced people, because the answer is different at each edge:

• At the eaves, the drip edge goes on first and the underlayment over it. Water running down the felt is then carried out onto the metal and into the gutter.
• At the rakes, the underlayment goes down first and the drip edge over it. Water blown up the gable edge is caught on top of the felt instead of behind it.

Get the eave one backwards and every drop that reaches the underlayment runs behind the fascia. Fasten at not more than 12 in on centre, and lap the rake pieces working from the bottom up so gravity is on your side.

What to buy

Profile matters more than price. Type C is the plain L, Type D has a kicked-out lower leg that throws water clear of the fascia, and the F profiles have an extended face for going over existing shingles on a re-roof. Whichever you pick, the code wants it to extend at least 1/4 in below the sheathing and at least 2 in back up onto the deck, in 26-gauge galvanized steel or 0.016 in aluminium — vinyl edging is not a compliant substitute.

Where the piece count needs your judgement

Two of the inputs are measurements this page can neither check nor convert.

The rake has to be measured up the slope. There is no pitch field here, so nothing turns a plan dimension into a sloped one — the 6/12 example above does that conversion by hand. Take a rake off a drawing or a footprint, enter it as read, and you are short on every gable: about 12% at 6/12, and more as the roof steepens.

Each total is treated as one unbroken run. Eaves and rakes are counted separately, which is right, but within each the arithmetic assumes the length is continuous. Two 40 ft eaves are not one 80 ft eave — each starts and ends on its own piece, so the pair takes ten pieces where a single 80 ft run takes nine. Four separate rakes behave the same way. Add a piece per extra run, or work each edge out on its own.

The lap has to be shorter than the piece, and the fields will not stop you breaking that. Set the piece length to 1 ft and the overlap to 12 in and each additional piece gains you almost nothing, so the count runs off into the thousands. Nothing flags it — a piece count that looks absurd is those two boxes disagreeing, not a long roof.

Not counted at all: the material lost where an eave meets a rake and the two pieces are mitred or lapped, which the waste allowance is quietly carrying; a gutter apron, if that is what is going on the eave instead; and any fastener beyond the field spacing, since the nail figure is the code maximum along each piece plus one at each end, with nothing for joints or corners.

The profile is your decision rather than an output. Nothing here knows whether your fascia wants a Type C, a Type D or an extended-face F, or what gauge to buy above the code minimum quoted here.