Roofing mistakes on an asphalt shingle roof, and what each one costs

Which roof do these numbers describe?

A plain gable over a 50 ft by 40 ft footprint, 2,000 sq ft of plan area, pitched 6/12, with a 12 in. overhang. The pitch factor is the square root of 1 + (6/12) squared, 1.1180, so the roof surface is 2,236 sq ft, 22.36 squares. Add 10 percent for waste and you buy 2,459.6 sq ft, which at 32.8 sq ft per Oakridge bundle is 75 bundles at $44.37, $3,327.75. Eaves run 2 x 50 = 100 lin ft, and each of the four rakes is the rafter length, 20 x 1.1180 = 22.36 ft, so 89.44 lin ft. Run your own house through the roofing cost calculator.

For labor, the BLS May 2025 mean wage for roofers is $27.95 an hour, and BLS employer-cost data for construction in June 2026 give a 1.4381 loading for taxes, insurance and benefits. A roofer therefore costs $40.20 loaded, a two-person crew $80.39, and at a 30 percent markup (my assumption) the crew bills $104.51 per crew-hour.

Why are my new shingles blowing off?

Nails above the nailing zone, or driven through the shingle

A shingle nail has two jobs, holding its own shingle and pinning the top edge of the course below, and both happen only inside the band the maker marks. CertainTeed's Landmark instructions give a 1-1/2 in. nailable area with four nails set 1 in. in from each edge and 12 in. and 14-3/4 in. between. Owens Corning says fasteners must land in the SureNail area and penetrate at least 3/4 in. into the deck. Both manuals print the same picture of failure: proper, crooked, under-driven, over-driven. A 1-1/2 in. band on a 13-1/4 in. shingle is 11 percent of its height, so an inch high misses. The cost is the whole roof, because Owens Corning excludes improper or faulty installation: that leaves $3,327.75 of shingles with no coverage and a re-nail you cannot do from on top.

Four nails where the warranty wanted six

IRC R905.2.6 requires the manufacturer's number of fasteners and never fewer than four per strip shingle, and the manufacturer's number is where the money is. Owens Corning's warranty table carries a footnote most people never read: 110 mph is standard with four-nail application, and 130 mph applies only with six nails plus Owens Corning starter shingles at eaves and rakes. Wind coverage runs 15 years either way. At 98.4 sq ft per square, 2,459.6 sq ft is 25.0 squares, and at 64 shingles per square that is 1,600 shingles, so 6,400 nails at four each against 9,600 at six. The 3,200 extra nails out of a 7,200-count box at $26.99 cost $12.00, and the starter the higher tier requires is two bundles at $62.48, $124.96. Total $136.96, 4.1 percent of the shingle bill, to buy 20 mph for 15 years.

Why is my new roof leaking at the eaves and walls?

No starter course, or a field shingle flipped over

The starter's sealant strip sits under the bottom edge of the first course and glues it down. Flip a field shingle upside down and the sealant lands in the wrong place, so the first course has nothing to bond to and the eave is the first thing the wind takes. Owens Corning is specific: use a starter product, trim 6-1/2 in. off the rake or hip end, overhang the drip edge 1/4 to 3/4 in., five fasteners per shingle. The only approved way to make a starter from a field shingle is to cut the exposure off entirely. Two bundles cover this roof for $124.96.

Reusing old flashing

IRC R908.5 says existing flashings, edgings, outlets, vents and similar devices shall be replaced where rusted, damaged or deteriorated. The warranty makes the same point from the other side: Owens Corning excludes damage caused by non-Owens Corning products, naming metal work, counterflashing, corroded roof nails and pipe boots. Reused step flashing is a leak with no owner. A 16 ft sidewall at the 5-5/8 in. exposure Oakridge uses takes 192 / 5.625 = 34.1, so 35 pieces, four packs of ten at $8.19, $32.76, against a leak that shows up inside a wall.

Skipping the drip edge

R905.2.8.5 requires a drip edge at eaves and rake edges, at least 1/4 in. below the sheathing and 2 in. back onto the deck, segments lapped 2 in., fasteners at no more than 12 in. on center. Underlayment goes over the drip edge at the eaves and under it at the rakes, and that order is the part people reverse. The roof needs 100 lin ft of eave plus 89.44 of rake, and a 12 ft piece lapped 2 in. contributes 11.833 ft, so 17 pieces at $10.77: $183.09, 5.5 percent of the shingle line. Without it the water wicks back onto the fascia, which you pay for twice, once in carpentry and once in the roofing over it.

Why does my roof leak in ice or wind-driven rain?

No ice barrier where the code requires one

R905.1.2 applies where there is a history of ice forming along the eaves and backing water up. Two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet, has to run from the lowest edge of every roof surface to at least 24 in. inside the exterior wall line, and at 8:12 or steeper it also has to reach 36 in. along the slope. Here the overhang is 12 in., so 24 in. inside the wall line is 36 in. horizontally, which on a 6/12 slope is 40.2 in. A 36 in. roll does not reach, which is why Owens Corning says most applications need two courses. Two courses over 100 lin ft of eave is 200 lin ft, four rolls at $74.98: $299.92. R908.3 gives one break on a re-roof: an existing ice barrier adhered to the deck may stay and be covered.

Single underlayment on a low slope

R905.2.2 allows asphalt shingles only at 2:12 and above, and from 2:12 up to 4:12 it requires double underlayment. Table R905.1.1(2) spells out the pattern: a 19 in. strip at the eave, then 36 in. sheets lapped 19 in., end laps 4 in. offset 6 ft. At 4:12 and above a 36 in. sheet lapped 2 in. leaves 34 in. exposed; below 4:12 it leaves 17 in., exactly half, so the roll count doubles. This 6/12 roof needs 2,236 sq ft of underlayment, three 1,000 sq ft rolls at $119.00, $357.00. The same area at 3:12 needs six rolls, $714.00. Get the slope wrong and the difference is $357.00 of material against stripping a roof that leaks in wind-driven rain.

Can I roof over the old shingles, and do the soffits matter?

Recovering over two layers

R908.3 says roof replacement includes removing existing layers down to the deck. R908.3.1 allows a recover in defined cases, and R908.3.1.1 bars one where the existing roof is water soaked or deteriorated, where the covering is slate, clay, cement or asbestos-cement tile, and where the roof already has two or more applications of any covering. The temptation is real money: tearing off 22.36 squares at one crew-hour per square (my assumption) is $2,336.84. If the inspector counts three layers afterward the tear-off happens anyway with new shingles in the pile, so the same decision costs that $2,336.84 plus a second $3,327.75 of shingles, $5,664.59. A deck you never saw is also a deck you never checked.

Insulation pushed into the soffit vents

R806.2 sets the minimum net free ventilating area at 1/150 of the vented space, with a 1/300 exception where a Class I or II vapor retarder is used in Climate Zones 6 to 8 and 40 to 50 percent of the venting sits high. On a 2,000 sq ft attic that is 13.33 sq ft, 1,920 sq in., or 960 sq in. under the exception. R806.3 is the one a bag of insulation violates: insulation shall not block the free flow of air, and at least 1 in. has to stay open between insulation and sheathing at the vent. With rafters at 24 in. on center and 50 ft of eave a side there are 50 bays, so 50 vent chutes at $2.69, $134.50. Owens Corning excludes damage caused by improper or inadequate ventilation, so a roof over a smothered soffit has a warranty that already stepped aside. Check what you are burying with the insulation R-value calculator.

Do I need fall protection on my own roof?

OSHA 1926.501(b)(13) requires anyone engaged in residential construction 6 ft or more above a lower level to be protected by guardrails, a safety net or a personal fall arrest system, with a written plan permitted only where the employer can show those are infeasible or a greater hazard; paragraph (b)(11) says the same for steep roofs. The rule binds employers, so a homeowner on their own house sits outside it, and the height the federal government thinks needs a system is still 6 ft. Maximum penalties after January 15, 2026 are $16,550 per serious violation and $165,514 per willful or repeated one. A Guardian fall protection kit is $114.99, 0.69 percent of one serious citation. I found no injury cost figure I would publish next to these prices, so that column stays empty.

What do the skipped parts cost against the shingles?

Read that column twice before comparing bids. A quote $1,200 cheaper on a roof this size is not a better price on shingles, because shingles are the one line both roofers pay the same for. It is a quote with this table deleted. Ask which of the eight items is in the price, in writing, and get the nail count and the starter product by name, because those two are what the wind warranty is keyed to.