How to shingle a roof?

What does shingling a roof actually involve?

Shingling a roof is not “nailing shingles down”. It is replacing an assembly of seven interacting layers, and the shingles are only the visible one. A crew tears off the existing covering to bare deck, inspects and replaces any sheathing that has gone soft, installs drip edge at the eaves and rakes, lays ice barrier where code requires it, covers the deck in underlayment, details every valley and penetration in metal, and only then starts laying courses from the eave upward.

The order is not optional. Each layer sheds water onto the one below it, and a layer installed out of sequence directs water underneath the assembly instead of off it.

Why can’t I just do this myself?

Three things you cannot check from the ground decide whether this job is survivable: the slope, the height, and the condition of the decking.

Slope is the obvious one. Anything at 6:12 or steeper is not walkable without fall protection, and by 8:12 it is not walkable at all in the ordinary sense. Height compounds it — a fall from a single-storey eave can be survivable, a fall from a two-storey ridge frequently is not.

Decking is the one people miss. A roof that has been leaking quietly around a chimney for six years can have sheathing that looks intact from below and gives way under a boot. You find that out by stepping on it.

What does a professional crew actually do differently?

They arrive with fall protection and anchor points, and they set them before anyone walks the roof. They tear off in sections rather than exposing the whole deck, so a surprise rainstorm does not put water in your ceiling. They know what a soft sheet feels like underfoot and replace it. They flash rather than caulk. And they carry liability insurance and workers’ compensation, which matters enormously if someone is hurt on your property.

Ask for proof of both before work starts. An uninsured crew on your roof is your problem, not theirs.

What should a shingle roof cost?

Roofing is priced by the square — 100 square feet of roof surface, not of floor area. A 2,000 square foot roof plane is 20 squares, and a 6:12 pitch means the roof surface is about 12 percent larger than the building footprint underneath it.

Architectural asphalt runs roughly $400 to $700 per square installed, including tear-off of one layer. Deck replacement is extra and should be quoted as a per-sheet price in the bid, not left open — because nobody, including the roofer, knows how many sheets are bad until the old roof is off.

Use the roof area and squares calculator to check that a bid’s square count matches your actual roof before you compare prices.

What separates a good installation from a bad one?

Flashing. Almost every roof leak is a flashing leak — the field of a shingle roof rarely fails, the interruptions do.

The specific thing to look for in a bid is whether flashing is being replaced or reused. Reusing old step flashing on a new roof is common, cheap, and the reason a two-year-old roof leaks at a wall line. Step flashing must be woven in piece by piece with the courses; a continuous strip of metal or a bead of roof cement in its place will fail, and it is invisible once the siding goes back.

Ask specifically: is new step flashing included, and is the valley metal new?

How is wind resistance decided?

By the fastening pattern and the sealant strip, more than by the shingle. Asphalt shingles are classified under ASTM D3161 (Class A, D or F at 60, 90 or 110 mph) or ASTM D7158 (Class D, G or H at 90, 120 or 150 mph basic wind speed), and IRC R905.2.6.1 sets which class is acceptable at which design wind speed.

But every one of those ratings assumes the shingle was fastened as the manufacturer specifies and that the sealant strip has bonded. Shingles installed in cold weather that never sealed have no meaningful wind rating at all. This is why we point at the manufacturer’s installation instructions rather than publishing a nailing pattern: deviating from them voids the material warranty, and that fact is more useful to you than the number.

Common mistakes

• Comparing bids on price per square without checking the square count. Two roofers quoting the same roof at different square counts are not quoting the same job. One of them measured wrong, or is measuring footprint instead of roof surface.
• Accepting an open-ended deck replacement clause. “Additional decking at cost” with no per-sheet number is an invitation to a surprise. Get the unit price in writing before work starts.
• Letting old flashing stay. It saves a few hundred dollars now and costs a wall repair later.
• Roofing over an existing layer to save money. It hides the deck condition entirely, adds weight, shortens the new roof’s life by trapping heat, and many jurisdictions limit how many layers are permitted before tear-off is required.
• Scheduling in cold weather without asking about sealing. Below roughly 40°F the sealant strips may not bond until the following spring, leaving the roof vulnerable through a whole winter of wind.