Plan area
Step one, the footprint
Someone has to establish the flat, plan view area of every roof plane. That can be done from a tape on the ground for a simple gable, from an aerial measurement service, from a satellite derived report, or by walking the roof with a measuring wheel and a sketch. All four are legitimate. They do not all produce the same answer on a complicated house.
Aerial and satellite reports are genuinely good on plan area and genuinely poor on everything an eye catches: a low slope porch roof tucked behind a gable, a second layer of shingle nobody mentioned, decking you can feel move underfoot, a chimney whose flashing was buried under cement ten years ago. A contractor who has only ever seen your roof in a report has measured it and has not inspected it, and those are not the same service.
Pitch
Step two, the multiplier
Plan area times a pitch multiplier gives surface area. The multiplier is pure geometry, the square root of one plus the rise over run squared, and it is not negotiable.
Reckoning 01 · footprint to squares
Waste is real, not padding. Every hip, valley, rake and penetration means shingles cut and offcuts discarded. A simple gable might run at five to seven percent. A cut up roof with several hips and dormers can run into the teens. A bid that shows no waste line has either buried it or is about to run short on a Friday afternoon.
Pitch also drives labour and safety. Above roughly 7:12 a crew cannot simply stand on the plane, so staging, harness anchors and roof jacks come into it, and the labour figure moves independently of the material figure. If two bids on the same steep roof are within a hundred dollars of each other, at least one of them has not priced the access.
The things a tape cannot find
Step three, the walk
Everything that actually changes the shape of a job is found by somebody standing on the roof and in the attic. In no particular order, on houses in this part of the county:
- A second layer. Two layers means double the tear off labour and double the disposal weight, and it is often invisible from the ground.
- Soft decking. Found by walking it. Cannot be seen in an aerial report and cannot be quoted honestly without an allowance.
- Low slope sections. Porches, additions and dormer roofs frequently sit below 4:12, where asphalt shingle is the wrong material and the permit rules change.
- Flashing that was face sealed. Roofing cement smeared over a chimney or a wall junction is a repair that has to be undone and rebuilt, not shingled over again.
- Ventilation that does not work. Discussed on the ventilation sheet. It is cheap to correct during a tear off and expensive to correct afterwards.
Allowances
The honest way to handle what nobody can see
A good proposal says out loud which numbers are firm and which are allowances, and it prices the allowance per unit rather than as a lump.
Decking is the standard example. The right form of words is something like: sheets or boards replaced as needed at a stated price per sheet or per linear foot, quantity confirmed and photographed on the day, homeowner notified before the count goes past an agreed figure. That is not a get out. It is the only structure that is honest about a thing nobody can measure through a roof.
The unflattering paragraph
An allowance is also the easiest place in a roofing contract to pad a job, and the industry knows it. Your protection is not to refuse allowances, which would mean refusing every honest bid on an older house. It is to insist on three things: a per unit price agreed in advance, photographs of the actual boards taken before they are covered, and a notification threshold above which somebody phones you rather than adding it to the invoice.
Ask for all three in writing. A contractor who resists any of them is telling you something useful for free.
Comparing two bids
Make them the same shape first
Two roofing bids are almost never comparable as written, because they are describing different scopes in different words. Before comparing price, normalise them.
- Same square count? If the numbers differ by more than a square or two, somebody has measured wrong or is excluding a plane.
- Same tear off? Full tear off to deck, or an overlay. An overlay is cheaper today and adds weight, hides the decking and shortens the life of what goes on top.
- Same underlayment and same ice and water coverage? Membrane at eaves, valleys and penetrations is a specification, not a courtesy, and it is a real cost difference.
- Same flashing scope? New step and counterflashing, or reuse what is there. Reuse is sometimes acceptable and it should be a decision, not a silence.
- Same disposal, permit and clean up? Including who pulls the permit if one is needed, which is covered on the permit sheet.
Once both bids describe the same job, the price difference means something. Until then it means nothing at all, and choosing the lower one is choosing the shorter document.