Planning a roof replacement

A roof is measured in squares of 100 square feet, and priced by the square. The covering is the visible part, but the roof works as a system: the deck, the underlayment, the flashing, the covering and the ventilation under it. A new roof on a poorly ventilated attic, or on a deck with rot, will not last as long as it should.

Most owners hire a roofer for the work, and use a plan like this one to check the quote and the quantities. The sequence below is the order the roofer will follow.

Step 1: Measure the roof and the pitch

The roof area is bigger than the footprint of the house because of the pitch, and hips, valleys, dormers and overhangs add to it. Find the pitch (the rise in inches for every 12 inches of run) and work out the area in squares. Safety matters: measure from the ground, the attic or photographs rather than climbing a steep or wet roof.

Step 2: Choose the covering

Asphalt shingles are the common choice and are sold in bundles, three to a square. Metal panels last longer and shed snow, and are ordered by panel length and coverage width. Whatever you choose, check the minimum pitch for it, because some coverings are not allowed on low slopes.

Step 3: Underlayment, ice barrier and flashing

Underlayment goes over the whole deck. In cold climates a self-adhering ice and water membrane goes along the eaves and in the valleys, often extending at least 24 inches inside the exterior wall line. Flashing goes around chimneys, vents, skylights and where roofs meet walls, and it is where most leaks start.

Step 4: Check the deck and add plywood where needed

Any rotten or damaged sheathing must be replaced before the new covering goes on. Sheets are 4 × 8 feet, and the roofer will usually quote a price per sheet for repairs.

Step 5: Ventilation and insulation

A roof needs air moving under it: intake at the eaves and exhaust at the ridge, in balance. Without it, heat and moisture shorten the life of the roof and cause ice dams and mold. While the roof is open is a good time to check how much insulation there is in the attic.

Before you start

• A permit if your area requires one for re-roofing, and an inspection of the deck.
• The number of existing layers: many codes limit a roof to two.
• Written quotes with the covering, the squares, the underlayment and the flashing listed.
• The roofer's insurance, and a plan to protect the garden and the gutters.
• Dry weather for the tear-off, and a tarp plan.

Mistakes that cost people money

• Re-roofing over rotten decking or old flashing.
• No ice barrier at the eaves where the climate needs one.
• Blocking the vents with insulation, or adding vents that fight each other.
• Choosing a covering that is not allowed for the pitch.
• Accepting a quote that does not state the squares and the materials.

Frequently asked questions