Flat roof replacement cost

A flat roof is not flat. Every roof that looks flat is built with a slight fall, usually a quarter inch per foot, so water runs to a drain instead of sitting. Whether that fall still works is the question that decides whether your job is a $10,000 membrane replacement or a $25,000 rebuild.

Flat roofing is also a different trade from shingle roofing. The material arrives in rolls or sheets and is welded, torched or glued into one continuous surface. There are no overlapping pieces shedding water downhill; there is one skin, and it either holds or it does not.

What the membranes cost

Installed over an existing deck, per square of 100 square feet, including tear-off of one old layer:

On a 1,500 square foot flat roof, which is 15 squares, that puts a TPO or EPDM replacement at roughly $8,000 to $13,000 and a PVC roof at $11,000 to $18,000.

Which membrane to pick

TPO is the default on new residential flat roofs. It is white, which keeps the building cooler, the seams are heat-welded into a continuous sheet, and it is the best value per year of service. Quality varies by manufacturer more than with other membranes, so the brand on the quote matters.

EPDM is black rubber, usually glued rather than welded, and it is the most forgiving to install well. It absorbs heat, which is a drawback in the south and an advantage in cold climates where it helps shed snow.

PVC costs more and is the right answer where the roof will see grease or chemical exposure, such as above a kitchen, or where a very long service life justifies the premium.

Modified bitumen is the modern descendant of tar and gravel, applied in rolls with a torch or cold adhesive. It is common on small residential flat sections like a porch or dormer roof, where a full membrane crew is overkill.

What pushes the price up

Insulation. If the existing insulation is wet, it comes out. Replacing rigid insulation board adds $150 to $400 a square, and on a roof that has leaked for a season or two, it is usually necessary rather than optional.

Deck repair. A flat roof that has held water rots its deck faster than a pitched roof does. Replacing plywood or planking runs $70 to $150 a sheet, and on a long-neglected roof it can be most of the surface.

Re-sloping. If ponding water is the reason the roof failed, replacing the membrane without fixing the fall just resets the clock on the same failure. Tapered insulation to rebuild the slope adds $200 to $500 a square. It is the most commonly skipped line item and the most commonly regretted one.

Drains, scuppers and parapet flashing. These are where flat roofs actually leak. Rebuilding them properly is a few hundred dollars each and is not a place to economize.

The tell-tale sign to check before you get quotes

Go up after it rains and look for standing water two days later. Water that has not drained in 48 hours is ponding, and it is a design problem, not a membrane problem. If a contractor quotes you a straight membrane swap on a roof that ponds, get another quote. The second one will cost more and last longer.

Frequently asked questions

Can a flat roof be coated instead of replaced?

Yes, if the membrane underneath is sound and dry. A silicone or acrylic coating runs $150 to $350 a square and buys 10 to 15 years. It is a genuine option on a roof that is aging but not failing. It is a waste of money over wet insulation, because the water is already inside and the coating seals it in.

How long does a flat roof replacement take?

A straightforward residential flat roof of 1,000 to 2,000 square feet is a two to four day job. Weather matters more than on a shingle roof: most membranes cannot be installed onto a wet deck, and adhesives have minimum temperatures.

Why do flat roofs have a bad reputation?

Because a pitched roof forgives a mediocre installation and a flat roof does not. Water sits on a flat roof and finds every weakness. Well-built flat roofs last decades; the reputation comes from the ones that were built cheaply.

Is a flat roof more expensive to insure?

Some carriers rate them slightly higher, and a few decline older tar and gravel roofs entirely. It is worth a call to your insurer before you commit to a membrane, because the answer sometimes differs by material.