Why does water pool on a flat roof in the first place?
Because the slope is small. A flat roof is built with a slight fall toward drains, but the fall is measured in fractions of an inch per foot, so a settled deck or a blocked drain can hold water in a basin.
Most flat roofs in this trade are built to drain by a slope that the eye cannot see. Over years the structure underneath settles, the insulation compresses in some places and not others, and a shallow basin forms. Water in a basin sits, it does not run off, and the membrane above the basin is under constant load and constant wetting. That is the pattern a roofer looks for first.
Ponding is often a drainage problem and not a membrane problem. A drain set higher than the low point of the roof will hold water no matter how good the membrane is, and clearing debris from a drain only helps for a while if the drain sits in the wrong place. The honest first answer on a ponding roof is often a change to the drain, which is cheaper than a new membrane.
- Measure where the water stands and how deep it is after a dry day
- Check the height of each drain against the low point of the roof
- Note any debris that collects at the same place every storm
How are seams and blisters checked and repaired?
Seams are probed along their length and the edges are lifted where the sealant has cracked. Blisters are opened and dried before any patch goes on, because a patch over a wet blister seals the water in.
A seam that has failed usually shows as a clean gap, a lifted edge or a line of cracked sealant. The roofer tests each seam by hand and looks for the places where the sealant has shrunk away from the membrane. Repairs are cleaned to bare membrane, primed where the product requires it, and sealed with a compatible patch. Using the wrong sealant on a membrane is a common cause of a second failure, so the product is named in the quote.
Blisters are different. A blister is a pocket of trapped air or water between the membrane and what lies under it. Opening a blister and finding it dry is good news and means a simple patch. Finding it wet means the insulation underneath has to come out, dry out or be replaced before the membrane goes back down, and that is the point at which the quote changes.
- Clean the membrane to bare surface before any sealant goes on
- Use the sealant the membrane manufacturer names for that membrane
- Open and inspect every blister, not only the ones that look large
What does a flat roof repair look like on site?
A morning on the roof, a set of photographs and a written list of what was done. Access and weather decide the timing, and the roofer says so when booking.
Most repairs on a flat roof are done in a single visit when the damage is limited to one area. Larger jobs, especially where insulation is replaced, are done over several visits so the roof is not left open overnight. The roofer carries the materials up, sets up edge protection where the building requires it, and keeps foot traffic to the path the roof can take.
Weather matters. Membrane and sealant do not bond well on a wet or very cold surface, and a roofer who works through a forecast of rain is taking a risk with your ceiling. The schedule is set with that in mind, and you are told when the roof will be open and when it will be closed.
- Set up edge protection and a clear walking path first
- Protect rooftop units and cover the openings the work exposes
- Photograph each repair before the surface is covered
When should a flat roof be replaced instead of repaired?
When the repairs keep spreading faster than they can be paid for, or when the deck or insulation under the membrane is wet over a large area.
Repair is the right answer for a roof with a sound deck, a membrane that is still bonded to its substrate, and a small number of failing points. Replacement becomes the better answer when the same area keeps failing, when the insulation is wet across a large part of the roof, or when the membrane has become brittle and splits when lifted. A roofer should be able to say which of these applies and show you the evidence, not just the photograph of the worst spot.
A roof replacement is a separate quote. This page covers repair, and if the inspection finds the roof is at the end of its life, the roofer says so and explains the options in writing, with the reasons.
- The same area failing after two or more repairs
- Wet insulation across a large part of the roof
- A membrane that cracks when it is flexed rather than stretching
