Guide
Repair or replace a flat roof?
Most people ask this after the second or third leak. Here is how an experienced roofer decides, and what you can check before anyone gets on the roof.
The question behind the question
A roof can be repaired if the problem is local and the rest is healthy. It should be replaced if the problem is everywhere or hidden. So the real question is: how much of this roof is still doing its job? Age matters, but condition matters more. A 12-year-old roof that has been maintained can be sound; a 7-year-old roof with ponding and open seams may not be.
The decision usually rests on four things: moisture in the insulation, the state of the membrane across the field, the number of layers already on the roof and the cost of repeat repairs.
A simple decision path
Is the insulation wet?
Wet insulation cannot be dried by patching. A moisture survey, with a meter or infrared camera, shows where. If a small area is wet, cut it out and repair. If large areas are wet, plan a replacement.
Is the membrane failing everywhere?
Cracking across the whole surface, widespread blisters or seams open all along the roof mean the material has aged out. See cracks and alligatoring and open seams.
How many layers are there?
Many codes and manufacturers limit how many layers can stack on a deck, and extra layers add weight. Ask the roofer to check, and see re-roofing permits.
How old is it, against its expected life?
Compare age to the usual range: EPDM 20 to 30 years, TPO 15 to 25 years, modified bitumen 15 to 25 years. See how long a flat roof lasts.
What have repairs cost so far?
Add up the last three years. If repairs keep returning, replacement is cheaper in the long run.
Signs that a repair is the right call
One leak, one cause
A failed flashing, a cut in the membrane, a loose seam or a blocked drain.
Dry decking and insulation
A probe shows the layers below are dry and firm.
A roof in the middle of its life
A repair buys several more years at a modest cost. See flat roof leak repair.
Damage from an event
A fallen branch, a dropped tool, an air conditioner install. The rest of the roof is unaffected.
Signs that replacement is the better call
Soft or spongy feel underfoot
This points to wet insulation or deteriorating deck.
Widespread ponding with membrane damage
Water still standing 24 to 48 hours after rain, with surface wear. See ponding water.
Multiple leaks in different places
Leaks in several rooms suggest the membrane is failing as a whole.
Repeated repairs in the same year
Patching is only a bandage if the surface around it keeps failing.
A building with renovation plans
If you plan to add a deck, a skylight or insulation, doing the roof first avoids tearing a new roof open.
The middle option: coat or overlay
If the membrane is worn but the insulation and deck are dry, a restoration coating can be a middle step. It costs from $3 per sq ft and commonly lasts 7 to 20 years, depending on the coating. A coating does not repair open seams or wet insulation, so the roof is repaired first and coated second. See roof coating.
In some cases a new membrane can go over an existing one. That depends on the roof being dry and flat, on local rules and on the manufacturer’s requirements. A roofer who proposes it should explain how they confirmed the roof is dry.
What to do before you call
Take photos of the ceiling stain and of the roof from a window or ladder, write down when it leaks (rain, snow melt, only in heavy wind) and note anything on the roof that has changed. Then request an inspection or a quote. See preparing for a roof quote.
Ask for both options in writing: a repair price and a replacement price. Seeing them side by side is the fastest way to decide.
Timing matters in Ontario
Replacement is easiest to plan for dry weather between spring and late fall. If you decide in winter, use the cold months to collect quotes, confirm the scope and book a start date, and ask for a temporary repair to hold the roof until then. A repair done in a hurry in February is a stopgap; a replacement planned in March can be done properly in May.
Also ask what else is planned for the building. A new rooftop unit, a solar or antenna install or a deck changes the roof, so it is cheaper to schedule roof and equipment work together than to cut into a new membrane later.
Questions about repair and replacement
How many repairs are too many?
There is no fixed number. When repairs in a year cost close to a third of a replacement, or leaks keep showing up in new places, ask for a replacement quote.
Can a roof leak without any visible damage?
Yes. Water can enter at a flashing or a seam and travel along the deck before it drips. See finding a flat roof leak.
Will a replacement fix ponding?
Only if the slope is corrected, usually with tapered insulation or by fixing drains. Otherwise the new roof ponds too. See tapered insulation.
Leak, damage or an old flat roof?
Send a few details and photos. We reply by email with next steps and a written quote.