Repair or Replace a Flat Roof? A Practical Way to Decide

Start with the whole roof, not the leak

A leak is a symptom. The real question is the condition of the entire roof assembly: membrane, seams, flashings, insulation, deck and drainage. A documented assessment is the best starting point, because two roofs with identical ceiling stains can need very different solutions.

When repair usually makes sense

• The problem is localized — one seam, one flashing, one penetration or drain.
• The membrane elsewhere is flexible, well-adhered and free of widespread cracking or blistering.
• Insulation is dry outside a small area.
• The roof is well within its expected service life.
• Drainage works, or can be improved locally.

When replacement deserves serious consideration

• Leaks recur in several different places.
• Large areas of the membrane are cracked, shrinking, blistered or worn through.
• Insulation is wet across significant areas — wet insulation loses much of its value and can damage the deck.
• The deck is deteriorating.
• Ponding is widespread and needs a new slope design.
• Repair costs are recurring year after year.
• You plan work that will disturb the roof anyway: new rooftop units, a deck, solar or an addition.

Middle-ground options

Between a spot repair and full tear-off there are options. Sections of a roof can sometimes be replaced while others are repaired. A recover — installing a new membrane over an existing roof — is possible only when the existing insulation is dry, the deck is sound, and the added weight and edge heights work; it is not a way to cover up a wet roof. Restorative coatings exist for some roof types in suitable condition; they don't fix wet insulation or structural issues.

Thinking about long-term cost

Compare options over the years you expect to own or operate the building, not just this season. Include likely future repairs, interior damage risk, energy performance and disruption. A replacement also lets you correct drainage and add insulation — in Ottawa's heating-dominated climate, insulation choices have long-term implications. The NRC's Energy RCI tool is a useful technical reference for roof insulation decisions.

Avoid decisions driven by pressure. A reputable contractor should explain the trade-offs in writing and give you time to compare.

Questions to ask any contractor

• What exactly is causing the problem, and how did you determine that?
• Is the insulation wet, and how much of it?
• What does the scope include and exclude?
• What membrane and assembly do you propose, and why for this building?
• How will drainage be handled?
• What permits, if any, may apply?
• What warranty applies, and who provides it?