What a terrace really is: a flat roof with a floor on top
A rooftop terrace or third-floor balcony over living space is a flat roof. Under the stone tile, pavers or decking there is a waterproofing membrane, a slope to the drain, and below that the structure and the ceiling of the room below. The tile you see is just a wear surface. It is not what keeps the water out.
That is why a terrace can look perfect and still drip into the living room. Water gets through grout, seams and edges, runs along the membrane, and finds the weak point. If that weak point is above a ceiling, you see a stain or a drip.
The most common causes
| Cause | How common | Typical repair |
|---|---|---|
| Failed seams in the membrane | Most common | $400 – $1,500 |
| Cracked or pulling flashing at walls, doors and rails | Very common | $500 – $2,000 |
| Ponding water in low spots | Common | $1,000 – $3,000 per zone |
| Clogged drain or scupper | Common | $200 – $800 |
| Punctures from furniture, planters or tools | Common | $300 – $1,000 |
| Membrane at the end of its life | Eventually | Replacement |
On a terrace the weakest points are almost always the details: the threshold at the door, the junction with the wall, the posts for the railing and the drain. Railing posts bolted through the membrane are a classic leak that homeowners rarely think about.
Why finding the leak is harder than on a bare roof
On an open flat roof you can see the damage. On a tiled terrace, the membrane is hidden. Water also travels, entering in one spot and showing up 5 to 15 feet away on the ceiling below. Good diagnosis means checking drains and flashing first, then using a flood test, moisture mapping or infrared to trace the path. Guessing and re-caulking the grout usually fails.
Caulking tile joints or painting a sealer on top treats the symptom. If the membrane under the tile is failing, the water is still getting in below the surface.
Repair or replace?
- **Repair** when the membrane is in good shape and the leak is at one detail, such as a drain, a flashing or a single seam. Often the finish surface only needs to be lifted in one area.
- **Replace the membrane** when there are multiple leaks, the membrane is brittle or past its service life, or ponding has damaged the layers underneath.
- **Rebuild the slope** when water sits on the deck. Adding tapered insulation to re-slope a zone costs $1,000 to $3,000, and structural deck repair runs $3,000 to $8,000+.
What a full replacement involves and costs
Replacing a terrace membrane is a bigger job than patching because the finish surface has to come off first. The sequence is: remove and set aside tile or pavers, inspect and repair the deck, install a new membrane with proper flashing and drain details, and then reinstate the surface.
Flat roof membranes cost about $5 to $17 per sq ft installed depending on the system: modified bitumen $5–$8, EPDM $6–$10, TPO $7–$12. A full flat re-roof typically costs $6,000 to $25,000+ depending on size. Taking up and reinstating stone tile or pavers is a separate line item that depends on the material and how much of it can be reused, so we price it after seeing the surface. Permit fees, where required, are billed at cost on top of the price, and HST is 13% on the full pre-tax amount.
How to protect a terrace once it is fixed
- Clear the drain and scupper in spring and fall, and after heavy leaf drop.
- Keep planters and heavy furniture on pads, not directly on the membrane.
- Do not drill into the deck or railing posts without resealing the penetration.
- Look at the ceiling below after heavy rain and during thaw periods.
- Watch the area at the parapet in winter, since ice dams there are a common source of leaks.
What to do right now if it is leaking
- 1Put a bucket under the drip and move belongings out of the way.
- 2Photograph the stain, the terrace and the date for your insurer.
- 3Clear the terrace drain and sweep standing water toward it.
- 4Do not seal the tile joints as an emergency measure.
- 5Call for a leak assessment, since water inside the ceiling can keep spreading for days.
FAQs about terrace and balcony leaks
Water coming through the ceiling?
We find the source first, then recommend the smallest repair that will actually work, with a written, itemized estimate. For terraces and flat decks across Toronto and the GTA, call (647) 254-0877 or send photos and your address.
Owner-Operator at Buildoreno with 25+ years of hands-on GTA construction experience. Last reviewed October 2026.