How to waterproof a flat roof under a low window
Standard membranes like TPO often cannot fit the tight gap beneath an existing windowsill, and customers usually refuse to remove or resize the window to make room. Furthermore, when building a deck over these flat roofs, crews often screw the decking directly through the waterproofing membrane, creating holes that leak into the room below.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Running a 2-ply modified bitumen system.Did this help you?
- 2Removing the window temporarily to wrap TPO into the bottom plate and reset the window.Did this help you?
- 3Replacing the siding near the window with a custom trim piece to wrap a membrane up the wall.Did this help you?
- 4Fabricating and installing a custom copper pan if the budget allows.Did this help you?
- 5Use TPO or EPDM for flat surfaces.Did this help you?
- 6Install the deck using methods that do not penetrate the waterproofing membrane.Did this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Customer does not want to remove the window or make it smaller. 1/12 pitch, cedar will be replaced on the siding as needed. Tpo won’t work I don’t think due to the gap under the windowsill. Reconstruction is not an option. Need to make this area watertight.”source ↗
Where this came up
People with this problem also raised
- 19How to temporarily waterproof a damaged roof until replacement
- 15How to fix a dipping flat roof that collects standing water
- 8How to fix a leaking skylight after rain
- 13Basement window level with the ground water leak prevention
- 2How to add a drip cap to existing windows without removing siding
- 3Best sealant for a roof pipe penetration