Water sitting on a flat roof
ARMA defines ponding as water that stays 48 hours or longer, and says it substantially increases the load on the deck and scrubs the membrane as ice moves.
What is probably happening
- A blocked drain or scupper. Leaves, a tennis ball or a bird nest is the most common cause.
- The deck has sagged into a low spot, or the insulation was never tapered, so the roof no longer drains at the quarter inch per foot ARMA calls positive drainage.
- The membrane is aging and the ponding is finding seams and pinholes.
Do this in the next five minutes
- From a window or the ground, photograph where the water sits and how deep.
- If you can safely reach a drain or scupper from a ladder at the edge, without stepping onto the roof, clear leaves and debris from it.
- Check the ceiling below the puddle for stains.
- Book a flat-roof contractor for an inspection. Ask specifically about drainage, not just patching.
- If the roof is holding a lot of water and you see any sag from below, treat it as structural and see the sagging roof page.
Do not
- Do not walk on a flat roof that is holding water. You cannot see the soft spots and a wet membrane is slick.
- Do not accept a coating as the fix for ponding. ARMA's fixes are drains, tapered insulation and crickets. Coatings do not change the slope.
- Do not let anyone drill a hole through the membrane to 'let it out'.
Who to call
A roofing contractor who does low-slope work
Membrane roofs are a specialty inside roofing. A shingle-only roofer is the wrong call. Illinois licenses this separately as an unlimited license.
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What it should cost
$350 to $2,200
Flat roof repair, national range. Full flat roof replacement averages $7,433, ranging $1,200 to $24,000.
Source: Angi, roof repair cost, updated August 2026 and Angi, flat roof replacement cost, updated July 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the roofing page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Why is the water not draining, and does your fix change the slope or only patch the membrane?
- Is tapered insulation or an extra drain part of this quote?
- What membrane is on my roof now, and how old is it?
- Will you check the deck from the inside for sag?
End the call if
- A coating sold as a cure for standing water.
- A contractor who has never done membrane roofs quoting a flat roof.