Build slope into the roof
Tapered insulation
Standing water is a design problem, not a membrane problem. Tapered insulation fixes it by giving the roof a gentle fall toward the drains.
- Ends ponding water
- Added during replacement
- Starting price listed
What it is
Tapered insulation is rigid board cut at a slight angle, supplied in a layout plan. Laid in sequence, the boards create a slope across the roof, with valleys running to the drains. It also adds insulation value, so a roof gets thermal and drainage improvements in one step.
It cannot be added as a patch. Because it sits under the membrane, it goes in when the roof is replaced or recovered. See flat roof replacement.
When it makes sense
Ponding that stays for days
Water that sits longer than a day or two after rain speeds up wear and attracts dirt. See ponding water.
Roofs with too few drains
Tapering can move water to the drains you have, so you do not need more.
New additions
Instead of framing a slope into the structure, a tapered layer can create it on top of a level deck.
Energy upgrades
The roof needs more insulation anyway, so adding it as a taper gives slope for little extra work.
Rooftop decks
Slope under a terrace membrane keeps the water out of the assembly. See rooftop deck waterproofing.
How it is designed and installed
Survey
Measure the roof, locate the drains and note where water collects.
Taper plan
A layout drawing shows each board, thickness and where the low points fall. Drains may be moved or added to suit.
Install
Boards laid in order over the base layer, with joints staggered and fastened or adhered.
Membrane
The new roof goes over the insulation. Flashings are raised so the changed height still works at walls and curbs.
Tapered insulation pricing
| Item | Starting price |
|---|---|
| Tapered insulation (added to a replacement) | from $2 per sq ft |
| House flat roof replacement (about 1,000 sq ft) | from $8,000 per roof |
| Roof drain or scupper repair | from $800 per drain |
Starting prices in CAD before 13% HST, based on typical Ontario contractor rates (mostly Toronto and the GTA). The final price depends on roof size, access, the layers to remove, the membrane, drains and flashings, and is confirmed in your written quote.
The tapered row is the added cost on top of a replacement. Thicker boards, complex layouts and raising flashings or parapets increase it.
Check the transition at walls and doors
A thicker roof makes the roof edge higher. Door thresholds, parapets and window sills need enough height above the finished surface, so the plan has to be checked against them before any boards are ordered.
Questions about tapered insulation
How much slope do I need?
A small, steady fall toward drains is the usual target. Your designer or roofer will give the figure for your project, since drain spacing affects the result.
Can you fix a ponding roof without replacing it?
Not with insulation. Short of replacement, options are an added drain, clearing blocked drains, or a coating that tolerates standing water. See roof coating.
Does tapered insulation change my energy bills?
Any added insulation helps, but the exact saving depends on your roof and building. We can specify the insulation value in the quote.
Will it work on a condo roof?
Yes, and it is common in large re-roofs. The planning side is covered in the condo roof planning guide.
Leak, damage or an old flat roof?
Send a few details and photos. We reply by email with next steps and a written quote.