Flat roof quotes vary more than pitched roof quotes, because the scope can differ enormously for the same visible area. One contractor is pricing a membrane over the existing boards; another is pricing a strip, new deck, insulation upgrade and correct falls. Both are quoting 'a new flat roof'. Only one of them is going to still be watertight in fifteen years.
Deck condition sets the floor price
If the existing boards are dry, sound and correctly fixed, a lot of the job is preparation and covering. If they are soft, swollen or sagging, they come off, and once they are off, joists, firrings and any wet insulation are all visible and may need addressing.
This is why we press the deck before quoting rather than pricing off a photograph. Discovering a rotten deck on the morning of the job helps nobody.
Warm roof or cold roof
A warm roof puts rigid insulation above the deck with the membrane over it. It performs better, avoids the interstitial condensation problems of poorly ventilated cold roofs, and it is what building regulations will normally push you towards on a replacement. It also raises the roof level, which affects upstand heights, door thresholds and the junction with any adjacent wall or window.
Insulation is a real cost, and it is one of the biggest differences between two quotes for the same roof. Ask which build-up each price is for.
Details, not area
The field of a small flat roof is quick. The time goes into the edges.
- Upstands against walls, and any cover flashing or chase needed
- Outlets, rainwater hoppers and gutter connections
- Soil pipes, vents and cable penetrations through the covering
- Kerbs to rooflights and lantern openings
- Junction with the pitched roof above where an extension abuts
Choosing the system on cost
Felt is generally the lowest initial cost, GRP sits in the middle with a hard walkable finish, and EPDM typically costs a little more than felt with a longer expected life. Over a twenty-year view the cheapest option at day one is not usually the cheapest overall. We price the alternatives against each other for your roof so the comparison is concrete rather than theoretical.
Common questions
Can you just lay a new covering over the old one?
Sometimes, if the existing roof is dry and sound. Usually we recommend stripping, because it is the only way to know the deck condition and it prevents trapping moisture under the new membrane.
Does a flat roof replacement need building regulations approval?
Replacing a significant proportion of a roof covering generally brings thermal upgrade requirements into play. We will tell you where your job sits and what that means for the build-up.
Why is a small dormer roof not cheap?
Because it is almost entirely details, upstands, cheeks, a junction with the pitched roof and often difficult access. Area is a poor guide to cost on small flat roofs.
Do you guarantee flat roof work?
Workmanship guarantees and full insurance are part of how we work, and the terms are set out on the written quotation rather than promised verbally.