Flat Roof Cost Calculator

Flat roofs are common on extensions, garages, dormers and porches across the UK. Modern materials last far longer than the old felt roofs many people remember, but choosing the right one, and the right installer, makes a big difference. This guide explains what a new flat roof costs and how the main options compare.

Estimate the cost of your own roof with the flat roof cost calculator.

Flat roof costs per m²

Typical 2026 prices for replacing a flat roof, including stripping the old covering and laying a new one, are:

These prices assume the timber deck is in reasonable condition or needs only small repairs. Replacing rotten decking or joists adds to the cost.

Insulation and Building Regulations

When you replace more than 25% of a roof's covering, it normally counts as renovating a thermal element. Building Regulations then require you to bring the roof's insulation up to current standards, where it's practical to do so. For most flat roofs, that means a warm roof: rigid insulation boards laid on top of the deck, with the waterproof covering on top.

Adding a warm roof typically costs around £25 to £45 per m² extra. It's money well spent, as it reduces heat loss and condensation. You'll usually need to notify building control, or use a contractor registered with a competent person scheme who can self-certify the work.

A worked example

A 5m by 4m extension roof, replaced in EPDM rubber with a warm roof upgrade:

• EPDM rubber: 20m² at a typical £90 per m² = £1,800
• Warm roof insulation: 20m² at about £35 per m² = £700
• Typical total: around £2,500

Smaller roofs like porches often cost more per m², because there's a minimum amount of time and materials for any job. Try your own measurements in the flat roof cost calculator.

Comparing the main flat roof materials

Torch-on felt

Modern felt roofs use several layers of reinforced bitumen membrane, heat-bonded with a gas torch. They're far tougher than older felt roofs, and the finishing layer often has a mineral surface to protect it from UV light. Torch-on felt is a good-value option, though the use of naked flames means it must be done by experienced, insured roofers.

EPDM rubber

EPDM is a synthetic rubber membrane, usually laid in a single sheet with no seams on most domestic roofs. It's flexible, handles temperature changes well and can last for decades. It has a plain black finish that some people find less attractive, but it's rarely visible on most extensions.

GRP fibreglass

GRP (glass-reinforced plastic) is built up from layers of resin and fibreglass matting over a timber deck, finished with a coloured topcoat. It creates a hard, seamless surface that can be walked on for maintenance and is easy to shape around details. It needs dry weather to cure properly and a skilled installer to avoid cracking.

What affects the cost?

• Size and shape. Lots of corners, skylights, vents and pipes add labour.
• Access. Roofs above conservatories or with poor access need more scaffolding or protection.
• Condition of the deck. Rotten boards or joists must be replaced before the new covering.
• Edges and drainage. New fascias, trims, gutters and outlets add to the bill.
• Rooflights. Fitting or replacing rooflights costs extra but can transform a room below.

Signs your flat roof needs attention

• Water stains or damp patches on the ceiling below
• Pools of water that remain on the roof long after rain
• Blistering, splitting or cracking in the surface
• A spongy feel underfoot, suggesting the deck is rotting
• Moss or plants growing on the roof
• Gaps where the roof meets walls or flashings

Small issues can sometimes be repaired, but widespread damage or an old roof nearing the end of its life usually makes replacement better value.

Repair or replace?

If your roof is less than halfway through its expected life and the problem is localised, a repair can be sensible. If it's older, has had several repairs or the deck is damaged, a full replacement is usually more cost-effective in the long run. A good roofer will explain which makes sense and why.

Choosing a roofer

1. Ask which system and manufacturer they'll use and whether they're approved installers.
2. Check what the price includes: stripping, disposal, deck repairs, insulation, trims and gutters.
3. Confirm they'll handle Building Regulations requirements.
4. Ask for a written guarantee, and whether it's insurance-backed.
5. Look at examples of their previous flat roofs.
6. Get at least three quotes.

Looking after a flat roof

A little maintenance helps any flat roof reach the end of its expected life. Check it at least once a year, and after storms, for debris, blocked outlets and damage. Clear leaves and moss so water can drain freely, and keep gutters clear. Avoid walking on the roof unless it's designed for it, and never place ladders or heavy items directly on the membrane. Catching small splits or lifted edges early is far cheaper than repairing water damage to the ceiling and timbers below.