How to Plan a Flat Roof Conversion Step by Step (2026)
A flat roof conversion — replacing a tired felt roof or re-roofing an extension or garage with a modern flat system — is straightforward if you plan it in the right order. The six steps are: survey the structure, confirm permissions, choose the membrane, specify insulation and falls, compare quotes, then build and sign off. This guide walks through each one.
Flat roof conversion — the short answer
- Step 1: Get the deck and joists surveyed for rot, sag and load.
- Step 2: Confirm permitted development / building control — most flat-roof replacements are PD but must meet Part L insulation.
- Step 3: Choose the membrane — EPDM rubber, GRP fibreglass or high-performance felt.
- Step 4: Specify insulation (warm-deck) and a minimum fall so water runs off.
- Step 5 & 6: Compare vetted roofer quotes, then build, inspect and get your guarantee.
The biggest mistakes on flat roofs are skipping the structural check and laying a new membrane over a wet or under-insulated deck. Getting the survey, falls and insulation right first time is what makes the difference between a 5-year and a 25-year roof.
The 6 Steps to Plan a Flat Roof Conversion
Survey the existing structure
Have a roofer check the deck, joists and any signs of rot, ponding or sag. This decides whether you can re-cover or need a new deck, and confirms the roof can carry a warm-deck build-up.
Confirm permissions & building control
A like-for-like flat roof replacement is usually permitted development, but more than 25% of the roof triggers Building Regulations (Part L) insulation upgrades. A new flat-roof extension needs its own approval.
Choose the membrane
EPDM rubber is single-ply, seamless on smaller roofs and long-lasting; GRP fibreglass gives a hard, walkable finish; modern torch-on felt is the budget option. Match the system to the roof size, traffic and budget.
Specify insulation & falls
Go for a warm-deck build-up with PIR insulation to meet current U-values, and make sure the design includes a minimum fall (usually 1:40 to 1:80) so water always drains to the outlet.
Compare vetted quotes
Get at least three quotes against the same spec — membrane type, insulation, guarantee length and whether scaffolding and disposal are included — so you compare like-for-like.
Build, inspect & sign off
Inspect the deck and insulation before the membrane goes down, check the falls and upstands, and keep the manufacturer-backed guarantee (often 20–25 years on EPDM/GRP) with your paperwork.
EPDM vs GRP vs Felt at a Glance
EPDM rubber
Single-ply rubber, often laid in one seamless sheet on smaller roofs. Flexible, long-lasting (25–30+ years) and good value — the most popular domestic choice.
GRP fibreglass
A rigid, seamless resin finish that is walkable and ideal for balconies and roofs with foot traffic. Needs dry weather and skilled installation.
Torch-on felt
Modern multi-layer felt is the budget option. Reliable when laid well, but typically a shorter life than EPDM or GRP.
Don't skip the falls
Standing water (ponding) is the number-one killer of flat roofs. A proper fall built into the deck or tapered insulation is essential whichever membrane you pick.
Common Questions
Related Guides
More how-to, compare and cost guides for your roofing project.
Get 3 Free Flat Roof Quotes
BestBuilders matches you with 3 vetted flat-roof specialists who price your conversion against one brief so you compare like-for-like — membrane, insulation and guarantee included. Insurance-backed workmanship.