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.
Planning the work? Check what it usually costs, or work out your own estimate. The free quote form is at the top of this page.
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. It also pays to estimate the flat roof replacement cost before work starts so your budget matches the spec.
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. You can also compare typical roof replacement costs before you decide.