How our commercial flat roofing process works
A commercial flat roofing project should feel controlled, not improvised. Every roof we specify follows the same six-step method, and the order matters: the survey and the design come first, the membrane comes last. That is the opposite of a patch-merchant, who names a product before anyone has read the deck. Here is exactly how the work runs from first visit to sign-off.
1. Survey and moisture / build-up assessment
The first visit reads the roof from the deck up. We confirm the deck type (metal, concrete, timber or an existing membrane), open up or scan the build-up to find where the insulation is wet, and record the falls, the outlets, the upstands and the details that leak. Ponding is mapped, because standing water is the visible symptom of a roof never laid to fall. On any building from before 2000 an asbestos survey is arranged before intrusive work, since the real risk on older commercial roofs is legacy asbestos insulating board and asbestos cement, not the modern felt itself. You come out of this stage with an honest condition report rather than a sales pitch.
2. Falls and U-value design
With the survey in hand, the roof is designed rather than guessed. Falls are set to BS 6229:2025, a minimum finished fall of 1:80, with the design fall derived from structural analysis or a level survey (commonly 1:40 or steeper) so the finished minimum survives construction tolerances and deflection. Where the existing roof ponds, tapered insulation is normally used to build the fall into the insulation layer without touching the structure. The U-value is set to Building Regulations Part L: renewing more than 50% of the roof surface, or more than 25% of the whole building envelope, triggers a thermal-element upgrade, typically to around 0.18 W/m²K on a re-roof, worked out by calculation against the current Approved Document L. Wind uplift is calculated to BS EN 1991-1-4 to set the fixing pattern and the enhanced perimeter and corner zones, and a vapour control layer is specified on the warm side to prevent interstitial condensation.
3. Strip-and-recover or overlay decision
Only now does the repair-or-replace question get answered, and it is answered on evidence. An overlay recovers a sound existing membrane, which is cheaper and faster, but it only works where the deck, insulation and falls are sound and the structure can take the added weight. A strip-and-recover removes the failed covering back to the deck, the right call where the insulation is wet, the deck is deflecting, the roof ponds or a Part L upgrade is due anyway. You get both options side by side, with honest costs and remaining-life estimates, on our repair or replace page and in your proposal, so the decision is yours and it is defensible to a board.
4. Membrane install and detailing
The chosen system is installed by a manufacturer-approved installer to specification. Single-ply membranes are laid in wide rolls with hot-air welded seams; reinforced bitumen is built up in layers; liquid-applied waterproofing is brushed or rolled seamlessly around congested details. On occupied and sensitive buildings we specify cold-applied or self-adhesive systems to remove naked-flame hot-works risk over your operation. The programme is phased bay by bay so a warehouse, school or shop keeps trading, teaching or picking below while the roof is renewed above, and every phase is protected and drained before the next is opened. Work at height, edge protection and fragile-roof precautions follow HSE roof-work guidance and CDM 2015 duties throughout. The warm-deck re-roof page shows the full build-up sequence.
5. Wet test and sign-off
Before anyone signs anything, the finished roof is checked. Seams are inspected, outlets and details are proven, and the roof is tested for water-tightness and confirmed to drain to the outlets with no back-falls or residual ponding. Any defect is corrected at this stage, not left for the first winter to find.
6. Guarantee and Building Regulations Compliance Certificate
Finally, the paperwork you actually need is issued. The manufacturer registers a single-point or insurer-backed guarantee, typically up to 20 to 30 years on single-ply, subject to system and approved-installer status, and you receive the wind-uplift and falls design and the operation-and-maintenance manual. Where the installer is CompetentRoofer-registered, the re-roof is self-certified and a Building Regulations Compliance Certificate is issued for your records, the document you will need at a sale, a lease event or an insurance review. What each guarantee covers, and what voids it, is set out on our guarantees page.
That is the whole commercial flat roofing process: survey and design first, an honest repair-or-replace decision, a phased install that keeps you operational, a wet-tested sign-off and the certification you can keep. To start with a free condition report, request a quote and we will book the survey.