Repairing a reinforced bitumen (felt) roof on a commercial building
Updated 23 October 2026 · SEO Dons Editorial
This guide covers repairs to reinforced bitumen membrane (RBM) roofs, the layered roofing commonly called felt, on commercial and industrial buildings. It does not cover shed or garage felting. The built-up felt and reinforced bitumen overview describes the system. BSI lists BS 8217:2005 as the current code of practice; it covers selecting and combining the membranes and applying them to flat and sloping roofs, including torch-applied, mechanically fastened and self-adhesive types. The wider commercial flat roof repair guide covers when to patch, overlay or replace.
Which felt roof defects can be repaired locally?
The table draws on the Department for Communities in Northern Ireland, whose maintenance checklist for bituminous sheet roofs lists causes and repairs, and on LRWA's refurbishment guidance (issued 2010). They describe causes and repair routes, not step-by-step instructions, and the work itself belongs to a contractor.
| Defect | Cause given | Repair route given |
|---|---|---|
| Small interlayer blisters | Inadequate solar protection (LRWA) | Can be left if small and the underlayers are sound (LRWA) |
| Blisters from trapped moisture | Water vapour pressure below the covering | Cut, trapped moisture allowed to escape, re-bonded to the underlay, moisture source traced and rectified |
| Punctures and rips | Impact from people or loose debris | A local patch built as described below (Sika) |
| Cracks and tears along joist lines | Thermal or moisture movement, saturated insulation or a sagging deck | Existing felt cut back, area left to dry, new high performance felt lapped onto the old |
| Cracks, splits and rucks | Thermal or moisture movement between substrate and membrane | Localised re-roofing with high performance felt is likely |
| Surface crazing | Lack of protection from the sun, or rarely chemical attack | Small patches re-checked; widespread areas re-roofed with specialist protection |
| Split or torn upstand skirting | Movement, poor key to the backing, or design or workmanship faults | Repaired or replaced by an experienced contractor |
| Softened covering round vents and flues | Condensed solvents leaching from the vent or flue | Repair with a membrane that resists attack |
Open laps do not appear as a separate item in the guidance read for this page, so ask the manufacturer's technical department how a lap on the specific cap sheet is to be re-made. Failed upstands are covered in parapets, upstands and copings, and how to read each symptom in common commercial flat roof problems.
What does a proper patch involve?
Manufacturers specify patches for their own products. Sika's published specimen guarantee says a patch to its reinforced bituminous membrane should comprise a layer of both the relevant underlay and cap sheet, sized to cover at least 150 mm beyond the damaged area, and finished with a consistent bitumen bead around the perimeter. The same document says damage is permanently repaired by a trained contractor. That shows the principle: the patch is built from the same layers as the roof. Other makers publish their own sizes, so the rules for the membrane actually on the roof apply.
Compatibility is also a guarantee issue. IKO's sample guarantee excludes damage resulting from products not supplied or approved by IKO, including works or modifications carried out after the date of the guarantee. Sealant-based fixes are covered in roof sealants on commercial roofs.
What hot works controls apply during a felt repair?
A torch-applied patch is hot work. The NFRC's Safe2Torch campaign says roof fires caused by gas torches, no matter how minor, pose a serious threat to life and property. Where any fire risk has been identified, or cannot be ruled out, the relevant parts of the specification must default to torch-free solutions. If a thorough inspection for fire risks is not practical, the guidance recommends a torch-free area of no less than 900 mm. Under no circumstances should a torch be applied direct to a timber roof deck or timber upstands, including timber fillets, even if primed, and torching directly to insulation is not advisable unless it is designed and tested for use with torch-on membranes.
Organisations that pledge support for the campaign commit to supervisors completing the Safe2Torch checklist before hot works start, and the guidance describes a permit-to-work as a formal written system to control work with a risk of fire. Permits, fire watch and insurer conditions are in the hot works guide, and flame-free alternatives in torch-on, cold-applied and self-adhesive felt.
Should asbestos be checked before felt is cut?
On older buildings, yes. HSE says buildings constructed after 2000 are unlikely to contain asbestos, and that people in certain trades are more likely to come across it on buildings that date from before 2000. Under the duty to manage, the dutyholder, who can be the person or organisation with clear responsibility for maintenance or repair, must presume materials contain asbestos unless there is strong evidence that they do not, and must give information on location and condition to anyone who may disturb it. Give the contractor the asbestos register before pricing. The asbestos in old roofing felt guide covers the material itself.
What do guarantees say about repairs?
Published guarantee wording asks for approval first. IKO's sample says the building owner shall not carry out any repairs, alterations, modifications or extension without the prior written approval of IKO. Sika's specimen says Sika must approve in writing any proposed repairs before they start. Bauder's maintenance guide says repairs should be carried out by a Bauder Approved Roofing Contractor, ideally the original installer, and that unauthorised alterations will invalidate the guarantee. It also asks for inspections after building or plant work and storms, or at least six-monthly, ideally in spring and autumn. Sika's appendix says to keep a record of the date, the party and a detailed account of each maintenance job.
When do repeated repairs signal a re-roof?
The Department for Communities says re-surfacing the entire roof may be necessary where surface problems are serious, and should be considered where patch repairs will no longer be sufficient to ensure the integrity of the covering. LRWA says trapped water must be allowed to dry out, or the wet areas cut out and replaced, before a new liquid system goes on, so wet insulation is the finding to look for before agreeing to patch. BRE's Good Repair Guide 16 (1998) accepts that most flat roof coverings need repair or replacement during the life of the building. The decision is set out in repair or replace. To ask about a repair, you can send roof details through the site's form, and the commercial flat roof repair guide explains what a repair should involve.
Sources
- BS 8217:2005 Reinforced bitumen membranes for roofing, code of practice, BSI Knowledge, published 26 January 2005, checked 22 October 2026.
- Maintenance checklist: bituminous sheet flat roofs, Department for Communities (Northern Ireland), checked 22 October 2026.
- Guidance Note 4: refurbishment, Liquid Roofing and Waterproofing Association, issued 2010, checked 22 October 2026.
- Guidance Note 1: inspection, Liquid Roofing and Waterproofing Association, issued 2010, checked 22 October 2026.
- Roofing, 20 year performance guarantee specimen, with maintenance appendix (Sika products only), Sika Limited, checked 22 October 2026.
- General maintenance, RBM waterproofing (Bauder products only), Bauder Limited, revision V9, May 2026, checked 22 October 2026.
- Sample single point guarantee (IKO products only), IKO PLC, checked 22 October 2026.
- Safe2Torch campaign page, National Federation of Roofing Contractors, checked 22 October 2026.
- Safe2Torch guidance, National Federation of Roofing Contractors, MRK057 v1.1, September 2020, checked 22 October 2026.
- Introduction to asbestos safety, Health and Safety Executive, updated 1 March 2021, checked 22 October 2026.
- The duty to manage asbestos in buildings: overview, Health and Safety Executive, updated 1 March 2021, checked 22 October 2026.
- Roof work, Health and Safety Executive, page dated 5 June 2026, checked 22 October 2026.
- Good Repair Guide 16: flat roofs, assessing and making repairs, BRE Group, published 6 June 1998, checked 22 October 2026.
Common questions
Can a felt roof be repaired without a torch?
Yes, where the specification allows it. BS 8217 covers self-adhesive and mechanically fastened reinforced bitumen membranes as well as torch-applied ones, and Safe2Torch says the specification must default to torch-free solutions where a fire risk cannot be ruled out. The maker's repair rules for the specific membrane still apply.
How often should a felt roof be inspected?
Bauder's maintenance guide for its reinforced bitumen systems asks for inspection at least every six months, ideally in spring and autumn, and after building or plant work and major storms. Other makers set similar conditions in their guarantees.
Is a small felt patch a job for the in-house maintenance team?
HSE says all work on roofs is highly dangerous, even if a job only takes a few minutes, and that those doing it must be trained, competent and instructed in the precautions. A patch also needs the right materials and, if a torch is used, hot works controls. Check any guarantee before anyone goes up.
What records should be kept after a repair?
Sika's maintenance appendix asks for the date, the party carrying out the work and a detailed record of what was done, with photographs where safe. Those records are what a guarantee provider or the next contractor will ask for.