Common commercial flat roof problems and what they mean
Updated 23 October 2026 · SEO Dons Editorial
This guide reads a flat roof's symptoms, noting which roofing system each applies to: the likely causes, how serious each is, and whether it points to repair, overlay or replacement. It does not cover finding where a leak enters, which is a separate job. The decision itself is set out in how to decide between repair and replacement.
How should a roof be looked at safely first?
HSE says all roofs, once fixed, should be treated as fragile until a competent person has confirmed that they are non-fragile. LRWA's inspection guidance (issued 2010) says that in cases of doubt, inspection must be confined to a visual inspection from the perimeter without accessing the roof. HSG33 lists a telescopic pole with a camera attachment as one way to inspect without going onto it.
What do blisters on a felt roof mean?
LRWA's refurbishment guidance separates two kinds. Interlayer blistering, between the cap sheet and the underlayer, often occurs as a result of inadequate solar protection. More severe blistering can occur as a result of interstitial moisture from water ingress. Blisters may also be a sign of trapped water below the surface, such as saturated insulation, or a very high vapour drive. The Department for Communities in Northern Ireland puts it as water vapour pressure below the roof covering.
Seriousness depends on the kind. LRWA says interlayer blisters can be left if they are small and the underlayers are sound; more severe ones need checking that they are not compromising attachment and wind resistance. For a blister that has to be dealt with, the Department's guidance is that it is cut, any trapped moisture allowed to escape, and the blister re-bonded, with the source of the moisture traced and rectified. Some manufacturers treat blistering as cosmetic: IKO's published sample guarantee excludes purely aesthetic issues that do not affect waterproofing integrity, including blisters. The felt roof repair guide covers the repair side.
Why do roofs split, crack or craze?
Reinforced bitumen. The Department for Communities says cracks and tears along the line of the support joists can be caused by thermal or moisture movement, saturation of insulation or sagging of the roof deck, and that the repair is to cut back the felt, let the area dry and lay new high performance felt with a suitable overlap. Cracks, splits and rucks from movement between substrate and membrane are likely to need localised re-roofing. Surface crazing comes from a lack of protection from the sun or, rarely, chemical attack: small patches need only re-checking, widespread crazing means re-roofing.
Liquid and coated roofs. LRWA reads cracks and splits in the main area as a sign that the membrane or coating was fully bonded to a substrate that has moved, such as timber decks, plywood, particle board or polystyrene insulation. Cracks at details indicate that no allowance was made for differential movement. The GRP roof problems guide covers fibreglass, and the mastic asphalt guide covers asphalt, which LRWA says may have aged or be subject to undue movement.
SPRA adds that when refurbishment is needed because a roof has failed, the cause should be fully investigated, and cracking of an old bituminous system due to building movement may necessitate revised detailing. Re-covering alone may not cure it. Many splits start at junctions; see parapets, upstands and copings.
What do open seams and lifted edges mean on a single-ply roof?
On single ply the seams between sheets do the waterproofing, which is why SPRA's seam guide treats their formation on site as critical. For hot-air welded thermoplastics it lists causes of weak welds that include low welding temperature and contamination of the lap with water, fines or chemicals. EPDM is bonded rather than welded, a different failure pattern covered in the EPDM guide.
SPRA's guarantee guide adds that defective workmanship is likely to show in the early years, through increased maintenance, poor appearance or, in the extreme, leaks. A seam open at one point suggests local damage; one open along its length is a question for the installer and the membrane maker. The EPDM repair guide covers repair materials and cover strips.
A membrane that looks pulled tight or lifts at an upstand has a different explanation. SPRA says suitable and effective restraint is always required at the roof perimeter, at the bottom of upstands and around details, so that tension in the field is not transferred to the upstand, where it could cause a tenting effect or high local stress.
Is ponding a defect or a symptom?
A symptom. LRWA lists the reasons: the original design did not include adequate falls, the structure has deflected, or the existing deck or insulation has failed, in which case the roof will need to be stripped. SPRA adds that if the membrane is damaged the interior suffers increased water ingress, and the load may cause progressive deflection of the deck. A new waterproofing finish cannot cure ponding alone, says LRWA. The NFRC's summary says BS 6229:2025 strengthens the design requirement to a minimum 1:80 finished fall for general roof areas. The ponding guide covers the design side.
How do wet insulation and ceiling stains show up?
Stains and wet insulation show where a problem becomes visible, not necessarily where it starts. SPRA says that on a warm roof, water ingress passes to the insulation, where it may be absorbed or pass down to the vapour control layer and spread to other areas. LRWA says deflection can indicate wet insulation, and that trapped moisture may come from leaks already patched or from condensation, so the first question is whether the roof is leaking or has a condensation problem. The condensation guide covers the second. If water is coming in now, see the first-day leak guide.
Which problems point to repair, overlay or replacement?
LRWA says isolated faults may need only local repairs, with extensive treatment needed where faults are widespread or leaks are difficult to trace. The Department for Communities says re-surfacing the whole roof should be considered where patch repairs will no longer be sufficient. BRE's Good Repair Guide 16 (1998) adds that most flat roof coverings need repair or replacement during the life of the building, anything from a small patch to rebuilding the whole roof structure.
| Symptom | Usual cause in the sources | Points towards |
|---|---|---|
| Small interlayer blisters, sound underlayers | Inadequate solar protection (LRWA) | Leave and re-inspect |
| Blisters from trapped water or vapour | Saturated insulation or vapour drive (LRWA) | Trace the moisture; local cut and re-bond if confined |
| Punctures and rips | Impact from people or loose debris (Department for Communities) | Local patch repair |
| Cracks along deck joist lines | Movement, wet insulation or a sagging deck | Cut back, dry and re-cover locally; deck and insulation checked |
| Widespread surface crazing | Lack of protection from the sun | Re-roofing with specialist protection |
| Weak or open seams | Welding set-up or contamination (SPRA) | Cover strip if local; installer and maker involved if along the seam |
| Persistent ponding | Falls, deflection, failed deck or insulation (LRWA) | Falls correction; stripping where deck or insulation has failed |
| Wet insulation | Leaks or condensation | Wet areas cut out and replaced, or replacement |
Whether a sound roof can take an overlay is covered in overlay versus strip-and-recover. 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
- Design guide (S1), Single Ply Roofing Association, 2020 edition, checked 22 October 2026.
- Securing seams in single ply membranes (S14), Single Ply Roofing Association, 2018, checked 22 October 2026.
- Guarantees: a guide for clients (S4), Single Ply Roofing Association, May 2017, checked 22 October 2026.
- Protection of single ply roofs (S13), Single Ply Roofing Association, July 2017, checked 22 October 2026.
- Design guide for specifiers, Liquid Roofing and Waterproofing Association, 2020, checked 22 October 2026.
- Guidance Note 1: inspection, Liquid Roofing and Waterproofing Association, issued 2010, checked 22 October 2026.
- Guidance Note 4: refurbishment, Liquid Roofing and Waterproofing Association, issued 2010, checked 22 October 2026.
- Maintenance checklist: bituminous sheet flat roofs, Department for Communities (Northern Ireland), checked 22 October 2026.
- Good Repair Guide 16: flat roofs, assessing and making repairs, BRE Group, published 6 June 1998, checked 22 October 2026.
- Sample single point guarantee (IKO products only), IKO PLC, checked 22 October 2026.
- BS 6229:2025 update, key points, National Federation of Roofing Contractors, 18 December 2025, checked 22 October 2026.
- Fragile surfaces, Health and Safety Executive, page dated 30 October 2024, checked 22 October 2026.
- Health and safety in roof work, HSG33, Health and Safety Executive, fifth edition, 2020, checked 22 October 2026.
Common questions
Does every blister on a felt roof mean it is leaking?
No. LRWA says small interlayer blisters with sound underlayers can be left, while blisters from trapped water or vapour need the moisture source traced. One manufacturer's sample guarantee lists blisters among purely aesthetic issues that it excludes, so check the wording of any guarantee.
Why do cracks keep appearing along the same line?
The Department for Communities links cracks along the line of the deck joists to thermal or moisture movement, saturated insulation or a sagging deck. Its repair starts by cutting back the felt and letting the area dry. If the cause is still there, a new patch over the crack is likely to open again.
Does the ceiling stain show where the leak is?
Not reliably. SPRA says that on a warm roof, water passes to the insulation and may spread to other areas through the vapour control layer. Finding the entry point is a separate job from reading the symptom.