Metal roof fixings and lap seals: why industrial roofs leak
Updated 23 October 2026 · SEO Dons Editorial
Water gets into a through-fixed metal roof mainly at its holes and its joints: the fasteners, and the side and end laps. Both depend on small parts working together, a sealing washer under the right compression, a butyl strip squeezed by enough fixings, a stitching fastener that pulls a lap tight. When one fails the leak often shows somewhere else. This guide covers how the two failure points show up, how a roof is surveyed for them, and when repair gives way to over-sheeting or re-sheeting.
Where does water get in?
LRWA's guidance on profiled metal roofs, issued in February 2014, says fixings are probably the main reason for water ingress in this type of roof. MCRMA's durability guidance quotes BS 5427, the code of practice for profiled sheet cladding, as noting that deterioration can be more pronounced at fixings, laps, edges and abrupt changes in profile. MCRMA's glossary separates the two jobs: a primary fastener makes the structural connection, while a secondary fastener secures joints between components and is usually part of weather sealing a joint, as side lap stitchers are.
Where the water appears is another matter. LRWA notes that even on a twin skin structure the leaks can end up showing at the gutter area, so a stain or drip is a starting point for tracing the cause, not a map of it. The gutter lining guide covers that end of the problem.
Why do washers and fasteners fail?
- Perished sealing washers. LRWA says the sealing washers deteriorate and perish over the years, leaving loose fixings that let water in. MCRMA's fastener guide, GD33, says ethylene-propylene-diene-monomer (EPDM) is generally recognised as giving the best all-round performance for the sealing element.
- Wrong driving depth. GD33 says a fastener must not be under driven, which can leave a gap that prevents the washer sealing, and must not be overdriven, which can strip the fastener in its support or damage the material being clamped. Washer compression is the visible check.
- Corrosion. GD33 says unprotected carbon steel corrodes in the atmosphere and that the coatings on carbon steel fasteners inevitably take some damage during installation, which reduces their durability. It places coated carbon steel in low-risk environments where the required functional life does not exceed approximately 25 years, and says suitable stainless steel grades can give enhanced durability.
- Too few fixings. LRWA says that if fixing spacings are not adequate, the sheets flex when people access the roof, which makes it extremely difficult to detail over the fixings and laps.
- Movement. GD33 says materials with a high coefficient of thermal expansion may need special provisions at fixings: aluminium expands at twice the rate of steel, and PVC and polycarbonate need predrilled oversize holes.
Why do side and end laps leak?
A lap seal is a butyl strip held under compression, not a bead of sealant on the surface. MCRMA's end lap guidance, GD19, says inadequately sized or positioned sealant, or insufficient or incorrectly positioned fixings, will result in poor sealing. Too few or badly placed fixings commonly leave inadequate pressure on the sealant, most noticeably at the corners of the trough. It finds that crown fixing performs poorly at end laps and that compression falls as the sealant moves away from the fixing.
For side laps, MCRMA's archived profiled metal roofing design guide (Technical Paper 6, June 2004, kept on its website for reference only) says trapezoidal sheets should be stitched through the top of the side lap at typically 450 mm centres with sealed stitcher screws or rivets. GD33 adds that stitchers must be purpose-designed to clamp and compress the sealant without stripping the thread. LRWA says a properly built pitched roof has 150 mm minimum end laps sealed with two butyl strips, and that not every roof was built that way, which a survey should establish.
Pitch matters as well. Technical Paper 6 notes that the risk of leakage decreases as the pitch increases, and MCRMA's over-roofing guidance, following BS 5427, says roofs over 4 degrees may be through fixed while shallower ones need a secret-fixed or standing seam system. A shallow roof with through-fixed laps may leak because of its design as much as its age.
How is a roof surveyed for fixing and lap failure?
LRWA's guidance calls for a full roof survey before any treatment. For these two failure points it looks at:
| Check | What the survey looks for |
|---|---|
| Washers and fastener heads | Perished or cracked washers, loose fixings, rust at the fixing, missing caps |
| Fixing number and spacing | Whether the pattern matches the manufacturer's details; sheets that flex |
| Side laps | Sealed stitchers present at the specified spacing, and laps that have opened |
| End laps | Sealant and fixing arrangement against the details above |
| Rooflight laps | Gaps between rooflight and metal sheet, which GD19 says can be larger and more variable |
Access is a safety question before it is a survey question. HSE says all roofs should be treated as fragile until a competent person has confirmed they are not, lists corroded metal sheets and liner panels among the surfaces likely to be fragile, and advises working from a platform beneath the roof where possible. The ACR's Green Book says even a simple inspection of a fragile roof is highly dangerous without adequate knowledge and planning, and LRWA warns against walking along lines of fixings. MCRMA publishes an inspection checklist for metal-based roofing and cladding systems (GD23).
What are the repair options?
| Finding | Repair route | Points to check |
|---|---|---|
| A few failed fasteners or washers | Replace the fasteners individually | Choose on the manufacturer's advice, because a visual check of the thread may not show the material thickness a fastener suits; stainless where corrosion risk is high |
| Washers perished or fixings rusting across large areas | Systematic re-fixing, or a coating system that treats each fixing | LRWA describes wire-brushing each fixing and bedding a reinforcement in the coating; ask what the system certificate says about fixings |
| Side laps opened or stitchers missing | Re-stitch with sealed stitchers and re-seal | Stitchers must be purpose-designed; check the rest of the lap against the details above |
| End laps leaking | Restore the sealant and fixing arrangement, or treat with a coating system that allows for movement | Sealant works under compression from the fixings; LRWA says some specifications need bond breaks at end laps |
| Shallow-pitch roof leaking repeatedly | Compare repair with a roof method suited to the pitch | Through-fixed laps have pitch limits, as above |
Any route that seals the outside more tightly can change how the roof handles condensation. LRWA warns that a seamless waterproofing skin over the existing structure may exacerbate condensation problems, so ventilation may have to be introduced. The metal roof condensation guide covers that side. Insulated panel roofs have their own lap and fixing details, covered in the composite panels guide. Rust along cut edges is a separate failure point (see cut edge corrosion), and rooflight laps are covered under rooflight replacement.
When do repairs stop making sense?
Repair suits discrete failures. It stops suiting a roof when leaks recur at many fixings or laps, when the fasteners are near the end of their functional life, when sheets are corroded around the fixings, or when the pitch is wrong for the lap details. The comparison then is over-sheeting or re-sheeting. MCRMA's over-roofing guidance says an over-roof should come with drawings showing fastener and sealant types and positioning, calculations covering wind loading and fastener suitability, and an agreed and documented life expectancy, with an additional design life of at least 25 years likely. The over-sheeting guide explains the method and its limits, and the industrial roof refurbishment page places fixings and laps among the five places an industrial roof fails.
Whichever route, read the warranty terms before relying on them. MCRMA lists conditions that some manufacturer and insurance-backed warranties carry, including formal documented inspections within strict time frames, annual maintenance programmes and prompt written notification of damage. If you want contractors to price a refix or lap treatment against a survey, you can send an enquiry through the site's form.
Sources
- MCRMA guidance documents GD33 fasteners (December 2019, updated May 2020), GD19 end laps (updated September 2019), GD38 over-roofing (January 2022) and GD02 advice on industry warranties (November 2012) (MCRMA, checked 22 October 2026)
- MCRMA Technical Paper 6, profiled metal roofing design guide (June 2004, archived), GD39 section 4, glossary and guidance list (MCRMA, checked 22 October 2026)
- LRWA, Guidance Note 10, waterproofing of metal profiled sheet roofing and cladding (LRWA, issued February 2014, checked 22 October 2026)
- HSE, fragile surfaces and roof work (HSE, checked 22 October 2026)
- ACR, the Green Book (Advisory Committee for Roofsafety, checked 22 October 2026)
- BSI Knowledge, BS 5427:2016+A1:2017 (BSI, checked 22 October 2026)
Common questions
Can extra sealant fix a leaking lap?
Not on its own. MCRMA's testing found that sealing depends on the sealant being the right size and close enough to the fixings to be compressed, and that too few or badly placed fixings leave inadequate pressure on the sealant. A survey should find out why the lap is open before more sealant is added.
Are stainless steel fasteners worth specifying?
GD33 says suitable stainless grades can give enhanced durability and corrosion resistance over coated carbon steel, and that a stainless steel fastener must be used with aluminium sheet to avoid galvanic corrosion. Environment and substrate matter, so the guide says to consult the fastener manufacturer.
Do rooflight laps need different sealing?
Yes. MCRMA's end lap guidance says rooflights are more flexible than metal panels and often leave larger gaps, and oversized sealant runs were seen to add to rooflight deflection and open the bottom of the lap. The detail should follow the rooflight maker's instructions.