What are the main garage roof replacement options?
For many garages, the practical comparison is between a GRP fibreglass system and a profiled metal roof, such as box-profile steel. GRP creates a continuous laminated roof finish; profiled metal uses formed sheets, laps and fixings. Choose only after the supporting structure, roof falls or pitch, drainage, edges and junctions have been assessed. If the existing roof may contain asbestos, its work category must be established before replacement work begins.
GRP, box-profile and metal are not three identical choices
GRP means glass-reinforced plastic. In garage-roof use, it is a fibreglass roof system formed as a continuous laminate over a suitable roof deck. Box-profile is different: it is a type of formed metal sheet, recognisable by its angular ribs. Other metal systems can use a different profile, coating or joint arrangement, so “metal roof” is a broad category rather than one specification. Cure It, for example, describes a GRP flat-roofing system with product specifications, while the NFRC publishes guidance on profiled sheet roof and wall cladding. Cure It: GRP roofing system NFRC: Guide to BS 5427
This distinction matters because the roof has to work as an assembly. A smooth GRP finish and a ribbed sheet finish ask different questions about the deck or support, falls or pitch, edge details, gutters and junctions. The table is a way to frame a conversation with the replacement installer; it is not a substitute for the manufacturer’s system specification or an assessment of the garage structure.
- Use “GRP” when you mean the fibreglass roof system, not a generic plastic sheet.
- Use “box-profile” when you mean a particular ribbed metal-sheet form.
- Ask what complete roof build-up is proposed, including supports, weathering details and water run-off–not simply which surface material is named.
| Decision point | GRP fibreglass system | Box-profile metal sheet | Other metal roof design |
|---|---|---|---|
| Basic form | A continuous glass-reinforced laminate formed over a roof deck. | A formed metal sheet with angular ribs; laps and fixings form part of the roof assembly. | A metal system with a different profile or joint design, specified for the particular roof. |
| Visible finish | A smooth, continuous finish. | A ribbed sheet finish with visible fixings or laps depending on the system. | Depends on the profile, coating and joint arrangement selected. |
| Key design check | Whether the existing or proposed deck and details suit the specified system. | Whether the supports, pitch or falls, sheet layout and drainage details suit the system. | Whether the manufacturer’s specification matches the roof geometry, supports and details. |
| Detailing to ask about | Edges, corners, penetrations and the junction with the gutter or wall. | Sheet ends, overlaps, fixings, flashings and the route for water run-off. | The system’s stated junction, flashing and drainage details. |
| Useful when | A continuous laminated finish is being considered for the proposed roof form. | A ribbed sheet roof is being considered for the proposed roof form. | The appearance or technical arrangement calls for a different metal profile or joint type. |
Compare systems, not slogans
A material label does not say whether the garage structure, drainage and junction details are suitable. Ask for the proposed roof build-up and the system specification before treating two options as like for like.
When a GRP fibreglass roof may suit the replacement brief
A GRP roof is a system, not just a top layer. The selection question is whether the proposed deck, edges, outlets and adjoining surfaces can be detailed as the system requires. A smooth, continuous appearance can be a reason to consider GRP, but it does not remove the need to establish sound support and water run-off. The manufacturer’s instructions for the particular system should govern the final design and installation.
Start by asking whether the garage roof is being retained in broadly the same form or whether the work changes its structure, weight or fire performance. Planning Portal guidance says that work to an existing roof may require building-control approval where there are structural alterations, a significantly different fire-performance covering, or a new covering that increases the roof-covering weight by 15% or more. The guidance is for England and local requirements can also apply. Planning Portal: Building Regulations–Work to an existing roof
- Ask whether a suitable deck is included in the proposed system, rather than assuming the existing surface is suitable.
- Ask how roof edges, wall junctions, outlets and gutter connections are to be detailed.
- If the roof form or structure is changing, check the building-control and planning position before work starts.
The replacement decision comes after the roof assessment
A new surface cannot compensate for an unassessed supporting structure or unresolved drainage detail. The replacement specification should state what has been assessed and what, if anything, needs alteration.
When box-profile or another metal roof may suit the replacement brief
Profiled metal roofs use formed sheets. Box-profile is one common profile; corrugated, standing-seam and other forms are separate designs with their own support and joint requirements. This makes it more useful to ask about the proposed sheet profile, coating, length, overlaps or jointing and flashings than to ask whether “metal” is better in the abstract. The NFRC’s guide to BS 5427 is specifically concerned with profiled sheet roof and wall cladding. NFRC: Guide to BS 5427
The important design issue is how water will leave the roof and how each sheet meets its neighbour, the perimeter and any wall. These are specification questions for the selected product and garage structure. Do not choose a replacement by matching the look of an old corrugated roof alone; the existing roof material and geometry do not confirm that a new system is suitable without an assessment.
- Confirm the proposed profile and whether it is box-profile or another metal design.
- Ask what supports, falls or pitch and drainage arrangement the specification assumes.
- Ask how flashings, sheet ends, laps or joints, fixings and gutter interfaces are included in the scope.
Box-profile is a metal option
A comparison headed “GRP versus box-profile versus metal” can be misleading if it treats box-profile and metal as unrelated materials. The more accurate comparison is GRP against a specified metal system, with box-profile as one metal-sheet design.
Questions that narrow the right replacement option
A useful replacement brief records the conditions that control the choice. It should identify the roof form, whether the garage is detached or attached, the available support, how rainwater is carried away, any wall or boundary junctions and the intended use of the garage. Those facts help a roofing professional decide whether a continuous GRP system or a specified metal-sheet system is technically appropriate.
For ordinary repair, replacement and maintenance work, formal building-regulations approval is often not needed, but GOV.UK advises checking with a building-control body if you are unsure and says safety and energy-efficiency standards still apply. Structural alteration or a change that falls outside the applicable exemptions should be checked before work proceeds. GOV.UK: When you do not need building regulations approval
- What is the present roof form, and will the new roof retain it or alter the structure?
- What supports and roof deck or purlins are present, and what can they safely carry?
- Where will water run, and how will gutters, outlets, flashings and wall junctions be resolved?
- Does the proposal change the roof’s weight, structure or fire performance?
- What manufacturer specification and written scope describe the complete new roof?
Do not make the material choice before the asbestos route is clear
Where an existing roof may contain asbestos, the assessment of the existing material, its condition and the planned work method comes first. Visual appearance cannot confirm asbestos; if a sample is needed, it should be analysed through the appropriate route rather than taken by the property owner. UKAS accredits laboratories for asbestos bulk-material identification to ISO/IEC 17025. UKAS: Laboratory Accreditation | Asbestos testing
How the asbestos route affects a garage roof replacement
Replacement and asbestos work are linked, but they are not the same decision. Under Regulation 5 of the Control of Asbestos Regulations 2012, an employer must assess whether asbestos is present, what type of material it is and its condition before work liable to expose employees to asbestos begins; where there is doubt, the employer must assume asbestos is present and follow the applicable regulations. Control of Asbestos Regulations 2012, regulation 5
HSE identifies carefully handled asbestos-cement roof sheeting as an example that may fall within non-licensed work, but it also identifies roof sheets that will be substantially damaged or broken up as notifiable non-licensed work. Higher-risk work has a different route. The category is determined by risk, material, condition and method, not by the fact that a new GRP or metal roof has been selected. HSE: Deciding if the work must be done by a licensed contractor
- Use the garage roof removal service for the existing-roof assessment and removal route, rather than treating this options guide as removal instructions.
- For wider roof and cladding replacement context, see asbestos roof removal and replacement.
- The asbestos removal services overview explains the wider service scope and helps keep garage-roof choices separate from other asbestos work.
Diamond’s scope decision
Diamond Asbestos Services handles non-licensed asbestos work. If assessment shows that the work is licensable, it is referred to an HSE-licensed asbestos contractor. This boundary means a replacement option is discussed only after the correct asbestos-work route has been considered.
Related garage roof decisions
These linked pages keep the next decisions distinct: identifying a suspect material, establishing the asbestos-work route, arranging replacement and requesting an enquiry. They are useful follow-ons, not a substitute for a site-specific assessment.
- How to tell if a garage roof may contain asbestos explains why visible clues call for caution rather than confirmation.
- Garage roof asbestos removal and replacement covers the service route for the existing roof.
- Asbestos roof removal and replacement provides the broader roof and cladding replacement context.
- Asbestos cement sheeting removal is relevant where corrugated asbestos-cement sheets form part of the question.
- Request a free quote when you can describe the existing roof, the intended replacement and any known access or structural issues.
Keep the specification focused
The best enquiry describes what is known about the existing roof and what the new roof needs to do. It should not assume that an old roof’s appearance proves its material or that one replacement system fits every garage.
Common questions
Frequently asked questions
Box-profile is a type of metal roof sheet, rather than a separate material category. Its angular ribs distinguish it from corrugated or other metal designs. The useful comparison is therefore GRP against a specified metal-sheet system, then between metal profiles where the appearance, supports, pitch or falls and detailing make that relevant.
Sources and further reading
- 1. Health and Safety Executive. Non-licensed work with asbestos Accessed 24 September 2026.
- 2. Health and Safety Executive. Deciding if the work must be done by a licensed contractor Accessed 24 September 2026.
- 3. legislation.gov.uk. The Control of Asbestos Regulations 2012, regulation 5: Identification of the presence of asbestos Accessed 24 September 2026.
- 4. legislation.gov.uk. The Control of Asbestos Regulations 2012, regulation 8: Licensing of work with asbestos Accessed 24 September 2026.
- 5. United Kingdom Accreditation Service. Laboratory Accreditation | Asbestos testing Accessed 24 September 2026.
- 6. Planning Portal. Building Regulations: Work to an existing roof Accessed 24 September 2026.
- 7. GOV.UK. When you do not need building regulations approval Accessed 24 September 2026.
- 8. Cure It Composites. Cure It GRP flat roofing system Accessed 24 September 2026.
- 9. National Federation of Roofing Contractors. NFRC guide to BS 5427: Use of profiled sheet for roof and wall cladding on buildings Accessed 24 September 2026.