--- title: "How to Replace Old or Damaged Roof Lights and GRP Sheets" url: "https://hardyroofing.co.uk/how-to-replace-old-damaged-roof-lights-grp-sheets.md" canonical: "https://hardyroofing.co.uk/how-to-replace-old-damaged-roof-lights-grp-sheets/" published: "2026-08-21" modified: "2026-08-21" author: "Hardy Roofing Team" description: "Learn what to check when replacing old or damaged roof lights and GRP sheets, including profile matching, measurements and surrounding roofing materials." tags: - "Roofing Guides" --- # How to Replace Old or Damaged Roof Lights and GRP Sheets **Replacing an old or damaged roof light starts with identifying its material, profile and relationship to the surrounding roof sheets.** A replacement GRP sheet must be compatible with the existing roofing profile and project details; a visually similar sheet may still have different dimensions and may not lap correctly. Clear photographs, careful measurements and information about the surrounding sheets give Hardy Roofing Supplies the best chance of identifying a suitable replacement. Roof lights are fragile, so inspection and replacement must also be planned safely. ## What are GRP roof-light sheets? GRP stands for glass-reinforced plastic. Profiled GRP sheets are commonly used within metal or fibre-cement roofs to allow natural daylight into agricultural, industrial, commercial and storage buildings. The roof-light profile generally needs to correspond with the surrounding opaque roofing sheets. Options and availability vary, so see our [roof lights and GRP sheets](/roof-lights-grp-sheets/) product page before requesting a quote. ## Why do older roof lights need replacing? Older GRP roof lights can become discoloured, brittle, cracked or damaged over time. Dirt and surface weathering can reduce the amount of useful daylight entering the building, while cracks, failed laps or damaged fixings may contribute to water ingress. A visible leak does not automatically mean the roof-light sheet itself is the only problem. The surrounding sheets, flashings, seals, washers and fixings should also be assessed before replacement materials are ordered. ## Safety comes before measurement Roof lights and surrounding roofing sheets can be fragile, particularly on older buildings. They may not support a person’s weight and can be difficult to distinguish from opaque roof sheets in some conditions. Do not walk on a roof light or rely on it as a working surface. Roof access, inspection and replacement should be planned by competent people using suitable access equipment, fall protection and safe working methods. Where possible, collect photographs and measurements from a safe location or ask an experienced roofing contractor to do so. ## Identify the surrounding roofing material Before looking for a replacement, establish what surrounds the roof light. It may be fitted within: - Box profile metal roofing sheets - Corrugated metal sheets - Fibre cement roofing sheets - Another proprietary or discontinued profile The material and profile affect how the roof light laps and how the assembly is fixed. If the roof uses metal sheeting, compare it with our [32/1000 box profile sheets](/32-1000-box-profile-sheeting/). If you are unsure which general shape you have, read [box profile vs corrugated roofing sheets](/box-profile-vs-corrugated-roofing-sheets/). ## Check for older fibre cement or asbestos-containing materials Older corrugated roofs may contain fibre cement or asbestos-containing materials. Do not drill, cut, break or otherwise disturb an unidentified sheet. If the material is not known, arrange for it to be assessed appropriately before work starts. Replacement planning should account for the condition of the whole roof area, not only the translucent sheet. Hardy Roofing Supplies can discuss [fibre cement sheet](/fibre-cement-sheets/) enquiries, but safe identification and removal require the appropriate competent advice. ## Why profile matching matters Roofing sheets that share a broad description can still have different profile dimensions. Two box profiles may differ in pitch, depth, crown width, valley width or side-lap detail. Corrugated profiles can also vary. If the replacement roof light does not match the surrounding sheet, the laps may not sit together correctly. This can make installation difficult and may create gaps or poorly supported areas. Accurate matching is therefore particularly important for repairs and partial replacements. ## What photographs should you take? Photographs are often the most useful starting point. Where they can be taken safely, provide: - A wide photograph showing the roof-light position and surrounding roof - A close photograph of the sheet-end profile - The side lap between the roof light and adjacent roofing sheet - The end lap, if present and safely visible - Existing fixings, washers and sealing details - Any cracks, splits, discolouration or damaged areas - Any manufacturer marks or profile references Include a ruler or tape measure in close-up photographs where practical, but keep the camera square to the measurement to reduce perspective distortion. ## What measurements are needed? The measurements required depend on the roof and profile, but useful information can include: - Overall sheet length - Cover width and overall width - Profile pitch from one matching point to the next - Profile depth - Crown and valley widths - Side-lap arrangement - End-lap length where relevant - Approximate thickness and material type, if known Our guide to [measuring replacement roofing sheets](/how-to-measure-for-replacement-roofing-sheets/) explains the main profile dimensions and the information to record. ## Can a damaged roof light be patched? Whether a local repair is appropriate depends on the material, extent and position of the damage, the overall condition of the sheet and the roof system. A temporary patch should not be treated automatically as a permanent solution. Cracked or brittle sheets may have deteriorated beyond the visibly damaged area. Ask a competent roofing professional to assess the roof and recommend a suitable repair or replacement approach. ## Should you replace one sheet or several? A single damaged roof light may be replaceable, but adjacent roof lights of a similar age may also be weathered or brittle. Their condition, daylight requirements and compatibility with available replacement profiles should be considered together. For a larger refurbishment, it may be useful to review the pattern and number of roof lights across the roof rather than replacing isolated sheets without an overall plan. Project-specific daylight, thermal and building requirements may require professional input. ## What fixings and sealing materials are needed? Replacement work may require suitable fixings, washers, fillers, sealants or lap tapes. The correct components and installation method depend on the GRP sheet, surrounding roof material, support structure and manufacturer’s details. Do not assume that old fixings should be reused or that a generic fastener is suitable. Hardy Roofing Supplies can discuss relevant [fixings and accessories](/fixings-accessories/) when the roof and sheet details are known. ## Could surrounding sheets also need replacement? Damage around a roof light may extend into adjacent roofing sheets, particularly where fixings, laps or supports have deteriorated. For older or unusual profiles, see our [replacement sheets and discontinued profiles](/replacement-roofing-sheets/) service. If metal sheets are also required, [cut-to-length roofing sheets](/cut-to-length-roofing-sheets/) may be available for suitable profiles. Provide separate dimensions and quantities for translucent and opaque sheets. ## Plan delivery and site access Profiled GRP sheets can be long and awkward to handle. Delivery depends on product availability, sheet dimensions, order quantity, location, vehicle access and unloading arrangements. Provide the delivery postcode and mention narrow roads, restricted entrances, farm tracks, limited turning space or other access constraints. Review Hardy Roofing Supplies’ [delivery information](/delivery/) when planning the order. ## What should you send for a replacement enquiry? To help Hardy Roofing Supplies investigate the requirement, send: - Clear roof and profile photographs - All available profile and sheet measurements - The number of roof-light sheets required - Details of the surrounding roofing material and profile - Known manufacturer names or references - Information about damage, leaks or discolouration - Required fixings or accessories - The delivery postcode and access information If measurements are uncertain, send photographs and drawings rather than guessing. The team can then explain what further information may be needed before quoting. ## Need help replacing a roof light or GRP sheet? Send Hardy Roofing Supplies your photographs, measurements, quantities and delivery postcode. The team can help investigate matching profile options and prepare a practical quotation, subject to product availability and confirmation of the project details. [Contact Hardy Roofing Supplies](/contact/) with your replacement enquiry, or return to the [Learning Centre](/learning-centre/) for more roofing and cladding guides.