If an existing flat roof is still structurally sound, an overlay system can create a new waterproof surface without the cost, waste and disruption of stripping off the entire covering. The difficult part is choosing the right system for the existing membrane, the roof details and the conditions in which it will be installed. HPX, Lava 20, Cure It ONE and Flexitec are all cold-applied waterproofing systems, but they are not interchangeable. HPX and Lava 20 form flexible liquid membranes; Cure It ONE and Flexitec are glass-reinforced systems that require fibreglass matting across the main roof area. Quick answer: choose HPX for the simplest ready-to-use application on a compatible, correctly prepared surface; Lava 20 for a highly flexible polyurethane system, including approved EPDM, TPO and PVC overlays with the specified primer; Cure It ONE for a fast, glass-reinforced GRP roof completed without a separate topcoat; or Flexitec for a flexible reinforced system with one- and two-coat specifications. The condition of the roof comes first. Do not overlay wet insulation, rotten decking, loose membranes or widespread blisters. A new coating can hide a defective roof build-up, but it cannot repair it. By the Apex Fibreglass Roofing Supplies editorial team. Last technical review: 23 September 2026. Prices, availability and product specifications can change; check the current product page and manufacturer instructions before ordering. Contents Can your existing roof be overlaid? The four systems at a glance Which system should you choose? HPX Hybrid Polymer Lava 20 liquid polyurethane Cure It ONE Flexitec 2020 Which system suits each substrate? Liquid membrane or reinforced GRP? Preparation, weather and roof details Building Regulations in England Current Apex guide prices Buying checklist Frequently asked questions Can Your Existing Roof Be Overlaid? An overlay is normally worth considering when the deck is firm, the insulation is dry, the existing waterproofing is securely bonded and the roof can be cleaned and prepared to the chosen system's requirements. Before selecting a product, inspect: The structural deck for softness, rot, corrosion, movement or deflection The insulation and roof build-up for trapped moisture The existing membrane for blisters, splits, delamination and loose laps Outlets, gutters and falls for blocked drainage or persistent ponding Upstands, wall junctions, kerbs, pipes and other penetrations Previous coatings, oils, moss and contaminants that could affect adhesion Whether the existing covering is positively identified and listed as a suitable substrate A small, straightforward garage roof may only need a careful visual inspection and adhesion test. Larger, older or repeatedly leaking roofs may require a moisture survey and core samples. The Liquid Roofing and Waterproofing Association design guide explains why the existing deck, insulation and waterproofing should be assessed as a complete build-up. When a strip-off is usually the safer option Do not treat an overlay as a shortcut when there is: Widespread wet insulation or trapped moisture Rotten, weak or distorted timber decking A loose or badly delaminated existing membrane Extensive blistering rather than isolated local defects Structural movement that has not been diagnosed Serious ponding caused by failed falls or deck deflection An unknown or contaminated coating that cannot be prepared reliably If the roof is leaking, find the cause before coating it. Water may be entering at an upstand or outlet and travelling within the roof build-up before it appears indoors. The Four Systems at a Glance System Technology Typical system build-up Preparation and reinforcement Best starting point for Current Apex price* HPX Hybrid Polymer Ready-to-use hybrid polymer liquid membrane One coat for the listed 10-year specification; reinforced two-stage build-up for the listed 20-year option No mixing; primer-free on several listed substrates, subject to compatibility and preparation Simple application without resin and catalyst mixing £166.80 per 14 kg tin Lava 20 Single-component polyurethane liquid membrane One or more coats according to substrate and required specification Substrate-specific primer; reinforce cracks, joints, outlets and other details Complex detailing and approved single-ply overlays From £130 Cure It ONE Glass-reinforced GRP system One reinforced coat with the finish incorporated; no separate topcoat Catalysed resin and fibreglass reinforcement; primer where specified A hard-wearing GRP finish completed in one installation session From £524.40 Flexitec 2020 — one coat / two coat Flexible, glass-reinforced liquid roofing system One-coat overlay or two-coat 20-year specification Primer, hardener, 225 gsm matting and detail bandage as specified A reinforced finish with more movement tolerance than traditional rigid GRP From £316.80 / £338.40 Displayed prices checked on 23 September 2026. Starting prices are not like-for-like because pack sizes and included components differ. Confirm the selected variant, availability, delivery charge and complete system build-up before ordering. HPX: ready-to-use hybrid polymer membrane. Lava 20: flexible polyurethane waterproofing. Cure It ONE: reinforced GRP without a separate topcoat. Flexitec: flexible, glass-reinforced liquid roofing. Which System Should You Choose? Your priority or existing roof Best system to investigate first Why Simplest product preparation, with no catalyst to measure HPX Ready to use from the tin and listed as primer-free on several common substrates Approved overlay over EPDM, TPO or PVC single-ply Lava 20 The manufacturer publishes dedicated preparation and primer routes for these membranes Numerous pipes, outlets, laps and awkward details Lava 20 or HPX Flexible liquid membranes are generally easier to work around complex geometry Fast, reinforced GRP installation without a separate topcoat Cure It ONE The colour and finish are incorporated into the single-coat reinforced system Flexible, reinforced overlay with one- and two-coat options Flexitec Combines fibreglass reinforcement with a more flexible resin system New board or concrete where Flexitec is preferred Flexitec two-coat system Apex states that new board and concrete require the 20-year, two-coat kit Regular foot traffic, a balcony or terrace A specifically designed trafficked-roof system A basic waterproofing membrane is not automatically suitable for regular pedestrian use This table is a shortlist, not a final specification. The correct primer, reinforcement, coverage and number of coats still depend on the substrate and the current application manual. HPX Hybrid Polymer TriRoof HPX is a ready-to-use hybrid polymer membrane. It requires no separate hardener and Apex lists it for common substrates including asphalt, rubber, concrete and existing GRP. It is self-levelling, solvent-free and non-flammable. Apex currently states that one 14 kg tin covers approximately 9.33–12 m², depending on the substrate. Treat that as a product-page guide rather than a universal coverage figure: rough surfaces, repairs, reinforcement and the selected guarantee build-up can increase consumption. The important distinction is between the two advertised specifications: 10-year option: local repairs and bandaging followed by the specified coat of HPX 20-year option: a reinforced system with additional HPX and 225 gsm fibreglass matting, followed by a further coat That means HPX should not be described simply as a “one-coat, 20-year system”. The longer option is a different build-up. Best for: buyers who want to avoid catalyst mixing and need a practical liquid overlay on a confirmed compatible surface. Watch-outs: “no primer on most substrates” does not mean “no preparation”. The roof must still be cleaned, repaired and tested where adhesion is uncertain. Check the latest TriRoof HPX technical data and guarantee method before work begins. View HPX at Apex → Lava 20 Liquid Polyurethane OWL Lava 20 is a single-component polyurethane membrane that forms a seamless, elastic waterproof layer. It is well suited to roofs with multiple junctions, cracks, pipes and outlets because the details can be reinforced within the liquid system. Lava 20 is also the clearest starting point of these four for compatible single-ply overlays, but the preparation route matters: TPO and EPDM require the specified Lava 20 EPDM & TPO Primer PVC requires the specified cleaner and PVC primer Bituminous and asphalt surfaces require the primer stated in the current system guide Absorbent surfaces may require a different primer and material allowance Structural joints, outlets, pipes, cracks and changes of plane require reinforcement as detailed by the manufacturer The cured membrane is promoted as retaining flexibility down to approximately −40°C. That is an in-service property, not the minimum application temperature. The current technical guide gives ideal application and curing conditions of 5–35°C for the cited system instructions; always check the exact product, substrate temperature, humidity and forecast. Best for: complex details, roofs where a highly flexible finish is wanted, and approved EPDM, TPO or PVC overlays prepared with the correct products. Watch-outs: Lava 20 is not a direct-to-everything coating. Primer choice, reinforcement and the specified consumption are part of the system. See the OWL Lava 20 technical documentation before pricing the job. View Lava 20 at Apex → Cure It ONE Cure It ONE is a glass-reinforced flat roofing system designed to complete the waterproofing and finish in one installation session. Its anthracite colour is incorporated into the system, so it does not need a separate topcoat after the laminate has cured. “Single coat” does not mean paint-like application. The resin must still be mixed correctly with catalyst, used within its working time and consolidated through the specified fibreglass reinforcement. Trims, corners, joints and penetrations also need the correct Cure It detailing method. Apex lists the system for existing felt, GRP, OSB3 and approved PIR applications, with suitable primer required for concrete and asphalt. Cure It states that the system carries a 20-year materials guarantee when its conditions are met. Best for: contractors and experienced installers who want a firm, reinforced GRP finish without a separate topcoat stage. Watch-outs: the result depends on accurate catalyst addition, full wet-out of the matting, removal of trapped air and careful timing. Check the latest Cure It ONE product and installation guidance before installation. View Cure It ONE kits at Apex → Flexitec 2020 Flexitec 2020 is a flexible, single-resin liquid roofing system reinforced with fibreglass matting. It combines a tough GRP-style finish with more movement tolerance than a conventional rigid fibreglass roof. Apex offers two distinct kit specifications: One-coat Flexitec overlay kit: resin is applied below and above the 225 gsm matting as one reinforced application Two-coat Flexitec system: the reinforced first coat is followed by a second coat; Apex specifies this build-up for new board and concrete The kits include the relevant quantities of primer, resin, hardener, 225 gsm matting and taping mat. The exact primer still depends on the surface. There is an important guarantee distinction. Apex markets the one-coat kit as a 10-year system and the two-coat kit as a 20-year system. Restec's current guarantee-registration information says its manufacturer materials guarantee is available for the two-coat, 20-year application and requires the installation conditions to be met. Confirm in writing what the stated 10-year protection covers before presenting it as a manufacturer-backed guarantee. Best for: buyers who want a reinforced finish with more flexibility, broad overlay capability and a choice of system build-up. Watch-outs: Flexitec still requires correct hardener dosage, matting, detailing and weather control. Read the current Flexitec application manual, not only the kit title. View one-coat Flexitec kits →View two-coat Flexitec kits → Which System Suits Each Substrate? Existing surface HPX Lava 20 Cure It ONE Flexitec Felt or bituminous membrane Listed for common felt/asphalt surfaces; clean, repair and confirm adhesion Suitable with the primer and preparation stated for bituminous surfaces Listed by Apex; follow the Cure It preparation and primer guidance Listed substrate; use the specified primer and system build-up Asphalt Listed; allow for substrate-dependent consumption Suitable with the specified primer Suitable with the specified primer Listed; confirm primer and whether the two-coat build-up is required Existing GRP Listed on a sound, correctly prepared surface Possible with the specified preparation and primer route Listed for overlay Listed for overlay; abrade, clean and prime as instructed Concrete Listed, subject to condition and preparation Suitable; primer need depends on substrate condition and the current guide Concrete primer required Use the specified primer; Apex directs buyers to the two-coat kit OSB3 or suitable timber deck Use only to the approved new-deck specification Listed subject to preparation, primer and coverage OSB3 is listed New board requires the two-coat system according to Apex EPDM or TPO Obtain written confirmation for the exact membrane and build-up Dedicated EPDM/TPO primer route is published Not the first choice without written manufacturer approval Follow the current manual and obtain confirmation for the specific membrane PVC single-ply Obtain written confirmation for the exact membrane and build-up Cleaner and PVC primer route is published Not the first choice without written manufacturer approval PVC single-ply is listed in current Flexitec literature; follow the precise preparation method Compatibility is roof-specific. Age, plasticisers, old repairs, surface coatings, contamination and the way the membrane is fixed can all change the answer. Carry out a test patch whenever the manufacturer or roof condition makes adhesion uncertain. Liquid Membrane or Reinforced GRP? The most useful dividing line is not simply “liquid versus GRP”, because all four products are liquid-applied. The practical difference is whether fibreglass reinforcement is installed across the whole roof area. HPX and Lava 20: flexible liquid membranes Faster around irregular details and penetrations No field matting across the main roof in their basic liquid specifications Good starting point where roof movement and complex geometry matter Local reinforcement, primers or additional coats may still be required Cure It ONE and Flexitec: glass-reinforced systems Fibreglass matting forms part of the main waterproofing layer Produce a tougher, more GRP-like finish Require resin or hardener control, full wet-out and consolidation of the matting Better suited to installers familiar with reinforced liquid roofing If ease of use is the main priority, start with HPX. If single-ply compatibility or intricate details drive the decision, start with Lava 20. If the desired finish is a reinforced GRP shell, compare Cure It ONE with Flexitec. Preparation, Weather and Roof Details Product choice cannot compensate for weak preparation. Allow time and materials for: Cleaning and fully removing moss, dust, oils and loose coatings Cutting out and repairing unsound areas rather than bridging over them Drying the roof where the system requires a dry substrate Priming porous, bituminous, metallic or single-ply surfaces where specified Reinforcing cracks, laps, joints, outlets, pipes, kerbs and wall junctions Checking dew point, substrate temperature, humidity and the rain window Keeping within the product's pot life or working time Measuring vertical surfaces and details, not only the flat plan area An overlay membrane will follow the existing roof shape. It will not create adequate falls or correct a sagging deck. Resolve blocked outlets, low spots and persistent ponding before waterproofing. Is any of it suitable for DIY installation? HPX has the simplest product preparation because it is used straight from the tin. Lava 20 is also supplied as a single-component membrane, but correct primer selection, reinforcement and material loading still demand care. Cure It ONE and Flexitec require greater installation skill because the resin, hardener or catalyst and fibreglass matting must be handled correctly within a limited working time. For a house roof, a complicated layout or any installation tied to a registered guarantee, use a competent installer. Cold-applied means no naked flame; it does not mean risk-free. Follow the safety data sheets, wear the specified PPE and keep the working area ventilated. Can you walk on the finished roof? Occasional access for maintenance is not the same as regular pedestrian use. A balcony, terrace, walkway or service route needs a system specifically designed for traffic, including the required wearing layer, slip resistance and detailing. Do not rely on a generic “walkable” description. Building Regulations in England Re-covering a flat roof can be building work even when the existing membrane is left in place. According to the Planning Portal guidance for work to an existing roof, a building control application is not normally required only when both less than 25% of the total building envelope and less than 50% of the roof are affected. Approval may still be required where the work: Alters the roof structure Significantly increases the loading Changes the roof covering's fire performance Renovates enough of a thermal element to trigger insulation requirements Forms part of work to a higher-risk or otherwise specially controlled building The waterproofing membrane is only one part of compliance. Structure, fire performance, drainage and thermal performance may all need consideration. Check the current Approved Document L guidance, speak to the local Building Control body and obtain project-specific advice before starting substantial work. Planning permission and Building Regulations are separate. Like-for-like repair is often permitted development, but listed buildings, Article 4 directions, material external changes and other site constraints can alter the position. Current Apex Guide Prices System Displayed starting price What the figure represents Main extras to check Lava 20 £130 Entry repair-kit variant; the listing spans much larger roof areas Correct primer, reinforcement, additional coats and tools HPX, 14 kg £166.80 One tin; Apex lists approximately 9.33–12 m² depending on substrate Detail bandage and the extra materials required for the 20-year build-up Flexitec one-coat kit From £316.80 Reinforced overlay kit, size dependent Substrate-specific primer, tools and the exact protection offered Flexitec two-coat kit From £338.40 Two-coat kit, size dependent Additional detail material, installer and guarantee requirements Cure It ONE kit From £524.40 Kit price varies by roof area Substrate primer, trims, tools and catalyst choice Prices shown by Apex on 23 September 2026. Product pages, stock, variants and promotions can change without notice. The cheapest starting price is not necessarily the cheapest completed roof. Compare systems using: True material cost per m² = total cost of every required system component ÷ actual roof area covered Include primer, reinforcement, detail materials, extra coats, catalyst or hardener, tools, delivery and an allowance for rough surfaces and vertical details. Do not compare a small repair kit with a complete larger-area kit as if they cover the same roof. Buying Checklist Before ordering, record these nine points: Existing membrane: felt, asphalt, GRP, concrete, EPDM, TPO, PVC or something else? Condition: are the deck, insulation and existing covering sound, dry and secure? Area: what is the plan area, plus upstands, kerbs and other vertical details? Complexity: how many outlets, pipes, rooflights, wall junctions, corners and repairs are there? Drainage: are the outlets clear and does water drain without persistent ponding? Use: maintenance access only, regular foot traffic, balcony or terrace? Weather: what are the forecast, substrate temperature, humidity and dew-point margin? System build-up: which primer, reinforcement, number of coats and coverage rate are required? Guarantee: is it a product description, expected service life or registered manufacturer guarantee, and who must install it? If the membrane cannot be identified or the deck feels soft, stop before ordering and obtain a roof assessment. Frequently Asked Questions Can You Overlay a Flat Roof Instead of Replacing It? Yes, provided the existing roof build-up is dry, stable, securely bonded and compatible with the new system. Wet insulation, rotten decking, loose membranes and widespread blistering normally need to be removed or repaired first. Which Flat Roof Overlay System Is Easiest to Apply? HPX has the simplest product preparation because it is ready to use and requires no catalyst mixing. The roof still needs inspection, cleaning, repairs, detail work and any adhesion checks specified for the substrate. Is Lava 20 Better Than HPX? Not universally. Lava 20 is the stronger starting point where a published primer route is needed for EPDM, TPO or PVC, or where there are many complex details. HPX is attractive where speed and simple, no-mix application on a compatible substrate matter most. Is Cure It ONE Better Than Flexitec? Cure It ONE is a good choice when you want a reinforced GRP roof completed without a separate topcoat. Flexitec is the better starting point when you want a more flexible reinforced system and a choice between one- and two-coat specifications. Substrate, installer experience and guarantee requirements should decide between them. What Does “One Coat” Mean for a GRP System? It does not mean applying ordinary paint. Cure It ONE and the one-coat Flexitec build-up still use fibreglass reinforcement that must be fully wetted and consolidated. “One coat” describes the system build-up or installation sequence, not the absence of reinforcement. Which System Is Best for Cold Weather? Use the manufacturer's permitted application conditions, not the lowest temperature the cured membrane can withstand. Lava 20's quoted flexibility down to −40°C is an in-service property. Flexitec, Cure It ONE and other catalysed systems may use season-specific hardeners or accelerators. Check air temperature, substrate temperature, humidity, dew point and cure time for the exact product. Can These Systems Go Over EPDM, TPO or PVC? Lava 20 publishes dedicated preparation routes for EPDM/TPO and PVC. Flexitec literature lists PVC single-ply among possible surfaces, subject to its current preparation method. For HPX or Cure It ONE, obtain written confirmation for the exact membrane rather than relying on a broad “rubber roof” claim. Do I Need Building Regulations Approval in England? Possibly. The affected area, structural changes, roof loading, fire performance and thermal renovation all matter. Planning Portal guidance says a building control application is not normally needed only where less than 25% of the total building envelope and less than 50% of the roof are affected, with no other trigger. Confirm the project with the local Building Control body. Does a 20-Year System Automatically Include a 20-Year Guarantee? No. A product's expected service life, a kit description and a registered manufacturer guarantee are different. The guarantee may depend on the full system build-up, substrate, installer training, batch records, registration and maintenance. Read the current terms before promising coverage to a customer. The Verdict There is no single winner for every existing flat roof: Choose HPX when simple, ready-to-use application is the main priority and the roof surface is confirmed compatible. Choose Lava 20 when flexibility, intricate details or an approved single-ply overlay route matter most. Choose Cure It ONE when you want a tough, reinforced GRP finish without a separate topcoat stage. Choose Flexitec when you want a flexible, glass-reinforced system with one- and two-coat options. Whichever system you choose, the roof survey and preparation are more important than the name on the tin. Identify the substrate, rule out trapped moisture, confirm the primer and build-up, then order enough material for the whole roof — including every upstand and detail. Ready to Choose Your Overlay System? Compare the complete Apex range, then select the kit size and system build-up that match your roof. Browse Complete Overlay Systems → Need help choosing? Call Apex on 0800 612 7903 with the roof dimensions, clear photographs, the existing membrane type and details of any repairs, outlets or upstands.
