A roof overlay system applies a new seamless waterproofing membrane directly over an existing roof — without removing the existing roofing material. The existing roof becomes the substrate. This eliminates strip-out costs, waste disposal obligations and the operational disruption of a full re-roofing programme — where the assessment confirms the existing roof is a suitable candidate. Overlay is not applicable to every roof. The assessment determines whether the route is viable.
These are distinct responses to different assessment findings — not a sliding scale of cost. Localised repair addresses isolated defects while the wider roof remains serviceable. Partial replacement and repair removes and replaces failed sheets, saturated insulation zones or defective details while retaining serviceable areas — it involves strip-out and reconstruction in the failed zones. Liquid-applied roof overlay applies a compatible system over a confirmed-suitable retained roof surface following full assessment and preparation. Comprehensive roof refurbishment is a wider coordinated programme combining preparation, local replacement, gutter work, rooflight treatment and membrane application across multiple zones — not another name for overlay. Complete roof replacement is appropriate where the existing build-up, substrate or structure cannot justify retention. The assessment determines which combination is appropriate — they are not interchangeable choices.
Overlay is appropriate where: the existing roof structure is sound with no structural cracking or deck movement; the insulation is confirmed dry — by measurement, not estimation; falls are adequate to prevent persistent ponding; the existing roofing material is compatible with the proposed overlay system; and detailing at upstands, outlets, penetrations and parapets can be addressed within the overlay programme. All conditions require confirmation by assessment. Overlay on a dry but structurally failing deck, or on a sound but saturated deck, will fail.
Overlay is not appropriate where: the structural deck is deflecting, cracking or failing; the insulation is saturated; there is unresolved moisture in the build-up regardless of surface condition; the existing waterproofing is extensively delaminated; drainage falls are inadequate; outlets are defective or undersized; widespread detailing defects cannot be addressed within the overlay programme; unstable historic coatings cannot be stabilised; contamination cannot be fully removed; the build-up composition is unknown; failed vapour-control layers are contributing to trapped moisture; or the overall extent of defects makes a warranted overlay outcome unlikely. The full suitability assessment determines the correct route.
A dry surface does not prove the underlying build-up is dry. A felt roof that looks intact at the surface may have saturated insulation distributed across a significant area. A concrete deck that appears sound may have active cracking at screed level. A metal roof that appears dry may have moisture tracking under failed lap sealants. Historic repairs, patch bonding and previous overlay layers introduce composition and compatibility unknowns that visual inspection cannot resolve. Assessment may require moisture surveying, core sampling, local opening-up, insulation inspection, vapour-control-layer investigation, deck inspection, existing membrane identification, adhesion testing and test areas — the appropriate scope depends on the roof type, age and build-up history.
Yes, on suitable substrates. Where the felt is well-bonded, the insulation beneath is confirmed dry and falls are adequate, HBS200 or SprayGrade may be considered for overlay. Core sampling at representative positions across the roof is the standard method for moisture investigation — not just near reported leak positions. Where the felt is extensively delaminated, blistered or saturated, or where the insulation condition is unknown, overlay is not appropriate.
Yes, where sheets are structurally sound, not perforated and the substrate is confirmed suitable. On metal roofs where the primary requirement is long-cycle surface protection, Rocathaan TopSpray35 may be considered as a spray-applied protective coating within a suitable metal-roof refurbishment specification — subject to substrate assessment, preparation to specification and approved build-up confirmation. TopSpray35 is a protective coating, not a waterproofing membrane. Where a waterproofing membrane is also required, SprayGrade may be considered for suitable areas — the build-up confirmed against the approved specification. See Metal Roof Coatings for the full substrate-specific assessment framework.
Yes, where the concrete deck is sound and the proposed system is confirmed for the specific substrate and condition. Concrete preparation is critical — laitance, contamination and carbonated surface layers must be removed mechanically. Structural cracks require investigation to determine whether deck movement is active — a surface membrane cannot bridge active structural movement. Where carbonation has contributed to concrete deterioration, repair of defective areas may be required before overlay. Primer requirements must be confirmed for the specific system and substrate.
Not automatically — and not without structured assessment. Existing liquid membranes require product identification, compatibility review, removal of loose or degraded material, cleaning, abrasion, primer trials, adhesion testing and test areas before any overlay can proceed. Universal adhesion to unknown historic membranes cannot be assumed. Where the existing system type is unknown, a test area is required to confirm compatibility before committing the full programme.
Saturated insulation is the primary reason overlay projects fail prematurely. Applying a sealed waterproofing membrane over saturated insulation traps moisture within the build-up permanently. As the roof heats in summer, trapped moisture becomes water vapour — and vapour pressure beneath the membrane causes blistering and delamination, typically within the first summer. A membrane that blisters has debonded and is no longer waterproof at the blister positions. Saturated insulation is not a problem that overlay resolves — it is a condition that makes overlay inappropriate until the insulation is confirmed dry.
Moisture content across the full roof area must be confirmed before any overlay is specified — not estimated and not inferred from the visible surface. Core sampling at representative positions provides physical samples of each insulation layer for assessment. Electrical resistance or capacitance probing gives faster coverage at more positions. Infrared thermal imaging at dawn reveals differential heat retention from wet insulation zones. On larger roofs, a combination of methods is standard. The investigation must cover the full area — moisture is frequently concentrated in zones away from reported leak positions.
A liquid-applied overlay membrane conforms to the existing roof surface — it does not correct falls, increase outlet capacity, repair defective drainage design, remove hidden water from saturated insulation or compensate for failed sumps or outlets. Where falls are inadequate, water will continue to pond after every rainfall regardless of membrane quality. Persistent standing water accelerates membrane degradation and may be a condition that invalidates warranty. Drainage alterations, tapered insulation, outlet enlargement or local reconstruction may be required before or alongside overlay work — these are design decisions that must be resolved at specification stage.
Preparation requirements depend on the substrate type, existing condition and approved build-up. Typically: mechanical cleaning to remove loose material, contaminated coatings, failed sealants and organic growth; treatment of any crack or defect positions; moisture confirmation at the required standard; primer application where required — type, dilution and coverage rate confirmed for the specific substrate and system; reinforcement at specified positions — upstands, penetrations, lap positions, detail transitions; and test-area adhesion confirmation on previously treated substrates. Preparation represents the majority of programme duration on most overlay projects and cannot be compressed without affecting system performance and warranty eligibility.
Primer creates the chemical and mechanical bond between the existing substrate and the overlay membrane. Primer requirements are substrate-specific — what is required on concrete differs from what is required on existing felt, existing liquid membrane or metal. The type, dilution rate and coverage rate must be confirmed against the manufacturer's current specification for the specific overlay system and substrate combination. Omitting primer where it is required is a preparation failure — it affects bond strength, may cause adhesion failure in service and will invalidate warranty eligibility.
These are different technologies — not interchangeable versions of the same product — each with its own substrate requirements, primer specification and application conditions. HBS200 is a single-component liquid-rubber system applied by hand for suitable overlay and detailing applications. SprayGrade is a two-component liquid-rubber membrane applied by cold-spray equipment for larger accessible areas — the same product family as HBS200 but with different application requirements. MonoSeal is a cold-applied pure-polyurea waterproofing membrane — a distinctly different chemistry. TopSpray35 is a spray-applied protective coating relevant to metal-roof refurbishment only — not a waterproofing membrane for flat, felt, asphalt or concrete roofing. Where more than one system is used within a project, each product must have a defined role, interfaces must be designed, compatibility confirmed, primer and recoat requirements confirmed, manufacturer approval obtained where required, and the warranty specification must identify the complete build-up.
HBS200 and SprayGrade are cold-applied water-based systems with no hot works and no fumes requiring building evacuation — suited to occupied buildings with appropriate operational planning including zone management, HVAC air-intake protection during spray, overspray controls and daily communication with the building operator. MonoSeal and TopSpray35 application conditions should be confirmed from the approved product documentation for the specific project. The absence of hot works is a significant advantage on occupied buildings where hot-work permit conditions apply.
Any warranty position is project-specific and depends on the substrate, existing condition, preparation, complete system build-up, application records and manufacturer acceptance. No warranty is automatic — all require full preparation to specification, application by a trained approved applicator, achievement of specified film thickness and documentary compliance. Where more than one system is used, warranty conditions for the complete build-up must be confirmed as part of the same approval.
Manufacturer warranty and expected service life are distinct figures and must not be conflated. A warranty is a conditional commitment by the manufacturer for a defined period, subject to specified compliance. An expected service life is a separately stated figure describing what the system may achieve under continued compliant conditions — it is not a guarantee. Any warranty position and service-life expectation for a specific system must be confirmed from the manufacturer's current documentation as part of the project assessment. In practice, the service life of any overlay is determined primarily by the detailing quality at upstands, outlets and penetrations — not by field membrane performance.
A useful assessment enquiry should include: the roof type and approximate area; the existing roofing system and its age; the known or suspected build-up type; any known leaks — location, frequency and suspected cause; whether previous repairs or coatings have been applied; whether the building is occupied during works; drainage outlet positions and any history of ponding; access constraints; whether gutters and drainage channels are in scope; and photographs of the roof surface, outlets, upstands, penetrations and visible defect areas. For larger or complex roofs, a site visit for substrate condition assessment and moisture investigation is the appropriate starting point before any specification or programme commitment.
