The short answer
- Coat the roof when the deck is sound, the insulation is dry and the existing covering is weathered, cracked or leaking at seams and details, but still bonded. A liquid silicone coating seals the whole surface in one seamless layer without tearing the roof off.
- Replace the roof when the deck is failing, the insulation is saturated, the covering has lost adhesion over large areas, or you need to add insulation to meet a U-value target.
- GacoFlex S42 is a 100% silicone, single-component, moisture-curing liquid roof coating. It is not a foam and not fibre-reinforced. It shows 0% water swelling (ASTM D471), which is why it suits roofs where water ponds after Irish rain.
- Warranty: S42 is a one-coat system. Sprayed at 0.90 mm wet film in a single pass, it carries a 20-year manufacturer-backed product warranty. A 0.5 mm build carries 10 years.
Why commercial flat roofs leak in Ireland
Ireland's weather is hard on flat roofs. It is not extreme heat or cold that does the damage. It is persistent rain, wind-driven showers, long periods of damp and repeated wetting and drying. A low-slope roof with shallow falls gives water time to sit and find weaknesses.
Most commercial flat roof leaks come from a short list of causes:
- Failed seams and laps in felt, single ply and older built-up roofs
- Details: upstands, parapets, outlets, rooflight kerbs, plant supports and pipe penetrations where the covering changes direction or is cut
- Ponding in low spots that never fully drain, where coverings degrade faster and every defect is under water
- Weathering: cracking, crazing and blistering of bitumen and older coatings from UV and age
- Mechanical damage from foot traffic, dropped tools, plant maintenance and birds
- Blocked outlets and gutters, which turn a shower into standing water
Metal and fibre cement commercial roofs share many of these problems at laps, fixings, rooflights and gutters, and they can be coated in the same way.
Coat or replace: the decision
The decision rests on what is underneath the surface you can see. A coating renews the waterproof surface. It does not fix a rotten deck or dry out soaked insulation.
| Finding at survey | Points to coating | Points to replacement |
|---|---|---|
| Deck | Sound, firm, no deflection | Soft, deflecting, corroded or rotten |
| Insulation (warm roof) | Dry | Wet or saturated over significant areas |
| Existing covering | Weathered, cracked, leaking at seams and details but still bonded | Delaminated, loose or blistered across large areas |
| Leak pattern | Localised at seams, details, penetrations | Widespread, with water tracking through the build-up |
| Ponding | Present; coating rated for it | Severe, caused by structural deflection |
| Insulation upgrade wanted | No, or insulation added from below | Yes, a new warm roof build-up |
| Building use during works | Must keep operating | Can accept a strip-off |
A survey checks these with visual inspection and, where needed, core samples or moisture testing of the build-up.
Beware the coating that is sold for every roof. A coating over wet insulation traps the water in. A coating over a failing deck fails with it. Anyone who recommends one system for every roof without looking underneath is selling product, not solving a leak.
Insulation and TGD L 2022
If a flat roof is being renewed in a way that counts as work to the thermal element, your designer may need to consider the TGD L 2022 Table 10 figure for flat roofs, 0.25 W/m²K. A coating renews the waterproofing surface. If insulation also has to be upgraded, the options are a new warm roof build-up or, on metal and fibre cement roofs, closed-cell spray foam applied from below. Our TGD L 2022 guide explains the triggers.
What GacoFlex S42 is, and what it is not
GacoFlex S42 is a 100% silicone liquid roof coating. It is supplied as a single component, with nothing to mix, and cures by reacting with moisture in the air. It can be applied by spray, brush or roller. It is sold in white (S4200), with grey, tan and dark grey options.
It is worth being clear about what it is not, because silicone coatings are often confused with other products:
- It is not spray foam. S42 is a thin liquid-applied coating, not an insulating foam. Where a roof needs insulation as well, foam is a separate layer (see external encapsulation).
- It is not fibre-reinforced. SF42 is the fibre-reinforced silicone in the range. S42 is not, and neither needs a separate reinforcing mesh or fleece.
- It is not a trafficked deck finish. S42 roofs are for maintenance access. Roofs that take regular foot traffic suit the GacoFlex U9 urethane system: a U91 base coat with a U92 top coat.
The properties that matter on an Irish roof
| Property | GacoFlex S42 | Why it matters |
|---|---|---|
| Water swelling (ASTM D471) | 0% | Ponding water after rain does not soften or swell the membrane |
| Cure | Moisture-curing, single component | Humidity works with the cure; no mixing ratios to get wrong |
| Coats | One-coat system | A single pass at the specified film build |
| Primer | None on most substrates | BleedTrap base coat required over bitumen-based roofs for warranted work |
| Solar reflectance (white) | 0.87 initial / 0.69 after three years | Lower surface temperatures in summer; see our BER guide |
| Solar Reflectance Index | 110 initial / 84 after three years | Weathered figures are the honest comparison |
| Warranty | 20 years at 0.90 mm wet, single pass; 10 years at 0.5 mm | Manufacturer-backed product warranty tied to film build |
Roof leaking now?
Send photos of the roof and the leak inside. We will arrange a free survey and tell you whether coating or replacement is right.
Which roofs can be coated
S42 is used over a wide range of commercial roof surfaces, once they have been cleaned and repaired:
- Bitumen felt and built-up roofs, including asphalt, SBS and APP modified membranes. The A4271 BleedTrap base coat goes on first where a warranty is required, to stop bitumen bleeding into and staining the silicone.
- Single ply and EPDM membranes
- GRP roofs
- Profiled metal roofs, including laps and fixings
- Fibre cement roofs, including asbestos cement. On asbestos cement the work follows the asbestos survey and HSA permit process. See our asbestos cement roofs guide.
Every substrate gets an adhesion check at survey. Old coatings of unknown type, contamination and surface chalking all affect adhesion, and they are identified before a specification is written, not after.
How a silicone roof coating job runs
- Survey. An approved installer inspects the covering, deck condition, drainage, details and penetrations. Where needed they take cores or test for moisture. The roof is measured and the results recorded.
- Specification and fixed price. Repairs, cleaning, any base coat, detail treatment and the S42 film build for the warranty you want, with one fixed price.
- Cleaning. The roof is cleaned with GacoWash, a concentrated cleaner used before silicone application, and rinsed. Loose material, dirt and organic growth are removed.
- Repairs and details. Blisters are cut and repaired, open seams made good, and outlets cleared. Seams, penetrations and details are sealed with SF4200 SeamSeal, a solvent-free silicone sealant.
- Base coat where needed. A4271 BleedTrap goes over bitumen-based roofs.
- S42 application. The coating is sprayed, brushed or rolled in a single pass to the specified wet film. For a 20-year warranty that is 0.90 mm wet (about 35 wet mils). Film thickness is checked with a wet film gauge as the work goes on.
- Handover. Application records with film build readings, product data and the manufacturer-backed product warranty documents.
Working with Irish weather
Every liquid coating needs a clean, dry surface and a suitable weather window, and the manufacturer's application guide sets the conditions. The installer watches the forecast, prepares and applies in dry spells, and protects work in progress. Because S42 cures with atmospheric moisture, damp air is not the enemy it is for some coatings. Rain falling on a wet roof before application is still a stop, and a good installer will not coat a wet roof to keep to a programme.
Looking after a coated roof
A coated roof still needs basic care, like any roof. Most problems we see on older coated roofs come from neglect rather than from the coating itself.
- Inspect it at least once a year, and after severe storms. Look at upstands, outlets, penetrations and anywhere plant or equipment has been added since the roof was coated.
- Keep outlets, gutters and scuppers clear. S42 tolerates ponding. The building below does not tolerate an outlet blocked by leaves and moss that turns a shower into a flood at the parapet.
- Control access. Contractors servicing plant, lights or telecoms equipment should use agreed routes and should not drag equipment across the surface. If the roof will see regular foot traffic, plan walkways, or specify a urethane system on those areas.
- Record new penetrations. A new pipe, bracket or cable through the roof must be sealed with a compatible silicone sealant. Do not let other trades patch a silicone roof with bitumen products.
- Keep the paperwork. The application record, film build readings and warranty documents are what a future buyer, tenant or insurer will ask for.
If your roof has already been coated with a product of unknown type and is failing, say so at survey. Silicone is only applied over surfaces it can be shown to adhere to, so an unknown previous coating is tested, not assumed.
Cost, disruption and roof life
We do not publish prices per square metre, because the real cost is set by what the survey finds: repairs, details, outlets, access and film build. Compared with stripping and replacing a commercial roof, a coating usually has these practical advantages:
- No strip-off. The existing covering stays, so there is no waste to remove and no open roof over the business.
- Less disruption. The building keeps operating, with work confined to the roof and access routes.
- Faster. With repairs done, a one-coat system on a prepared roof moves quickly in a good weather window.
- A warranty tied to what was applied. The warranty term follows the film build, which is recorded.
The trade-off is honest: a coating extends a roof that is fundamentally sound. It does not rebuild one that is not. If the survey shows the deck or insulation has failed, we will recommend replacement, and a coating quote in that situation would not be in your interest.
We survey commercial roofs across the Republic, including Dublin, Cork, Galway, Limerick and Waterford.