Commercial Flat Roof Overlays: GRP, Felt or Liquid Rubber?

If the deck underneath is dry and sound, an overlay is usually the fastest and most affordable way to give a tired commercial flat roof another 20 years. As a guide, a high-performance felt overlay typically costs £45 to £70 per square metre fitted, GRP fibreglass around £70 to £100, and liquid rubber £35 to £60. For most units and warehouses around Doncaster, we recommend felt first, because it balances cost, lifespan and speed.

An overlay means installing a new roofing system over the existing one, without stripping back to the deck. You save on skip hire, labour and downtime, and your business keeps trading while we work. The catch is that an overlay only suits a roof that is dry underneath, so the deck condition matters more than the system you choose.

We have fitted overlays on commercial buildings for over 30 years, from single-shop units to large industrial roofs. Below is the honest comparison we give every customer who calls us about commercial flat roofing in Doncaster.

The Verdict at a Glance

Here is the side-by-side comparison most guides skip. Prices are supplied and fitted, as a guide, for a straightforward commercial roof with reasonable access in the South Yorkshire area.

No single system wins every job. Felt wins on speed and track record, GRP wins on durability, and liquid rubber wins on awkward details and budgets.

Can You Overlay a Commercial Flat Roof?

Usually yes, but only after an honest inspection of three things: the deck, the moisture content, and the number of existing layers. We check every roof before we quote, because overlaying the wrong roof just hides a problem for a couple of years.

The deck is the foundation of the whole system. We look for:

• Timber sarking boards or OSB decks that are rotten, springy or delaminating
• Concrete screeds that are cracked, crumbling or breaking up
• Metal decks with corrosion around fixings and seams
• Existing membranes that are lifting, bubbling or breaking apart underfoot
• Trapped water or wet insulation beneath the current surface

Moisture entrapment is the big risk. If you seal a wet roof under a new membrane, the water stays there. It turns to vapour in warm weather, blisters the new surface from underneath, and rots a timber deck from within. That is why we take moisture readings on commercial decks before recommending an overlay, full stop.

When an Overlay Is the Wrong Choice

Strip and recover is the better investment when:

• The deck is wet, rotten or unstable in more than small isolated patches
• The roof already has two or more built-up layers and adding a third adds weight without benefit
• The insulation is saturated and has lost its thermal value
• The fall is so poor that ponds form after every shower, because an overlay copies the existing falls
• Deck repairs would cost more than stripping, once scaffolding and labour are counted

In those cases we strip to the deck, renew insulation to the current standard set out in BS 6229, the code of practice for flat roofs, and install a full new system. It costs more upfront, £70 to £100 per square metre, but it is the only honest fix. Our commercial roofing service in Doncaster covers both routes, and we will tell you plainly which one your roof needs.

Felt Overlays: The Workhorse

A high-performance felt overlay bonds a new two or three layer built-up system directly onto the sound existing surface using torch-on membranes. It has been the default commercial overlay in Britain for decades, and for good reason.

The strengths are speed and certainty. On a typical unit roof we can overlay hundreds of square metres in a day or two, the surface is waterproof as soon as each roll is torched down, and there is no cure period before the roof can take foot traffic. Modern high-performance polyester-reinforced felts regularly last 20 to 30 years.

The weakness is workmanship. A felt overlay lives or dies on preparation and detailing at upstands, gutters and outlets. It also needs dry conditions to install, so winter jobs depend on weather windows. As a guide, expect £45 to £70 per square metre supplied and fitted on a straightforward commercial roof.

GRP Fibreglass Overlays: The Durable Option

GRP, or glass reinforced plastic, is a resin and glass mat system that cures into a single, joint-free shell. On a commercial roof it is the toughest of the three, shrugging off foot traffic, plant equipment and standing water better than felt.

GRP suits roofs that take abuse: plant rooms, balconies over commercial premises, and roofs with regular maintenance traffic. A properly laid GRP roof has no seams to fail and typically lasts 25 to 30 years or more.

The trade-offs are cost and conditions. GRP needs a dry deck and stable temperatures to cure properly, which limits winter use, and it is usually the most expensive option, typically £70 to £100 per square metre as a guide. It also demands a highly trained installer, because a miscured laminate cannot be patched cosmetically.

Liquid Rubber Overlays: The Problem Solver

A liquid rubber or liquid plastic coating is roller or spray applied over the existing surface, bonding to felt, asphalt, concrete and metal. There are no seams at all, and the liquid finds its way into every detail, upstand and awkward junction that roll materials fight against.

This is the system we reach for on complicated commercial roofs: roofs full of plant, ductwork and penetrations, parapet-heavy older buildings in Doncaster town centre, and roofs where hot works are not welcome. It is also the kindest on budget, typically £35 to £60 per square metre as a guide.

The catch is preparation and curing. The surface must be clean, dry and primed, and the coating needs 4 to 8 hours before rain can touch it, then 24 to 48 hours to reach full strength. Lifespans sit a little below GRP, usually 15 to 25 years depending on the system and the number of coats.

A Worked Cost Example

Take a fairly typical commercial job for us: a 400 square metre warehouse roof on an industrial estate outside Doncaster, with sound decking, reasonable access and straightforward detailing. As a guide, the overlay options would look like this. If a skip needs to sit on the highway, a City of Doncaster Council permit usually takes a few days to arrange.

These are guide figures for illustration, not a quote. Access, height, upstands, outlets, skylights and board condition all move the price. The point of the table is the shape of the decision: liquid and felt overlays sit close together, while GRP overlay costs about the same as a full strip and recover, so GRP only makes sense over stripping when the deck is genuinely sound and disruption must stay minimal.

Commercial Overlays vs Domestic Overlays

Domestic flat roofs, the garage and extension roofs we handle through our trusted roofing company in Bentley and Roslyn, are smaller and simpler. GRP is popular there because a tidy finish matters and the areas are manageable.

Commercial roofs change the calculus. You are working at height on bigger areas, so our IPAF and CSCS trained teams use access machinery rather than scaffolding wherever possible. Fire rating to BS 476 matters on larger buildings.

Cure times matter because the roof may need to take contractors the next morning. And downtime matters most of all: an overlay that keeps a factory trading beats a cheaper quote that closes it for a fortnight. More detail on these systems is on our commercial felt roofing in Doncaster.

Matching the System to Your Building

The right overlay usually announces itself once you list the facts of the building. Work through these questions with your roofer and the shortlist tends to narrow itself:

• What sits on the roof? Heavy plant and regular foot traffic point to GRP or a reinforced felt system.
• How complicated is the shape? Dozens of upstands, ducts and penetrations point to liquid rubber.
• How soon must it be watertight? Felt is waterproof the moment it is torched; liquids need curing time before rain.
• When is the work happening? GRP and liquids both dislike cold, damp conditions, while felt can go down in more weather windows.
• How long do you plan to hold the building? A 10 year plan and a 30 year plan justify different spends.

Budget usually settles any tie. If two systems suit the roof equally, take the cheaper one and protect it with a planned maintenance visit every couple of years. A £300 inspection catches a failed seal long before it becomes a £3,000 wet deck.

What a Proper Overlay Quote Should Include

We see a lot of commercial overlay quotes that are one line and a number. Before you sign anything, check the quote states all of this:

• The exact system and manufacturer, including membrane thickness or coating build-up in millimetres
• What happens to existing outlets, upstands, gutters and edge trims
• Whether insulation forms part of the job, and whether the Part L 50 per cent rule applies and Building Regulations sign-off is included
• Access method, programme in working days, and what happens if it rains
• The warranty length, what it covers, and whether it is insurance-backed
• Proof of public liability insurance, ours is £5M, and relevant training such as IPAF and CSCS cards

If a quote skips the moisture survey or deck assessment, ask why. Overlaying blind is the single most common cause of premature commercial flat roof failure in our experience, and no warranty fully protects you from a roofer who never looked under the surface.

Our Straight Recommendation

After 30 years and more than 2,500 customers, our default advice is simple. If the deck is sound and dry, overlay with high-performance felt, and spend the money you save on good detailing and regular inspection. Choose GRP when the roof takes heavy traffic or you want the longest possible service life from one job. Choose liquid rubber when the roof is complicated, hot works are a problem, or the budget is tight.

The only wrong answer is overlaying a wet or failing deck. We survey commercial roofs across Doncaster and South Yorkshire with no call-out fee, take moisture readings, and give you a written recommendation, overlay or strip, with honest pricing for both. All our work is covered by £5M public liability insurance and guarantees of up to 20 years.

Common Mistakes to Avoid

We see these five errors on commercial overlay jobs more than any others. Each one shortens the life of the new roof.

• Overlaying wet decking. Trapped moisture blisters the new membrane and rots timber decks from within. Take moisture readings before any overlay is quoted.
• Ignoring the Part L 50 per cent rule. Re-covering more than half a roof layer triggers the thermal upgrade requirement. Budget for insulation, not just the membrane.
• Laying GRP in cold or damp conditions. Resin laid below the manufacturer's temperature range fails to cure properly and cannot be patched cosmetically. Wait for the right weather window.
• Poor detailing at upstands and guttering. Overlays fail at details, not in the field. Outlets, upstands and edge trims need proper trims and sealant, not a rushed fillet.
• Skimming over drainage faults. An overlay copies the existing falls, so a ponding roof stays a ponding roof. Correct the falls or accept a shorter life.