Commercial Flat Roof Refurbishment

Refurbishment, repair and replacement are three different jobs

These words get used loosely, and the looseness costs building owners money. A repair addresses discrete, identified defects — a failed lap, a split at an upstand, a blocked outlet — and leaves the rest of the roof alone. A replacement takes the build-up back to the structural deck and starts again. Refurbishment is the middle ground: the waterproofing is renewed across the whole roof, but the existing build-up is retained wherever it is sound.

The middle ground is where most commercial flat roofs of fifteen to twenty-five years old actually belong, and it is the option most often skipped. A contractor whose default is repair keeps coming back. A contractor whose default is replacement quotes a number that gets the project deferred for another two winters. Refurbishment, correctly specified, gets a roof with a genuine manufacturer guarantee for materially less than a strip — but only when the survey supports it.

This page deals with membrane flat roofs on solid decks: concrete, timber, or metal deck with a warm-deck build-up above it. Profiled metal industrial roofs work differently and are covered on industrial flat roofing and over-sheeting. If you are still deciding between fixing and replacing at all, start with repair or replace.

The core sample decides everything

A refurbishment proposal that has not opened the roof up is not a proposal. The core sample — a small cut through the entire build-up, taken in several places and made good the same day — tells you the four things every subsequent decision depends on: what the existing membrane is, how many layers are already there, what the insulation is and how thick, and critically whether that insulation is wet.

Wet insulation is the finding that closes off refurbishment. Water in the build-up does not dry out under a new membrane; it stays, it keeps degrading the boards, it corrodes fixings and a metal deck, and it will blister the new waterproofing from beneath. Sealing over it converts a roof problem into a hidden roof problem. Where a core comes back wet across the roof, the honest recommendation is replacement, and any contractor willing to overlay it anyway is selling you a two-year reprieve at refurbishment prices.

Where wetness is localised, a partial strip of the affected bays combined with refurbishment elsewhere is a legitimate and common answer. An electronic moisture survey across the whole roof, calibrated against the cores, is what maps that boundary. It is inexpensive relative to the works and it is the difference between a scoped job and a gamble.

The three refurbishment routes

Liquid coating over the existing membrane. A cold-applied PMMA or polyurethane system taken over the whole roof, including the details, upstands and outlets. It adds negligible height and weight, avoids hot works entirely — which matters where the insurer has restricted them — and it can be laid around plant and services that would be difficult to work under. It needs a sound, dry, compatible substrate and meticulous preparation; the coating is only ever as good as what it was applied to. Indicatively £100–£180/m² for liquid and GRP systems, with guarantees commonly in the 20–25 year range.

Overlay onto a recovery board. The existing membrane stays, a recovery or overlay board is fixed over it, and a new single-ply or reinforced bitumen membrane is installed on top. This is the route where the old surface is too irregular or too incompatible to coat directly. It adds height at every upstand, kerb and door threshold, so those details have to be checked before it is specified — a threshold that was compliant at the old level may not be at the new one.

Strip-and-recover. The covering comes off but sound, dry insulation stays, and a new membrane is installed — often with a tapered layer added to correct the falls at the same time. It is the most invasive of the three and the only one that lets you inspect and repair the deck. Mechanically-fixed single ply on a warm deck runs around £90–£160/m² with a 20–30 year guarantee where an approved contractor installed it; reinforced bitumen sits around £90–£150/m². Our overlay versus strip-and-recover guide works through the choice in more depth.

Correcting falls during a refurbishment

Refurbishment is the natural moment to fix a roof that ponds, because a tapered insulation scheme can be introduced without a full rebuild. BS 6229:2025 — which superseded the 2018 edition at the end of 2025 and removed the old design-to-1:40, finish-at-1:80 convention — asks for a finished fall no flatter than 1:80, with the design fall established from structural analysis and a level survey of the deck as built.

That level survey is what turns "it ponds over there" into a tapered scheme. It maps the actual deflected surface, and the taper is designed to that rather than to the drawing the building was originally built from. A zero-fall design — between 0° and 1:80 with no back-falls and a build-up specified for standing water — is a legitimate choice in some situations, but it should be a decision recorded on a drawing, not the accidental outcome of a roof that was meant to fall.

Adding taper adds height, and height changes every detail: upstands, door thresholds, rooflight kerbs, plant plinths and the level at which the roof meets a parapet. Those are the details that leak on an otherwise well-executed refurbishment, so they belong in the specification rather than being resolved on site.

Where Building Regulations come in

Renovating more than 50% of the roof surface — or more than 25% of the whole building envelope — makes the roof a controlled thermal element that must be upgraded to current standards, which for a re-roof means around 0.18 W/m²K established by calculation for the actual build-up.

This is where the three routes diverge sharply. A liquid coating adds no insulation, so where the trigger applies a coating alone may not satisfy it. An overlay or a strip-and-recover can carry additional insulation and therefore can. That is a genuine constraint on the cheapest option, and it is worth establishing early rather than discovering after a contractor has priced a coating.

Where the work is notifiable, a contractor registered under a competent person scheme such as CompetentRoofer can self-certify and issue a Building Regulations Compliance Certificate rather than you making a separate building control application. We are a lead-generation service rather than the installing entity, so that is a capability we look for in the contractors we introduce you to — not a registration we hold.

What refurbishment costs, and what doing nothing costs

Full ranges are on the cost page. In summary: localised repair and overlay work runs £25–£70/m² over the treated area; single-ply and reinforced bitumen warm-deck systems £90–£160/m² and £90–£150/m² respectively; cold-applied liquid and GRP £100–£180/m². Service lives run roughly 25–35 years on single ply and reinforced bitumen, with guarantees shorter than that and always conditional.

The comparison that decides most refurbishment projects is not between contractors, though. It is between doing the work and continuing as you are. A commercial roof being patched reactively can absorb an indicative £6,000–£9,000 a year indefinitely without ever improving, and that figure excludes the cost of an ingress over stock, plant, IT or a tenant's fit-out. Over a ten-year horizon the reactive route is frequently the more expensive one, and it carries a risk the planned route does not.

VAT is charged at the standard 20% (VAT Notice 708) and is recoverable as input tax by a VAT-registered business. On capital allowances, insulation added to an existing building generally falls to the special-rate pool rather than the main pool, with the Annual Investment Allowance the usual practical route — confirm the treatment of a particular scheme with your accountant before relying on it.

What a refurbishment proposal should contain

Ask for these six things, and compare proposals on them rather than on the bottom line. The core sample results and what they showed. The moisture survey and the map of any wet areas. The named system and manufacturer, with the number of layers. The U-value calculation for the proposed build-up. The falls strategy, including any taper and the level survey behind it. And the guarantee — its term, what it covers, what conditions attach and whether it survives the contractor ceasing to trade.

A proposal that will not name the system cannot be compared with one that does. And be cautious of anything presented as a lifetime guarantee: a guarantee with no stated term cannot be checked, priced or claimed against, which is why the manufacturers who actually stand behind their systems always state a number.