620 m² office and plant-deck PMMA liquid overlay

This is a representative, modelled scenario rather than a named client, but it reflects a common refurbishment: a 1990s office block with a plant-congested roof, where a sound but tired single-ply membrane was reaching the end of its guarantee. The roof was crowded with upstands, rainwater outlets and service penetrations around rooftop plant, and offices were occupied directly below. Two constraints framed the job from the start — every detail had to be waterproofed without a seam or lap that could fail, and no naked flame could be used over the working floors beneath.

Why an overlay, not a strip

A survey confirmed the deck was sound, the insulation dry and the falls broadly adequate — only the surface had aged. Those are the conditions under which an overlay is the honest call rather than a full strip. Stripping the roof back to the deck would have spent capital solving a problem that was not there, and opened an occupied building to the weather. The framework for that decision is set out in repair or replace: where the build-up beneath is sound, you treat the covering, not the whole roof.

Cold-applied PMMA, no naked flame

The roof was overlaid with a cold-applied PMMA liquid-applied waterproofing that encapsulated the existing membrane and dressed every upstand, outlet and plant plinth seamlessly — no welds or laps to fail on a detailing-heavy roof, which is exactly where liquid systems earn their place. Cold application was the decisive reason liquid was specified: with no naked flame there was no hot-works fire risk over the occupied offices below. PMMA’s fast cure allowed a rapid return to service between weather windows, and adhesion was tested to the substrate before application, installed to an LRWA-referenced specification by a manufacturer-approved installer. The cover is backed by up to a 20-year system guarantee, subject to system and approved-installer status.

No strip, no disruption

The works took about two weeks, sequenced in zones around live plant so the offices carried on beneath. Because the roof was never opened to the deck, there was no window where a phase was vulnerable to rain — the building stayed watertight throughout. And because this was an overlay renewing the surface and adding no insulation, it did not trigger a Part L thermal-element upgrade; the U-value was unchanged, which the owner should note honestly rather than assume a thermal improvement was delivered. The trigger thresholds are in Approved Document L.

What it bought

The overlay stopped the leaks and extended the roof’s life without a full strip, deferring the eventual capital re-roof to a financial year of the owner’s choosing rather than an unbudgeted mid-year ask. That is the value of the honest overlay: it is cheaper and less disruptive now, and it does not pretend to be a re-roof it is not. The cost guide shows how liquid overlay pricing compares with a full warm-deck strip, and the competence framework behind the specification runs through the NFRC. For a detailing-heavy roof over an occupied building, book a free survey and get the overlay and strip routes costed side by side.