Worked specification example, not a completed project
Cold-applied PMMA liquid overlay on an office and plant deck: a worked specification
How the specification works for a detailing-heavy 1990s office roof with a plant deck: a cold-applied PMMA overlay, with no hot works over occupied offices, every upstand and penetration dressed seamlessly, and no strip.
- Situation
- Sound but tired roof crowded with plant, upstands and outlets
- System
- Cold-applied PMMA liquid-applied waterproofing, overlay
- Thermal effect
- Unchanged: an overlay does not trigger a Part L thermal-element upgrade
- Building
- Office block with rooftop plant deck
This is a worked example of the method, not a completed project or a named client. It reflects a common refurbishment question: a 1990s office block with a plant-congested roof, where a sound but tired single-ply membrane is reaching the end of its guarantee. The roof is crowded with upstands, rainwater outlets and service penetrations around rooftop plant, and offices are occupied directly below. Two constraints frame the job from the start: every detail has to be waterproofed without a seam or lap that could fail, and no naked flame can be used over the working floors beneath.
Why an overlay, not a strip
A survey has to confirm that the deck is sound, the insulation dry and the falls broadly adequate, so that only the surface has 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 spend capital solving a problem that is not there, and open 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
A cold-applied PMMA liquid-applied waterproofing encapsulates the existing membrane and dresses every upstand, outlet and plant plinth seamlessly, with no welds or laps to fail on a detailing-heavy roof, which is where liquid systems earn their place. Cold application removes the hot-works fire risk over the occupied offices below, and PMMA’s fast cure allows a return to service between weather windows. Ask for adhesion testing to the substrate before application and for installation to an LRWA-referenced specification by a manufacturer-approved installer. Guarantee terms depend on the system and approved-installer status; see the guarantees page.
What an overlay does and does not do
Because the roof is never opened to the deck, there is no window where a phase is vulnerable to rain. And because an overlay renews the surface and adds no insulation, it does not trigger a Part L thermal-element upgrade, so the U-value is unchanged: the owner should note that honestly rather than assume a thermal improvement was delivered. The trigger thresholds are in Approved Document L. An overlay stops the leaks and extends 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.
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, get a quote with the overlay and strip routes costed side by side.