A 1,600 m² school re-roof phased across the summer holiday

This is a representative, modelled scenario rather than a named client, but it mirrors a re-roof that school business managers and multi-academy trust estates teams face regularly: a 1970s teaching block with a leaking flat roof, a tight capital budget, and a building that cannot simply close while the work is done. The figures are indicative and illustrate the method, not a named school.

The problem: a cold deck that condensed as well as leaked

The building was a 1,600 m² concrete-deck teaching block from the 1970s, with a cold-deck built-up felt roof that had begun leaking into classrooms. The obvious reading was a worn-out covering, and a cheap overlay quote proposed exactly that — lay a new membrane over the old one and move on. But the survey found the real problem was not only the failed felt. The roof was a cold-deck construction, with the insulation below the structural deck and no effective vapour control, and it was showing interstitial condensation: warm, moist air from the occupied classrooms was reaching the cold underside of the deck and condensing there.

That changes everything. An overlay over a cold deck with a condensation problem seals the fault in rather than fixing it — the moisture keeps forming, the insulation stays wet, and the building pays for the same problem again within a few years. The condensation, not just the leak, is what ruled out the cheap option.

The specification: strip to the deck and rebuild as a warm deck

The honest route was a full strip to the structural deck and a rebuild as a warm-deck flat roof. A vapour control layer was laid on the warm side of the insulation to stop moist internal air reaching the cold zone, insulation was laid above the deck to bring the U-value up to 0.18 W/m²K under the Part L thermal-element upgrade triggered by renewing the roof, and tapered insulation designed the falls into the build-up to a 1:80 finished minimum, draining to the outlets so the roof would no longer pond. The covering was a reinforced-bitumen membrane — a robust, multi-layer system with a redundancy of layers that suits a school roof — installed to the manufacturer’s specification. The Part L standard the upgrade had to meet is set out in Approved Document L.

Delivered around the school year, without naked-flame risk

The programme was built around the school calendar. The noisiest and most disruptive work was phased across the summer holiday, with additional works in evenings and quiet periods, so classrooms below stayed in use through term-time and no area was left exposed to weather between phases. Critically, the system used cold-applied, self-adhesive and torch-free detailing rather than naked-flame hot works, removing the fire risk of an open flame over an occupied school — a decision that matters far more on a school than a headline price does.

The work was carried out by a manufacturer-approved, NFRC-accredited installer, self-certified against the Building Regulations where CompetentRoofer-registered, with a Building Regulations Compliance Certificate issued for the trust’s records — the document asked for at a funding audit, insurance review or condition survey. It was backed by up to a 20-year single-point manufacturer guarantee, subject to system and approved-installer status.

The lesson

A cheap overlay quote and an honest strip-and-recover quote on the same school roof are not two prices for the same work — they are two different jobs, and only one of them addresses the condensation that was damaging the building. The survey, not the headline number, is what tells the two apart. Where the capital is tight, the works can be phased by roof area across financial years so the spend lands in stages. If your school or trust is weighing a re-roof against another patch, request a survey-based quote and get the repair, overlay and replace numbers side by side before authorising anything.