Re-roofing an industrial unit for a future ballasted solar array: a worked specification

This is a worked example of the method, not a completed project or a named client. It answers a question that comes up on almost every large re-roof: can this roof carry solar later? The owner of a manufacturing unit wants a rooftop array, but the profiled-metal-and-felt roof is near the end of its life. Installing panels first would mean lifting the whole array again within a few years to re-roof underneath. The honest sequence is the reverse: renew the roof, and design it from the outset to carry the future PV load.

Roof first, then panels

A ballasted or fixed array adds dead load and wind uplift to the roof, and it sits on the membrane for decades. Put that onto a tired roof and you have bought a membrane failure you cannot reach without craning the array off. Sequencing the roof before the PV closes a gap that many roofers and many solar firms leave open. The structural engineer, not a rule of thumb, sets the allowable load for the specific array and ballast design. The load logic is covered in can a commercial flat roof carry solar panels.

The survey and the structural check

The specification starts with a survey of the existing build-up, confirming the deck and assessing the full roof load and build-up profile: dead, live, wind-uplift and snow loads. The residual structural capacity is then confirmed for the combined weight of the new roof plus a future ballasted array, so the array can be added later without disturbing the membrane. That structural question is the one a specialist asks that a generalist does not, and it is why a warm-deck re-roof engineered for the future load is not the same job as a like-for-like recover.

The specification

A mechanically-fixed single-ply warm deck is the lighter suitable system, which leaves the most residual capacity for the array. Tapered insulation sets the 1:80 finished minimum in BS 6229:2025, with the design fall taken from structural analysis and a level survey so the finished minimum survives tolerances and deflection. Renewing more than 50% of the roof surface triggers the Part L thermal-element upgrade to 0.18 W/m²K (Approved Document L), and the fixing pattern is calculated to BS EN 1991-1-4 with enhanced perimeter and corner zones. Single-ply practice is governed by the Single Ply Roofing Association; see the guarantees page for what a manufacturer guarantee does and does not cover.

Ready for PV

The aim is a roof that is watertight, thermally compliant, laid to a designed fall and structurally proven for a ballasted array, so the PV can be installed now or in a later financial year without touching the membrane. That is roof-before-PV done in the right order. The cost guide shows how a re-roof engineered for future load is priced, and any building weighing up the two projects together should have a survey first, so the membrane and the structure are right before an array ever goes up. Start with a quote.