What roof covering should i use on my flat roof? – CIJA | Ep.44

Flat roofs are one of the most technically demanding roof types to get right, and the membrane you choose has a bigger impact on long-term performance than most people realise. In this episode of Can I Just Ask?, Chris and Andy work through the key considerations for unventilated flat roofs — the roof type BTE most commonly specifies with wood fibre insulation.

The conversation covers why condensation risk remains the leading cause of flat roof failure, and how an unventilated build-up relies on an annual cycle of moisture absorption in winter and drying in summer to stay safe over time. Andy explains why that summer drying process depends heavily on solar radiation — and why the colour of your roof covering is therefore a critical specification decision, not an aesthetic one. Two flat roofs with identical specs and identical orientation can have vastly different moisture profiles depending on whether the membrane is dark or light.

Chris and Andy also cover SD values and membrane types in practical terms — why a target SD value of 20 to 30 metres is the sweet spot for most unventilated flat roofs, why ePDM membranes (despite their green credentials) can be problematic if they’re too vapour-tight, and why variable diffusion membranes on the room side are the right call for unventilated build-ups, switching behaviour between winter and summer conditions.

If you’re specifying a flat roof with wood fibre insulation, this episode gives you the technical framework to choose a membrane with confidence rather than defaulting to whatever the roofer suggests.