PIR or XPS for a flat roof: which layer determines the choice?

Distinguish a non-accessible roof, a terrace and a roof carrying equipment. Each has different mechanical demands. A planned change in use belongs in the renovation design. Do not select material solely by price and the thickness that fits beneath a door.

Examples of aluminium-faced PIR boards for flat roofs are intended for conventional assemblies beneath waterproof roofing. The specific system’s instructions define the substrate, vapour control, fixing and compatible waterproofing. This does not approve permanent use above waterproofing in rainwater.

Keep boards dry during construction. Low declared core water absorption does not turn thermal insulation into waterproofing. An opened pack or transport film does not protect the whole roof. Finish every work stage in a way that protects the layers from rain.

Inverted roofs use XPS grades intended for that application. Insulation lies above the waterproofing and faces different moisture conditions. Suitable separating layers, drainage and protection against movement are needed. Not every board called XPS suits every inverted-roof assembly.

Thermal calculations must account for inverted-roof conditions, including required corrections. Check ballast or wearing-layer loads structurally. In older buildings especially, the weight of the complete solution may determine the construction method.

Calculation example: 120 mm of insulation with λ = 0.022 W/(m·K) gives R of about 5.45 m²K/W. Achieving that resistance with λ = 0.034 requires approximately 185 mm. This compares the layers alone at assumed values, excluding water, fixings and other roof components.

A smaller thickness can help at low parapets or terrace thresholds. The complete build-up must still fit, including falls and waterproofing. Thin insulation does not justify an insufficient waterproofing upstand. If the detail needs alteration, include it in both options’ costs.

CS(10) gives stress at a specified deformation in testing, not a ready-made long-term roof load capacity. Assess permanent loads, creep and concentrated support pressures separately. Planters, equipment feet and terrace slabs may need forces spread over a larger area.

Selection must include both insulation and the structural deck. During renovation, do not assume the existing slab has spare capacity for ballast. Measurements and structural assessment may be necessary. The same applies to storing packs and materials during the work.

Keep roof outlets, overflows and equipment accessible. Plan servicing without damaging waterproofing. In a ballasted system, leak repair may require exposing a larger area. In a conventional assembly, protect the roof covering against damage from walking and installation work.

Compare the complete cost: removing wet layers, falls, vapour control, fixings, roof covering and flashings. Cheaper boards do not necessarily mean a cheaper roof. The most useful option suits the existing structure and allows durable drainage.

For roof terraces, check access to outlets after the wearing surface is installed. Insulation and drainage must still match the design after planters or furniture are added. Such items can impose concentrated, sustained loads and should not be equated with ordinary use of an empty roof area.