When should you choose PIR or XPS for a flat roof?

Waterproofing above the insulation defines the warm roof. Where it lies below, select insulation approved for inverted use. Reordering the layers to use surplus boards changes the design. Check moisture, drainage, restraint and loads throughout the assembly.

On a suitable warm roof, PIR provides high thermal resistance within limited depth, which can help at parapets and door thresholds. The facing must suit the waterproofing attachment method. Check compatibility with the specified adhesive, membrane and installation technique.

Check whether the design calls for one layer, multiple layers with staggered joints, or tapered boards. Each has its own fixing requirements. Keep insulation dry and protected from rainfall during construction. Wet material sealed beneath the covering can complicate later diagnosis and repair.

An inverted roof needs an XPS grade suited to water exposure and the intended loading. Check water absorption, freeze behaviour and long-term properties. The letters XPS on a label alone do not establish suitability beneath a terrace or green roof.

The thermal calculation must reflect these conditions, including the effect of water under the chosen method. Layers above the insulation provide drainage, separation and stability. Their selection and weight are design inputs. Loose boards alone do not perform all these functions.

Compressive stress at 10% deformation is not a permissible permanent roof load. For terraces, equipment and ballast, assess creep and pressure distribution. A small equipment foot can apply much higher local pressure than the total equipment weight divided by the roof area suggests.

Compare products at equal loads and durations. Check compression of intermediate layers and structural capacity. Wind uplift restraint is a separate design check. The insulation rating cannot establish the load capacity of the slab, structural deck or connections.

Insulation thickness affects the space left for falls and waterproofing upstands. Survey deck levels, thresholds, parapets and outlets. A few centimetres may require a different detail. In refurbishment, check that enough height remains to terminate the new covering correctly.

Falls and drainage must still work after deflection and construction tolerances are allowed for. A thinner board cannot correct a poorly planned outlet. On a terrace or green roof, maintain inspection access to outlets and overflows and check them regularly for blockages.

Tapered insulation varies in thickness across the roof. Use a thermal calculation method for variable thickness. Entering only the thickest point may make performance look better than it is.

Include insulation, waterproofing, separating layers, fasteners, flashings and labour in both estimates. For the inverted roof, include ballast or wearing layers and their structural impact. For the warm roof, include compatible fixings and protection of the membrane in service areas.

Record overall depth and how leaks could be repaired. Roofs with similar U-values may offer very different access to the waterproofing. Choose for the use, available levels and maintenance arrangements. Board prices alone do not capture these differences.