Polyurethane Foam Roofing, The Basics

Polyurethane boards vs. sprayed urethane foam

Polyurethane, as a roofing material, can be applied in two different ways:
- as the PU boards (urethane is foamed into the molds where the foam solidifies into the required shape);
- as a SPF or spray polyurethane foam (where the foam - a liquid substance under pressure - is sprayed directly on the roof surface).

Polyurethane boards

Apart from the simple PU boards that need to be laid onto the roof where already the vapour barrier material was applied, there are numerous types of much more universal urethane foam boards, e.g.:
- boards where the bottom part of the board was covered with the thin ALU foil or asphalt sheet (no vapour barrier is necessary);
- boards where the plastic or metal canvass was used (fixed in the mold and sprayed all around with the foam) to achieve harder foam blocks;
- if even firmer polyurethane foam segments are needed, the polyurethane can be foamed between two profiled steel plates (sandwich insulation).

Spray polyurethane foam (SPF)

The sprayed PU is applied onto the asphalt vapour barrier, usually in 2 or 3 layers.
The SPF has one substantial disadvantage - it is not stable if exposed to UV (ultraviolet) radiation. Therefor the SPF roofing should be treated with some protective coating (acrylic or PU based).

Differences between PU boards and sprayed PU foams

Even though you cmight think that both polyurethane roofing types offer the same features it´s not true. The main differences are as follows:

Volumetric weight

The PU boards are heavier as there is more material squeeezed in the mold. You shall consider this factor when calculating the maximum load of your roof (don´t worry, still PU is much lighter than majority of all roofing materials).

Absorptivity

SPF is less absorptive as it forms a hard crust on it´s surface during it´s solidification on air.