This flat roof assembly consists of rigid insulation placed on the exterior of insulated roof framing. High effective R-values are achieved by using continuous insulation outside of the structural framing in combination with thermally efficient attachments, and supplemented by insulation in the framing cavity.

In most cases the roofing substrate (such as protection board) can be fastened directly through the rigid insulation into the roof sheathing and framed assembly. The waterproof roofing membrane above the insulation controls all exterior moisture. The sheathing membrane over the roof sheathing is used as the air barrier and vapour retarder. This split-insulated approach must be carefully designed to ensure the interior insulation does not result in condensation risk at the roof sheathing.