Roof underlay is a protective layer beneath the visible covering of a pitched roof. Its specific function and properties depend on the selected product and complete construction.
What does this mean on your roof?
Underlay is not a complete product specification. A membrane and a board-based solution may have different properties.
Ask which product is present or proposed and how it connects at eaves, ridges, walls and openings. Those transitions belong in the assessment alongside the uninterrupted surface, rather than being treated as unrelated finishing work.
For insulation projects, its relationship with other vapour-control or vapour-permeable layers matters. Do not identify an existing material solely by colour. Product records or further investigation may be needed. A renovation proposal should identify renewed and retained underlay sections so later moisture questions can be related to the actual construction instead of a generic roof diagram.
Frequently asked questions
Is every underlay vapour-permeable?
Do not assume that from the name alone. Check product properties and the actual assembly.
Can underlay replace ordinary tiles?
It forms part of a system beneath the visible covering, not a general substitute for it.