Roof underlay is a layer or provision beneath the visible covering that performs an additional role within the roof system. Arrangements differ, so the word underlay alone is insufficient; establish which layer is actually present in the building.
On a tiled roof, membranes and other underlying components may remain largely concealed. Connections at eaves, ridges and penetrations deserve attention. One recognisable piece of material in a photograph does not prove the layer continues in the same way throughout.
Where a proposal retains existing layers, the basis for that decision should be clear. Ask which parts were assessed and which still require investigation. The term underlay is not automatic confirmation that a future assembly will be suitable in building-physics terms.
Worth asking
Frequently asked questions
Is every underlay the same membrane?
No. Material, arrangement and function need to be established from the actual situation.
Do new tiles prove the underlay is sound?
No. The underlying layer requires its own assessment.