Ask which layer exists beneath the tiles, what role it serves in the assembly and how its junctions are formed. The visible tiled roof or one photograph from the attic cannot provide a complete picture of the underlay.
What can the existing record establish?
Look for drawings and records of earlier roof work describing the underlying assembly. Check that they concern the correct roof area and note their date. A property description from a purchase is not automatically a current technical survey of every layer.
Distinguish information established by documents from assumptions. Later work around a dormer or roof section may have created local differences. Ask for an assessment that allows for those transitions. The aim is not merely to fill in one material name, but to understand which layer exists where and how it connects to adjoining components. Historical information should guide investigation without replacing confirmation of the actual arrangement.
Why do junctions belong in the underlay question?
An underlying layer ends or changes at eaves, ridges, penetrations and other transitions. Ask how those locations will be included in the investigation. Recognising one flat piece of material does not establish that the entire layer is continuous or appropriately connected.
Manufacturer information for pitched roofs distinguishes underlay membranes from tiles and other system components. This helps explain why a proposal for new tiles does not automatically resolve every underlying question. State explicitly whether the underlay will be investigated, retained or renewed. If retention is proposed, ask which assessment supports it and which sections were not visible, rather than assuming a small observed sample represents the entire roof.
How do insulation and use affect the assessment?
Record whether the space beneath the roof is heated or used differently from before, where that information is known. Earlier internal insulation can also be relevant background. These details are not a diagnosis, but they help prevent a proposal from considering only the outside surface.
Ask who assesses relationships between layers and moisture behaviour when the assembly changes. A general instruction to add a membrane is not a building-specific design. The solution should suit the actual roof and its use. This guide therefore provides neither a universal layer sequence nor a DIY procedure for opening the construction, even where a small section appears accessible from inside.
How can an incomplete assessment still be useful?
An investigation may be limited because components are inaccessible or concealed. Record those limitations specifically: which roof area, layer or junction remained unseen? That information can define a useful follow-up step instead of leaving a general note that inspection was incomplete.
Ask which additional assessment is required before deciding on retention or renewal. An explicit open question is more useful than an apparently certain statement without support. Keep findings with locations and dates in the roof dossier. Future work then need not begin by guessing which underlying components were previously examined and which were not, or on what basis the earlier choice was made.
Worth asking
Frequently asked questions
Is underlay the same as roof decking?
No. The terms refer to different components or functions. Have the actual assembly identified in the proposal.
Does a new tile resolve every underlay question?
No. Underlying layers and their junctions need a separate assessment.
Should I open a section for a photograph?
No. Record what is safely visible and have further investigation arranged appropriately.