Paper Under Roof Tiles. They are there to protect the building paper from the sun etc. Basemat the flexible base layer is saturated with asphalt for water resistance.
To a somewhat lesser degree. Rafters spouting fascia board roof tiles underlay over anti-ponding board and lapped over spouting edge spouting roofing tile battens over underlay spacing to suit roofing tiles fascia board underlay lapped over gutter edge purlin rafter anti-ponding board. It will prevent the passage of water in its liquid form.
They are there to protect the building paper from the sun etc.
Less in hot climates under the sun. Tile battens fixed over underlay. The underlayment is a layer of paper that helps keep the seams of the roof from leaking. It can help prevent the picture framing effect or the pattern of your wood decking telegraphing through your shingles by.