Roof underlay: bitumen felt vs breathable membrane

The underlay is the actual waterproof line of defence on a pitched roof. Tiles shed the bulk of the water; the underlay handles what gets past them.

How old felt fails

Traditional bitumen felt embrittles with age and UV exposure at the eaves. It tears along the batten lines, sags between rafters and eventually holds water rather than shedding it into the gutter.

What breathable membranes change

They allow water vapour out while keeping liquid water out, which reduces condensation risk in the roof space and, with the right specification, can reduce the need for some ventilation openings.

The eaves detail

Underlay must discharge into the gutter, not behind it. An eaves protection strip carries the water over the fascia line and is cheap to fit while the fascia is off.

You can't replace it without stripping

This is the single strongest argument for a full re-roof over an overlay. If the underlay has gone, patching tiles is treating a symptom.

Specification matters

Not all breathable membranes suit all situations, particularly at low pitch or in unventilated cold roofs. The product should be chosen for the roof, not the van.

FAQs

How do I check my underlay?

Look up from inside the loft with a torch. Sagging, tears, daylight or crumbling material at the eaves are the signs.

Is breathable membrane always better?

For most re-roofs, yes — but it must be paired with the right ventilation strategy for the roof build-up.

Can underlay be repaired locally?

Small localised repairs are possible where tiles are lifted, but general failure means the roof needs stripping.

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