Question 1

1.What is a membrane on a roof?

Accepted Answer

This normally depends on the pitch of the roof. For a pitched roof the roofing membrane is a flexible sheet laid on the roof deck beneath tiles, slates, or shingles. It acts as a secondary barrier to prevent water getting in, and also manages moisture in the space. This type of roofing membrane is also called a roof underlay. On a flat roof, the membrane is the main (and often only) waterproof layer for the roof, exposed to the elements.

Question 2

2.Are roof membranes and roof underlays the same thing?

Accepted Answer

It depends on the type of roof. On pitched roofs (with tiles, slates or shingles), the terms roof membrane and roof underlay are often used interchangeably. They refer to the secondary waterproof layer installed directly beneath the outer roof covering. This type of underlay is also sometimes referred to as roofing felt underlay, or roof underlayment.

On a flat roof however, there’s usually only a single roofing layer, which is known as the roof membrane. This single layer is the main waterproofing layer for the roof, with no separate ‘underlay’ layer beneath.

Question 3

3. Is roof membrane material waterproof?

Accepted Answer

Most membranes for pitched roofs offer water resistance rather than being fully waterproof. However, membranes for flat roofs, like rubberised asphalt and PVC membranes are considered waterproof when installed correctly.

Question 4

4. Do I need underlay for a shed roof?

Accepted Answer

Underlay isn’t strictly essential for shed roofs, but it’s recommended if you want prolonged protection and moisture control. Using underlay adds a secondary waterproof barrier, and helps reduce condensation, especially on low pitch roofs or in harsh weather.

Question 5

5. Do you need underlay for roof shingles?

Accepted Answer

Yes - underlay is generally needed under all UK pitched roofs to meet good practice and standards. Use of underlay complies with BS 5534 guidelines, and the underlayment forms a secondary moisture defence should shingles fail or be lifted by wind. It also offers temporary protection for the roof structure during installation.

Question 6

6. How long does roof underlayment last?

Accepted Answer

Lifespan depends on material, installation quality, climate, and exposure. Asphalt‑felt typically lasts 12–20 years. Synthetic underlay may last 25–40 years or more.

Question 7

7. Can you use roofing membrane on walls?

Accepted Answer

Some breathable roofing membranes may be suitable for vertical use, but that depends on the product. Always check manufacturer guidance before applying roofing‑grade membranes to walls, as not all are tested or approved for vertical installations.

Question 8

8. Should roof membrane go into gutter?

Accepted Answer

No. The membrane should stop just short of the gutter unless the design allows direct drainage. Letting it overhang can cause water back-up or sagging.

Roof membranes and roofing underlay

Tyvek Supro Breather Membrane 1m x 50m

Tyvek Supro Plus Breather Membrane 1.5m x 50m

IKO Glass Universal Torch-on Felt Underlay 1m x 16m

Danosa Esterdan 30 P Elast Bituminous Membrane 1m x 12m

Novia VC200 Reflective Vapour Control Layer 1500mm x 50m

Novia STRP Pro Vapour Control Layer 1600mm x 50m

Novia Building Wrap 2.7m x 100m

Calder Standard Building Paper Grade A1F Lead Underlay 1m x 25m

Klober Permo Air Breather Membrane 1m x 50m

Tyvek Supro Breather Membrane 1.5m x 50m

Roofshield Breather Membrane 1m x 50m

Powerlon UltraPerm Lite Breather Membrane 1m x 50m

Danosa Polydan Pro 50 GP Green Capping Sheet 1m x 8m

Wraptite Liquid Flashing 600ml

Calder Geotec 220PY Lead Underlay - 1m x 25m

Calder Polybit Moisture Resistant Lead Paper Underlay - 25m x 1m

Powerlon UltraPerm Premium Breather Membrane 1m x 50m

Cromar Vent3 Light Roof Underlay 1.5 X 50m Grey

Roof Pro Torch-on underlay, (L)8m (W)1m

Calder Geotec 220PY Lead Underlay - 1m x 50m

Powerlon UltraPerm Standard Breather Membrane 1m x 50m

Powerlon UltraPerm Standard Breather Membrane 1m x 25m

Powerlon UltraPerm Standard Breather Membrane 1.5m x 50m

Cromar Vent 3 Breather Membrane 1.5m x 50m