How Long Can I Leave a Pitched Roof Membrane Exposed?

A ‘breathable’ membrane is more accurately referred to as a low resistance (LR) membrane. It allows the passage of moisture vapour from inside to outside, reducing condensation risk in the roof space.

Some underlays, like Proctor Air®, also allow air to pass through them. These are known as air permeable, low resistance (APLR) membranes. Benefits of an APLR membrane include better and more uniform ventilation of the roof space, without the need for well-sealed ceilings or eaves/ridge ventilation solutions.

In terms of water resistance, the chosen LR or APLR underlay is a secondary line of defence. Water can get into the space below a tile or slate roof finish, so an underlay must also be capable of resisting water ingress, in addition to allowing vapour (and possibly air) to pass through it.

As there is an inevitable period during installation of a roofing system where the underlay is in place with no covering to protect it, a product has to be capable of providing some temporary weather protection – with the emphasis on temporary.

As a secondary defence, there is always a possibility for membranes to allow water ingress, because they are not waterproof. They are water resistant.

NFRC Technical Bulletin TB06 is referenced in BS 5534, the code of practice for slating and tiling. It describes fitness for purpose of pitched roof underlays and says, “a water hold out over 1m when tested to BS EN 20811 will be adequate. Class W1 membranes are to be preferred as defined in BS EN 13589-1.”

(Note: since the withdrawal of BS EN 20811 in 2018, the relevant test standard is now BS EN ISO 811.)

The combination of these two test results gives a realistic overview of a membrane’s performance. It’s important to check both the BS EN 13589-1 classification of a membrane, and its declared hydrostatic head (water hold out) . A product might achieve a W1 classification, but not the 1m hydrostatic head.

Broadly speaking, yes. The recommended maximum exposure of three months is actually about protection from UV rather than direct weather protection.

Exposure to UV can degrade a membrane’s performance over time; beyond three months is the point at which water hold out performance can start to become affected. We have carried out internal testing that shows between six and nine months of exposure can cause a membrane’s BS EN 13589-1 classification to drop from W1 to W2.

Ultimately, a pitched roof underlay should be left exposed for as little time as possible. Consider the specifics of the site, the type of weather you expect, and the potential knock-on effects if water was to reach the inside of the building.

If a membrane has been left exposed and particularly bad weather is forecast, then use temporary protection like a tarpaulin – especially in refurbishment projects where the building is occupied or you need to protect internal finishes from water damage.

There will always be examples of sites where a membrane has been left exposed for months, or even years, with no issue. Maybe the site is particularly sheltered, or the roof is steeply pitched and sheds water well. There is no universal answer and, while we offer the best general advice we can, there is no substitute for project-specific common sense.

In many ways, pitched roof underlays have been a victim of their own success. It can take just one day of the wrong sort of weather to cause water ingress. It may not even be the fault of the membrane or the length of exposure, if water is blown up the overlaps through untaped joints.

The climate is changing, leading to more frequent and more intense periods of rainfall. The urgent need for a fresh approach to our built environment was highlighted at the start of this year with the publication of new recommendations by the National Fire Chiefs Council (NFCC). Among the data the report shares, UK winters are anticipated to be 30% wetter than in 1990, by 2070.

Building fabric generally, and roofs in particular, therefore need to be designed, specified and constructed for the future climate. They must be capable of protecting people and buildings from more extreme weather.

As a manufacturer, when we design a product like Proctor Air, we do so with the future in mind. We are continually evaluating and seeking to understand what issues could occur with underlays. Water ingress can occur for a host of reasons, all of which feed into our product development process, and into what we communicate to the industry through articles like this one.