Keeping the rain out

The UK is seeing a bigger push for Modern Methods of Construction (MMC) than ever before. With benefits such as increased efficiency and productivity; accelerated delivery; higher energy performance and improved quality, it’s clear to see why.

While the use of MMC - and specifically timber frame - is the solution to achieving energy efficient buildings with much shorter lead times, the realities and practicalities of this kind of building must be considered. On paper MMC is unbeatable, but, if not used correctly, our unpredictable climate can cause problems with build quality.

As more homes are being built using MMC, BM TRADA is receiving a rise in calls about the challenge of trapped moisture in timber frame buildings. If moisture damage occurs and drying out is needed, it counteracts the reasons for using MMC. Either the build programme is affected while the wet materials are dried or removed (which also increases costs) or the contractors plough on and hope for the best, negating the quality of the build and risking future structural failure if timber decay takes hold. 

The solution is a tricky one. Developers are being encouraged to manufacture as much as possible offsite, yet the reality when the components arrive on site can be challenging. It’s crucial that everyone on site knows how these materials should be handled so that moisture stops being an issue for current and future buildings.

With this in mind, we take a look at some our top tips for getting the most out of MMC and preventing trapped moisture compromising build quality. 

Top Tips

1 - Keep it dry

It sounds obvious, but the key is to keep the materials dry, and if they do get wet, measures must be in place for drying. This is not just the case for timber, but also for other materials, particularly plasterboard and mineral wool.

2 - Plan weather protection

Weather protection should be in place pre-site. Storing under a tarpaulin outside a factory and transporting it in the wind and rain means that we sometimes see closed panels arriving on site wet. The kit needs to be stored inside and well covered for the journey. Polythene should be used with caution, as it may keep water out, but if not used carefully, it can also trap water in.


3 - Limit weather exposure

With innovations in timber building, structures are getting bigger and taller, which makes it even harder to make weathertight during construction. The build needs to happen vertically in phases to limit weather exposure, and weather sensitive materials should only be introduced when the roof is on and the building is dry enough.


4 - Think about water run-off

The management of water run-off is a challenge. If polythene is laid horizontally, water collects on it and can run through the building until it meets a hole or edge, zoning the damage into dry areas and really wet areas. It’s also important to consider robustness during the construction phase – foot traffic and trades pulling away protection for access can lead to holes and laps which can let rain in. Water trapped under polythene protection can cause gradual damage which may not be discovered until much later.

5 - Consider ventilation

Factory fitted windows save time, but also reduces ventilation drying. BM TRADA's timber consultancy team has been in several cold, damp and humid buildings during winter, with mould growth due to high humidity caused by erecting buildings in the wet with a lack of ventilation.

Although achieving the solution may have its hurdles, the basic message is simple – arrive just in time, lift straight into final position and carefully protect on site.

For more detailed guidance, our popular book ‘Timber Frame Construction (5th edition)’ discusses the issue of party thermal wall bypass and the potential risks involved, and looks at alternative methods. This is a must read for those considering building with timber frame, offering advice and best practice for the entire build process.

If you would like more information on this, please get in touch with one of our timber specialists on +44 (0) 1494 569 601 or email us.