Safeguarding structural timber against damp

Alex Massingham of RIW offers advice on the waterproofing strategies needed to help meet increasingly popular engineered timber buildings’ design lifespans

Some of history’s most iconic structures – from Westminster’s Great Hall to the architecture of Venice – make extensive use of structural timber, presenting those who care for them with multiple concerns such as damp-related deterioration and fungal attack. Today we are witnessing a renaissance in the materials use for load-bearing applications; thanks to developments in timber engineering, such as cross-laminated timber or CLT and glue-laminated members.

Recent forecasts by Allied Market Research indicate that the engineered wood market is projected to grow by 24.8 per cent by 2022. However the perennial ‘Achilles heel’ of wood, i.e. damp, and the industry mantra ‘to keep its feet dry’, remain as necessary today as it ever was.

Through the centuries, most of the timber used in construction was literally hewn on site – with tools as rudimentary as an adze (early axe) – while the UK industry today uses the very latest advancements in modern timber engineering. In addition to open or closed panel, and even dual-frame systems steadily increasing in popularity for housebuilding, solutions such as CLT and laminated veneered lumber, or LVL, are all gaining traction for commercial work.

While contemporary timber-based systems are generally supported by sophisticated design software, there remain very significant challenges for clients and their consultants in achieving an integrated, weatherproof and watertight structure.

Basically, no matter how quickly and slickly the BIM model might indicate that the engineered timber elements will go together, involving an experienced waterproofing manufacturer at the outset can save the project team significant delays or other headaches on site.

In fact, if the price premium for timber systems is to be justified, the potential programme advantages of a prefabricated timber solution must be properly exploited. And as part of that, the interfaces between the timber elements and the supporting sub-structure must be waterproofed quickly and effectively.

So whether the frame members are being located on a simple slab, or installed on top of some type of concrete undercroft, the barriers to protect them from damp must fit with the method statement as well as the geometry of the interface.

The pragmatic option is to engage with a specialist capable of providing fully compatible and dependable solutions; suited to the project’s specific technical and other challenges. These might relate to site topography and ground conditions, as well as the timber elements’ size, loadings and proximity to other parts of the structure. The window of opportunity for applying waterproofing systems can also have considerable influence on their final specification.

There are significant similarities between employing engineered timber and SFS construction, and an early decision needs to be made on whether to create the waterproof barrier beneath the slab or above it. This is an illustration of why a specialist manufacturer can provide crucial guidance to the project delivery team.

Reinforced concrete floors can be cast across seam-welded membranes fixed on top of a blinding layer, though many groundworkers prefer the user-friendly installation characteristics of bentonite impregnated membranes, whose overlaps do not require taping or welding. Used horizontally or vertically over the face of piled retaining walls, the bentonite material swells on contact with groundwater and is effectively ‘self-healing’.

However, where the groundworks solution features a series of pile caps and ground beams, or possibly lift-pits, installing the waterproofing barrier beneath the slab may become prohibitively problematic. However, although creating a raft foundation may consume a slightly higher volume of reinforced concrete than individual pads with a floor slab cast between them, it permits a far more straightforward waterproofing solution, likely to offer overall savings, while simplifying the construction sequence.

Atop the slab, the waterproofing strategy will normally use a combination of liquid-applied and sheet membranes to achieve the best solution. This said, the most efficient designs will always consider each projects proposed construction sequence and the building materials selected. A rugged two-coat product is often used to create a strip along the line where the frame is to be erected: allowing resin anchors to be installed as required, but maintaining the integrity of the

damp-proof membrane (DPM), before the sheet applied membrane overlaps the strip to complete the waterproofing barrier and/or gas protection.

In addition, project design and delivery teams should also bear in mind the likelihood that no matter how much thought and effort has gone into the pre-construction period, major contracts almost always throw up unforeseen situations which have to be swiftly dealt with. This is where the experience of the waterproofing specialist will pay dividends, in being able to provide on-site assistance, including technical or installation advice and, often, customised CAD drawings or hand sketches of workable details.

There is no doubt that, supported by its sustainability credentials and the work of industry bodies such as the Structural Timber Association, the use of engineered wood is set to increase still further. Accordingly, through the use of correctly specified and expertly applied waterproofing solutions, such structures will be able to perform for the full extent of their design life.

Alex Massingham is national sales manager for RIW