– Pitching of roof panels: All flat roofs should be installed with a minimum pitch of 1:50. This accounts for the deflection of large roofing panels, helping to prevent water pooling. Pitched panels also facilitate roof drainage in the event of waterproofing membrane failures, minimising damage during installation or repair. – Focus on temporary and permanent rainwater outlets (RWO) design: Rainwater outlets (RWOs) and downpipes may be part of the permanent rainwater system or serve as temporary solutions. If RWOs are positioned above the roof insulation in the final design, they may sit too high to drain water during construction. For flat CLT roof terraces and floor slabs, rainwater can track across the surface, pool, and seep through slab joints unless the joints are extensively taped. – Focus on roof panels service penetration design: Service penetrations should generally be minimised and located in designated areas treated with end-grain sealer at the factory. Even small holes and cut-outs, whether temporary or permanent, require attention. For instance, lifting holes must be sealed with wood plugs or taped immediately after assembly. Temporary holes used only during installation can be sealed with breathable tape, while larger holes can be covered with a breathable membrane and tape, temporarily closed with plywood sheets, or used as temporary rainwater outlets. – Focus on identifying system failures to ensure prompt remediation: Install tell-tale drainage points (small holes in the timber roof structure) that allow water to drain through to the ceiling’s internal face, making Common Perception: Timber structures are too susceptible to water damage, making them a risky choice for long-term use. Reality: While timber is more sensitive to moisture than some other materials, this risk can be mitigated with proper design, installation, and maintenance. Effective moisture management strategies ensure timber structures remain robust and durable.
water ingress clearly visible to occupants. Place leak detection systems at the roof’s lowest points, which will notify building management if water starts to pool. Install moisture sensors at the lowest edges of roof panels to alert the building management system if timber moisture levels exceed 14%. Capillary action – wall bases Wall bases are vulnerable to deterioration from capillarity action when in contact with a wet substrate, especially at the ground floor level where they may be exposed to severe wetting from standing water and groundwater pooling. This risk should be addressed during the design phase, with the following mitigation measures incorporated into the design. – Specify a DPC to the underside of timber wall panels where these are in contact with grout, screed or concrete to prevent moisture rising via capillary action through wall panels. However, relying on damp or water proofing membranes is not enough as most of them have a shorter lifespan than the structure itself and may not be capable of fulfilling their function once they deteriorate. A secondary mitigation measure should also be employed. – Apply end-grain protection to wall panels in contact with the ground. – Raise timber components from the external ground and finished floor level where water could potentially accumulate and trigger a localised wood decay process over time. In the UK, there is no specific regulatory guidance on how or by how much the timber should be raised. Documents such as STA good practice and the BM Trada technical sheets propose the minimum support height of timber (or any other degradable material) is 150 mm. Escape of water - leaks In timber buildings, special consideration must be given to the design and monitoring of water carrying systems such as heating, cooling, fresh or wastewater and sprinkler systems. These are the main sources of leakage, causing localised but significant durability problems to the timber structure.
DURABILITY
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