Designing for timber frame with render Timber frame construction is on the rise in the UK, with shorter build times and cheaper materials meeting a demand for more housing. When rendering timber frame the problems of fire risk, insulation and frame movement can be overcome with the right input at the planning stage through to application. Here, Adam from the technical team at SAS Europe takes us through some of the challenges and solutions for rendering on timber frame.
The use of render carrier boards
Choosing a render solution for timber frame was once a choice of either a block skin, or render onto lathe, but these both had serious disadvantages that could often deter architects from specifying timber frames with rendered facades. Issues with window frames moving and cracking were once seen as a necessary evil when rendering timber frames. Now, we are seeing a rise in the use of render carrier boards that make it easier to render timber frames. Modern render carrier boards can still pose problems for timber frame constructions if you do not pay attention to the correct combination of board, renders, thickness and detailing.
Battening and timber grain
Often overlooked, some seemingly unimportant things can make the biggest difference when rendering timber frame. Simple things like: batten widths, batten direction, expansion, contraction, cross grain timber, ventilation, fire, fixings and flexibility can all play a part in defining a good system. Using the incorrect batten width can often lead to splitting of the battens, and to reducing the efficiency of the board fixings as they can be too close to the batten edge. The correct batten widths can be obtained from British Standard BS 5268-2:2002 part 2 section 6. This provides valuable information to allow more effective and stronger fixing of render carrier boards. Three imperative design considerations to consider when rendering timber frame are: expansion, contraction and movement. A common practice that can often hinder the performance of a render system onto timber frame is cross battening. While essential for hanging tiles, it can have drastic implications on a rendered system as the cross battening hampers the natural movement of the timber, causing undue stress on the render and inciting cracking. Movement of cross grain timber at floor levels needs consideration. This can lead to cracking, separating and spalling render at floor zones as the timber frame moves more at areas of cross grain timber. Eliminate any issues by including joints in the render at these areas that can accommodate for the lateral movement.
A rainscreen cladding solution
Adopting the principles of rainsceen cladding with a rendered timber frame system will create a fully ventilated and drained cavity that can remove any moisture and condensation that can occur, eliminating damp, mould and rot. The ventilated cavity also reduces the temperature difference between the render surface and the air, minimising surface condensation that subsequently leads to less algae, biofilms and metabolites thus reducing the frequency of the render’s maintenance cycle.
With a ventilated cavity can come the risk of fire spreading undetected to other floors. Intumescent materials that expand and swell as a result of heat exposure can be utilised in the cavity at floor levels allowing the cavity to remain clear and can seal the cavity in the event of a fire, stopping the spread of fire to other floors. Here at SAS (Europe) Ltd we have dedicated years of knowledge, research and development into creating the perfect system for timber frame. We have studied the British Standards and other publications, assessed the rate at which timber expands, contracts and moves, adopted the principles of rainscreen cladding, performed countless tests with some of the UK’s leading test houses and obtained BBA certification to produce ProWall
– the ultimate render system for timber frame.