Shrinkage Movement
All timber frames decrease in height over the first 2 years of use. During construction this is primarily caused by the frame tightening up under load as finishes are applied. Following completion the primary mechanism is a decrease in cross grain timber section due to moisture content loss from the original moisture content and the final equilibrium content of approximately 10%. For this reason it is good practice to put roof claddings on and load out at least some of the internal finishes before advancing too far with masonry cladding.
Whilst this change in height is not discernable internally it is necessary to accommodate the movement where it occurs differentially against the external cladding. Additionally brick cladding tends to expand in height which also needs to be considered during the joint design.
Prestoplan has developed a "low movement" frame solution to minimise the movement joint sizes required below roofs, windows and flues. This has been achieved by a combination of minimising cross grain timber, and the use of "engineered timber" of low moisture content in floor ring beams.
As a matter of policy, Prestoplan uses "Ultra joist" ring beams on three and four storey projects (except three storey housing). "Ultra joist" is a super dried timber with an average moisture content of only 14% compared with the 20% of ordinary kiln dried timber. On five storeys and above, Prestoplan use Glulam ring beams with only 10% moisture content.
This overall reduction of moisture content combined with the reduction in cross grain timber significantly reduces the degree of downward movement. Nevertheless Prestoplan advises all Clients to take full advantage of this attribute by selecting a stable cladding with low thermal and moisture expansion characteristics.
We recommend that joints are sealed using a "Compriband" type strip for proper joint performance. (Silicone sealant is not a suitable material). Architects also need to consider joints within stairwells where full height plasterboard finishes need to allow for height reduction.
As part of our design service we provide specific guidance on joint sizes and positioning. In addition the UKTFA has an excellent publication written by our Technical Director providing detailed guidance on areas to be considered , good design practice and how to avoid problems on site. (Refer to the UKTFA website)
