Timber-frame construction requires the designer and builder to be aware of and make provision to accommodate differential movement between the external masonry skin and the internal timber skin. The extent of total movement increases as the building gets higher, and also as the cavity width gets wider. Wherever there is connection between inner and outer skins there is potential for movement conflict to manifest as cracking. The most common connection areas are window and door openings (reveals and sills).
The way in which the reveal is closed can reduce the potential for cracking. There are now Caviclosers with integral movement provision.
The WCA Cavicloser (WCA stands for wide cavity applications) uses accompanying ties to secure it to the outer masonry skin. The side of the closer nearest the inner leaf has a friction fit relationship with the rest of the closer body. This side can be screwed or nailed directly into the timber inner leaf through the face flange. The arrangement can thus benefit expansion and contraction provision.
The wider the cavity width the greater the differential movement potential. This is because of the see-saw effect wherein upward or downward movement becomes more pronounced as it becomes more distant from the pivot point.
The differing expansion and contraction rate between timber and masonry can be generalised: as a rule of thumb a masonry external skin has a tendency to slightly rise as it dries out (2.5mm per storey of clay masonry) whereas an internal skin of timber will shrink and move downwards. (See UKTFSA publication for advice.)
Externally if the construction features a masonry or tile sill to the window opening there is potential for movement to fracture or crack the arrangement. The obligatory expansion gap (subsequently soft-pointed using a flexible sealant) is required under every sill observing the NHBC recommendation. The sill itself when created using a preformed Undersill Cavitray will benefit integral damp arrestment plus further movement provision can be provided via a variable link where the Undersill Cavitray passes through the cavity.
With a range of caviclosers and cavitrays for traditional and timber frame construction accommodating cavity widths up to 330mm, Cavity Trays of Yeovil has a unique range of solutions. On display on stand A118 at the National Homebuilding and Renovating Show.
