Cavity Wall Openings, Wider Cavities and the See-Saw Effect

Anticipating movement expansion and contraction movement is necessary when constructing cavity walls consisting of a timber inner skin and a masonry outer skin.

The differential movement potential between skins increases as the building gets higher and also as the cavity gets wider.

Wherever there is connection between inner and outer skins the likelihood for movement conflict manifesting as cracking increases.

The most common connection areas are window and door openings.

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 thus move downwards. (See UKTFSA publication for advice).

Much of this movement occurs in the months immediately following construction of the wall, which is when a cavity closer with movement provision can be used to advantage by helping limit the eventual cracking potential. 

The WCA Cavicloser (WCA stands for wide cavity applications) uses ties to secure one side of it to the outer masonry skin. 

The other side of the closer can be screwed or nailed through its edge moulding into the timber inner skin.

This edge moulding has a friction fit relationship with the rest of the closer body, so movement from one skin need not be fully conveyed to the other.

The arrangement can thus contribute to the limitation of initial potential anticipated movement across the reveal.

It can also help when a cavity width gets wider.

Movement of skins in opposite directions creates a see-saw effect with any connecting medium.

As a cavity gets wider, so can the apparent manifestation of movement at the pivot point between them.

The Type WCA with friction fit movement provision is now available to suit cavity widths up to 330mm wide,

and is one of a range of closers offered by Cavity Trays of Yeovil.

See it at the forthcoming Build Show during Construction Week  -  NEC Birmingham – 6th, 7th & 8th October 2015.

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