CPD Specifier May 2015 issue January 2015 | Page 21

Walls

Cavity Wall Lateral
Transference The Sideways
Soak
Lateral transference is the term used to describe wet ingress
progressing ‘sideways’ and in so doing permeating parts of a
structure that are intended to remain dry. The traditional bay
window is a good example of part of a building susceptible to
lateral transference.
Before going further it is prudent to remember absorption
testing standards of masonry do not replicate the severity of
rain experienced in the UK. For example masonry tested to BS
4315 is often described as indicating ‘very low levels of rates of
rain penetration’. But is this a realistic description where the test
involves spraying water on to masonry for one minute only, at half
hourly intervals for 48 hours? This equates to 96 minutes spread
over two days - with consistent drying periods between each
spray.

The bay roof intersection level and the bay lintel spanning level are separated by a number of courses

In comparison the UK weather experienced during the winter
of 2013/14 subjected structures to continuously long periods of
wind-accompanied saturating rain - with few intermittent drying
opportunities.
Structures should be built with accompanying DPC’s in place to
accommodate the worst case scenario - long periods of winddriven rain that really saturate the masonry.
With regards to the bay window, as bay fascia/facade heights
increase, so do the number of courses between the bay roof
intersection level and the bay support lintel level. The greater the
vertical distance between them, the greater the bay susceptibility
to lateral transference because the exterior masonry skin on
the outside of the bay runs uninterrupted onto the support lintel
that is inside the building. The construction permits a ‘sideways
soak’.

Whether a flat or sloping roof, the absence of protection linking the bedding level
of the roof tray/flashing with the bay-spanning lintel bedding level
leaves the structure susceptible to horizontal transference

Horizontal transference between intersection level and lintel level can be
prevented by building in a preformed DPC cavitray that vertically connects
two levels within the same masonry skin without adversely interrupting
bonding or coursing. Its presence prevents horizontal damp transference.

the lowest cavitray at roof intersection level. Wind-driven rain saturating
masonry to the sides of the bay is thus prevented from continuing soaking
inwardly where the exterior skin becomes an internal skin inside the bay.
When used with approved tray types X, G and C, the Type BWVC forms
part of a warranted performance system for the benefit of architect, builder
and client. Full details are in the latest Technical Manual from Cavity Trays
of Yeovil.

Called a Bay Window Vertical Cavitray and manufactured by Cavity Trays
of Yeovil, Type BWVC units are handed and available to suit brickwork
and block work coursings. Units are introduced at each end of the lintel
and provide a cleverly shaped permanent DPC connection upwardly to

CAVITY TRAYS OF YEOVIL

TEL: [phone]

WWW.CAVITYTRAYS.CO.UK

READER ENQUIRY NO: 25

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