Cavity Impingement Affects Most Construction
Impingement of the cavity width occurs naturally whenever a cavity wall is constructed. The specification will call for a minimum cavity width but it is rarely maintained for three principal reasons:
1)Bricks manufactured to the European Standard BS EN 771-1 are permitted to vary in size. The tolerance within the R1 category allows a brick 102.5mm in width to have a 6mm tolerance (mean range measurements over given number of bricks). The mason levels and aligns the brick face so the variance in width affects the back of the brick – the cavity face.
2)The internal skin is usually built of blocks, and these also have permitted tolerances.
3)Both skins are reliant on the mason maintaining accuracy and alignment. In practice, some irregularity will occur. NHBC Standards 1.2 S1 for example stipulates a ±8mm maximum deviation in on plan straightness measured over any length of wall up to five metres.
All three aspects affect the resultant wall and the cavity width can be impinged. This can affect the actual cavity space available into which any intended dpc arrangement must fit. Investigations carried out by Cavity Trays of Yeovil have highlighted the advantage of dpc arrangements having clear cavity compartment areas.
Approved cavitrays have such open compartment areas - free of mouldings and protrusions that otherwise would encroach the water arrestment and flow path.
‘As soon as you introduce protrusions into the cavity they provide opportunity for mortar to become trapped’ commented a Company spokesperson. If the masonry protrudes into the tray, the remaining tray space must be sufficient to permit mortar to be easily removed and arrested water to free flow. One must also anticipate mortar evasion will be present on the inside face of the external skin.
Cavity protrusion caused by the shape of the actual dpc tray can be hinder / restrict the evacuation path. If you have impingement from both these aspects – masonry and tray - the result could be far from ideal’.
Approved cavitrays are moulded using a polymer dpc that is sufficiently substantial and robust to maintain shape and provide a completely open and unobstructed cavity compartment area. During heavy rain conditions, up to 2.25 litres of water per square metre can permeate and end up running down the internal (cavity) face of the external skin. Where arrestment of water is required, an unrestricted arrangement promotes efficient functionality.
Cavity Trays Ltd is the longest established in its specialised field and advises an open compartment area is ideally placed to accept masonry module variances and workmanship variances – and permits easy compliance of the NHBC directives and Euro Standards. A product performance undertaking accompanies its approved cavitrays.
Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU
T: 01935 474769
F: 01935 428223