Parapet Walls?

The right specification can ensure a correctly protected structure and one in which the structural continuity is not diminished because of dpc slip planes.

 

Parapet cavity walls behave differently because both faces are exposed. Such dual exposure dictates the protection against damp penetration takes into consideration more variable elements than a conventional cavity wall. And if the parapet is curved, the behaviour of the masonry itself can be more problematic if certain aspects are ignored.

 

When a traditionally constructed parapet wall saturates with rain and in extremes of heat and cold, expansion of the masonry can prejudice the masonry structural continuity. When this occurs, the dpc bedding course in a parapet offers the least line of resistance and acts as a slip-plane, so movement at dpc level in the outer and inner skins is commonly witnessed.  Where the amount of expansion is not fully compensated by contraction, progressive movement over the years will occur mainly as a result of ratcheting.  (Where the expansion movement is not fully matched by the retraction movement, and masonry above and below the dpc progressively moves out of alignment). This is easily identified initially by witness lines and eventually by horizontal cracking along the courses in which the parapet dpc is incorporated.

 

Curved parapets can accentuate the problem especially if the curve is between two resilient structural points. Thrust direction can then move at 90º to the arc and a radius eyebrow ledge can result. Such movement is commonly accompanied with coping dislodgement where coping end pinching occurs.

 

Standard detail depicts an outward stepping dpc supported in both the inner and outer skins of the parapet wall. Using a preformed Type P parapet cavitray by Cavity Trays of Yeovil offers a different approach. The Type P is already profiled and self-supporting, so requires building into one skin only. It provides the requisite outward stepping profile but is enveloped in one skin only, so the opposite masonry skin can remain fully bonded. Consequently the solidity of parapet arrangement is considerably improved.

 

A spokesman for Cavity Trays of Yeovil explained: ‘Originally the British Standard showed an inward stepping dpc rather than an outward one and initially parapet walls built to that detail functioned correctly. But in time parapets tend to crack along the bedding courses and eventually water would permeate the outer skin and track inwardly on the underside of that inward stepping dpc. As soon as that tracking reached the inner skin, the damp problem became visible.

 

Our approach was the opposite.
We designed an outward stepping arrangement - the approach subsequently adopted within the latest British Standard.  However, limiting our design to being built into one skin only, improves the structural integrity. And externally the Type P has a projecting turn-down lip the shelters the flashing/dpc bed – a further protective feature not possible when using conventional roll dpc.Parapet design must embrace the coping also, as coping joints eventually weather and fissures provide entry opportunities for water.  We offer an accompanying product for use under copings’.

 

Parapet walls need not be problematic.
The right specification can ensure a correctly protected structure and one in which the structural continuity is not diminished because of dpc slip planes.

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