Determining Damp Course Dimensions

Growth Rate of 2.5 mm per Year!

It’s unusual to provide a lineal measurement to demonstrate trend growth, but 2.5mm per year is approximately the extent some dampcourses have grown in size over the past few decades. To put it another way, the original Code of Practice 121 for building showed dampcourses and trays in cavity walls rising in the cavity just 75mm – or 3inches as it was then. Today the total rise within the cavity wall is usually required to be not less than 150mm – a 100% increase.

The same might be said of cavity widths. When CP 121 was in place the cavity width was normally 50mm / 2 inches. That measurement has similarly increased, and today 100mm is one of the most popular dimensions.

It was Cavity Trays Ltd, the specialist dpc and tray manufacturer that promoted the larger dpc dimensions, demonstrating that the measurement that then featured within Code of Practice CP121 was inadequate. Similarly the Company proved that damp course trays made of the right material did not require building into both skins, as trays could be pre-shaped and self-supporting.

Cavity Trays Ltd also recommended that some British Standards were in need of review and a more functional and reliable build detail could be achieved by changing the shape of some dpc arrangements. Subsequently dpc dimensions increased and shapes changed.

A good example is the original BS parapet detail that showed the dpc built into both skins and stepping inwardly. Thus the entire parapet was structurally separated and sitting on dpc. Such parapet construction was prone to what is termed ratcheting – where masonry expansion and contraction are not equally matched and movement /cracking results – particularly along the dpc bedding courses. Wind-driven rain could penetrate such movement weaknesses, then under-track the dpc and gravitate downwardly towards the inner leaf.

The alternative pioneered by the Company maintained structural bonding in one skin and eliminated the under-tracking weakness. This was achieved by reversing the dpc shape so it stepped outwardly and manufacturing in a material that held its pre-shaped profile, so it required building into one skin only.

Back to the subject of the extent a dpc rises in a cavity and according to Cavity Trays Ltd 150mm is the minimum one should see in construction today. It quotes past tests in which the Company compared performance in a variety of simulated and practical on-site appraisals.  “Whenever we are asked our opinion of a problematic construction suffering damp ingress we first check the dimensions inside the cavity” commented a spokesman. “We have encountered many cases where the culprit has been inadequate dpc base and height dimensions to service the cavity width. If the rise in the cavity is not 150mm then it is below the currently accepted standard”.

When asked to comment about possible changes in the future, a spokesman for Cavity Trays Ltd advised the Company had already (in 2010) increased some dimensions in anticipation of continuing trends with cavity widths over the next decade. “As the cavity widths grow wider, the angle of genuflection reduces and greater upstand dimensions are required to reach the minimum rise requirements called for by the latest British Standards and NHBC directives. It’s important to get the details right and we design to accommodate the most onerous of site conditions and applications”.


© Cavity Trays Ltd 2009.