In traditional building the masonry of the inner skin is dry above its dpc level. The problem is the interfacing of any flooring arrangement terminating at threshold level commonly results in the depth of that arrangement descending below that dpc and coming into contact with the adjacent wet external leaf.
So doing provides an opportunity for damp transference upwardly and inwardly. Additionally the inner skin reveals of any opening must unite the reveal vertical dpc with the dpc at oversite / flooring level and the dpc at threshold level.
The correct coming-together of these three elements is essential. Level thresholds that fail to address all the above are likely to be problematic in the future.
Whether the abutting floor is constructed of timber or concrete, eventually any damp ingress will affect the structure, its surface and its decorative finish.
To remove such risk one can build in a threshold dpc. The Type LTT (Level Threshold Tray) by Cavity Trays of Yeovil may be introduced onto the inner skin during construction.
It can also be applied to the exterior skin so both are enveloped. The function of the Type LTT is to act as an extended dpc hat that covers the top and the sides on the masonry.
So doing permits the flooring arrangement to interface with those skins at threshold level. But because the masonry is enveloped, dampness cannot translocate.
Where the Type LTT meets the reveal masonry either side of the opening, it does not terminate but continues upwardly and around both faces of the reveal skin, extending into the actual cavity.
This permits reveal closing using a cavicloser to descend and overlay the protected masonry. If moisture is not isolated, dampness and calcium sulphate from building material can be translocated and can subsequently manifest on the wall surface as efflorescence.
Whilst hygroscopic salts in plaster are not a significant structural issue, any dampness will catalise eventual deterioration. Any actual moisture scource will translocate salts - nitrate and combinations of chloride salt being typical.
Where such ingress is at flooring level and the reveals, rapid deterioration is enevitable. The gravimetric moisture content of building materials varies, dictated by differences in density and a materials ability to contain and bind water hygroscopically.
BRE Digest 245 provides guidance and the formulae for determining moisture content. Defined as the total gravimetric moisture content less the hygroscopic water, it provides a useful indicator of water available supporting deterioration and biodegradation processes.
The British Standard released in December 2009 (BS 8102) provides the Code of Practice regarding protection from water from the ground. The Type LTT by Cavity Trays of Yeovil promotes a route to compliance. v
In isolating one of the areas most susceptible to damp transference, the Type LTT permits abutting flooring, abutting insulation and dpc’s / membranes to continue over an inside skin and converge with an exterior skin that itself is isolated from dampness at the point of convergence.
Cavity Trays Product Guide number 21.
British Standard 8102.