Where Timber Frame and Floors Meet - Best Practice
Your Risk and Cost Considerations
The way in which timber frame construction and SIPS construction integrate with concrete foundations and floors is flawed. Reliance is placed on a number of separate elements working together as one, but they are actually unconnected so the resultant build detail cannot provide optimum functionality.
At ground level the operative is required to establish three protective dpc’s: One in the outer skin, one in the inner skin and the oversite membrane. But continuous protection requires all three to be linked, and failure to do so means the build detail has three principal weaknesses.
The three principal weaknesses are damp ingress, hygroscopic absorption and contaminated land gases entering the building envelope.
How the last of these weaknesses is best eliminated is currently the subject of the review of Part C of the Building Regulations which is focussed on making new buildings safer for the inhabitants.
The inner skin dpc relies on correct placement to act as a barrier against rising damp. The oversite membrane does not physically link up but merely terminates adjacent to the dpc by turning up against the inner skin. Consequently, contaminated land gases can enter the building envelope via the most common entry point – the floor/wall junction. There is no connected protection and settlement and movement cracking occurs - albeit often hidden under the skirting. Pending the placement accuracy of the inner skin dpc, this is also the area where dampness can be experienced if the inner skin dpc positioning is inaccurate. But contaminated land gases can also enter by rising up through the cavity and the current Part C consultation data refers to oversite membranes linking with protection in the wall. By having separate dpc’s in the inner and outer skins, there is nothing to address the cavity when building the conventional way.
The alternative approach offered by Cavity Trays Ltd is called the Type TFC – Timber Frame Cavitray. It combines the inner and outer skin dpc requirements and does so in a way that offers several important accompanying benefits. The one-placement - one-solution Type TFC establishes unpunctuated protection.
It protects the outer skin and then rises transversing the cavity and at a higher level projects inwardly. The inward projection extends through and beyond the inner skin, providing substantial inboard presence to unite horizontally with the oversite membrane. All the separate unconnected weak areas associated with separate dpc’s are eliminated.
The arrangement also arrests rising contaminated gases from progressing upwardly in the cavity. Instead they are safely discharged out of the wall via cavibricks located below TFC level around the base of the building. The cavity spanning portion means water penetrating the building envelope at higher level is arrested on top of the barrier where caviweeps located in various perp joints provide evacuation routes. This arrangement eliminates volume pooling within the base of the cavity, and this can be advantageous if the land under the building offers poor drainage or is of rock or has a high water table.
Within the cavity the Type TFC profile has a vertical projection that locates against the inner timber frame skin. Apart from aiding optimum placement, this laps and covers the timber that frequently remains partly exposed to the cavity along the bedding course. Hygroscopic exposure of the timber to the usual cavity moisture is thus prevented.
By combining the dpc requirements for both masonry skins with a cavity barrier that extends upwardly and inwardly, the resultant Type TFC provides functionality, an improved standard of build detail and an arrangement that offers greater protection against contaminated land gases.
The Type TFC is manufactured from gas-grade resistant Petheleyne in various profiles. The inboard projection can also be cranked to suit different floor/slab arrangements. Profiles are semi-rigid and supported with a range of preformed angles and steps to suit. Adjoining sections link using gas-grade anchoring strip. Non-standard profiles and angles are normally available within five days.
The current main technical manual of Cavity Trays Ltd is number 22.
Details of the Type TFC are on page 30 in the current Book of Wise Decisions / Best Practice Guide, volume 40.
Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU
T: 01935 474769
F: 01935 428223