While the recession continues to rumble on, there are signs that a number of timber frame projects that stalled several months ago are now restarting. This is good news indeed, but it is important to review each project carefully before construction begins again, to identify any damage or deterioration to the structure caused by long-term exposure to weather.
Short-term wetting of timber on site is not the issue here. Provided timber is sufficiently dry when it is delivered, it will quickly dry down again following exposure to rain.
What happens then to building materials, designed to be used within the building’s weathertight envelope, when they have been exposed to the weather for several months? One of the main concerns about exposing timber products long-term to the weather is that it raises moisture content over the decay threshold of 20% for a long period. This applies equally to the timber frame in a building that has not been made weathertight and to any timber products stored on site unprotected for an extended period.
Areas of a building such as sole plates and timber floor/roof decks are particularly at risk from long-term wetting since these are poorly ventilated or drained. Also, areas of failed weather protection, such as holes in roofing membranes, incomplete window installation or defective temporary downpipes can zone water into particular areas, increasing wetting locally. Trapped water pooling on or against timber will raise its moisture content to levels where decay can begin. If buildings have progressed to being insulated, or have had linings installed before a weathertight shell has been achieved, then these materials, once wetted, will elevate, then maintain, high moisture contents in adjacent timbers.
Often when building work stops, there is no time or money available to provide adequate temporary protection. Achieving a weathertight shell, yet allowing for ventilation, is therefore an important first step towards restarting a project and assessing the structural integrity of the timber frame.
There is a perception that wetted timber may lose its strength as a result of decay if its moisture content remains above 20% for an extended period. This, however, is not inevitable, as framing timbers are provided some protection by preservative treatment. An independent inspection, for example by one of TRADA Technology’s consultants, can identify whether decay is present and whether the frame is still fit for use.
Even if some decay is present, by assessing its extent it may be possible to arrest it through treatment with a remedial preservative until the timber can be dried to below the threshold where decay will be stopped.
Various drying techniques can be used to bring down the moisture content of timbers wetted on site. This may involve stripping back areas of the building to expose timber surfaces and introducing air movement to ensure the air humidity is kept low adjacent to timbers. Again, an experienced consultant can advise on the most efficient, cost effective way to dry out a timber framed structure, so that other site work can proceed as soon as possible. Care must be taken if the building work continues to close up the structural elements before the timber has dried down to 20% moisture content, as there is a risk of decay. Moisture content measurements can be taken on site using insulated electrical probes that allow for measurement of moisture throughout the timber section, or if a wholly accurate reading is required, samples can be removed for oven drying and weighing.
TRADA guidance on storing wood-based panels on site states that external storage should be avoided wherever possible to avoid damage from wetting or high humidity. Panels should be stored flat and dry, off the ground to avoid any contact with moisture. Where possible, panels should be fully supported or, where this is not possible, on battens at maximum 600mm centres. In the case of thin panels (6mm or less) it is recommended that they be supported on a thicker panel (for example, 18mm) under the whole area.
It can be difficult for panel product manufacturers to guarantee the structural integrity of their products once they have been exposed to moisture for more than a short term. Localised cyclic wetting and other site conditions can cause variable deterioration.
Prolonged wetting of board products such as OSB and chipboard may cause swelling and loss of strength. As part of a research project carried out by TRADA, extensive testing into the performance of sheet materials in contact with water was conducted. Most of these tests exposed the materials to wetting for long periods. While this may not resemble the real life situations experienced on stalled sites, the research helped to identify ways in which samples may be removed from site and tested in the laboratory to ascertain the strength and other properties of whole sheets. Taking samples from site for laboratory testing can help ascertain structural performance and while it may seem costly to enlist consultants to carry this out, the ultimate savings on cost and time can easily outweigh the outlay of their replacement.
Of course, it is not just timber that can deteriorate with long-term exposure to the British weather. UV degradation of breather membrane reduces its performance, mineral wool cavity barriers can become dislodged and saturated, fixings may corrode and plasterboard wall linings disintegrate. For many of these materials, the practical and economic answer is to remove them and, once the building is weathertight, replace them.
The longer a building is left exposed to the elements, the greater the risk of damage. Quickly establishing the condition of the structure, planning the replacement of damaged materials and achieving a dry, weathertight shell are all key stages which have to be handled effectively, seeking expert advice where necessary. It may be all too tempting to start building again as soon as possible, but in our experience time spent reviewing the project first is time well spent.
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A floor deck showing green mould where more extreme conditions have allowed growth |
TRADA Technology intends to repeat its successful seminar ‘Restarting timber frame projects’ aimed at timber frame manufacturers, builders, contractors, warranty providers, local authorities and housing associations later this year. Contact rlancashire@trada.co.uk to register interest or visit www.trada.co.uk/framecheck for further details on timber frame consultancy.
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It's raining inside! |
