Leaving frames exposed to the weather
April 07, 2020
In the event that you may need to pause the construction of a house for an extended period of time, due to circumstances beyond your control, you may have questions regarding temporary bracing of structure to ensure it remains unaffected.
Another question may be in relation to exposure of frames and other materials such as structural flooring that may be open to the external environment and how long is permissible before it effects the structural integrity of the building.
This information sheet provides guidance on these matters which has been sourced from relevant industry sectors and product manufacturers.
Assessing the effects of moisture to timber framing
Ideally, covering the frame for the duration of any extended period of time left exposed or reaching for example partial lock up to ensure the weatherproofing of the structure would be best case to reduce the impact of moisture, however this is not always possible or feasible.
The last thing also you want to be doing is having to constantly be coming back to check and adjust tarps.
When does timber framed construction need to be assessed for moisture impacts when left exposed for extended periods of time or after extreme weather events? There is no simple solution for this. Each individual site would need to be assessed for the potential degradation of timber components and any rectification would then need to be considered.
Items that can affect a timber structure, but are not limited to:
- Time of exposure
- The protection of the structure
- Amount of rain days
- Time allowed for timber to dry (sun exposure)
- Climate temperature.
When a partly built structure is exposed to the weather the level of degradation will depend on the effects listed above. Some timber may need to be replaced after a few months of exposure where other parts may be sufficient for use after 12 months of exposure.
Where timber has 20% or more of moisture content for prolonged periods of time, fungal degradation or decay may be present, especially in untreated timber. Fungal degradation should not be confused with mould, this would only be on the surface and would not affect the underlying timber.
Lightweight metal wall and roof frames
Metal wall and roof frames generally have an aluminium/zinc/magnesium alloy coating, this is similar to a metal roof that is designed for outdoor exposure where there would be little impact if frames are exposed for a few months.
If the structure was close to breaking surf, washing of the frame after a period of weather exposure before adding cladding and internal linings is recommended to remove any salt deposits.
The two most common types of sheet bracing used are structural plywood and orientated strand board. An important part of the sheet bracing which requires it to be fit for purpose are the areas were fixings are placed and at the sheet edges.
If these are to delaminate it may affect the structural adequacy and the sheer capacity of the fasteners and require partial or replacement of the sheets in more extreme circumstances.
As soon as particleboard flooring has been fixed it should be protected from the weather. If this is not possible it is recommended that the flooring not be left uncovered for more than 3 months.
However, if the 3 months is exceeded the flooring manufacturer should be called upon for an opinion on if the boards are still satisfactory.
Exposed Metal Connections
Metal components used in structural timber applications shall be corrosive protected suitable for the particular conditions of use. The minimum protection as per AS 1684-2010 describes a protection of Z 275 galvanised. In the short to medium term (6 months) there would be minimal concern of the connections becoming corroded.
When timber is repeatedly wet and dried this can lead to the nailplates ‘withdrawing’ from the timber due to the timber contracting and expanding in the timber. It could be possible for the plates to completely remove from the timber. If a gap of 2mm> occurs the integrity of the structure would be affected.
Straps used in the structure may become loose due to the expansion and contraction of the timbers from continuous wetting and drying. These all would need to be checked on returning to site.
When leaving a site for extended periods of time there needs to be adequate temporary bracing applied to the structure. AS 1684-2010 describes temporary bracing shall be equivalent to at least 60% of the permanent bracing required.
Also temporary bracing may form part of the installed permanent bracing. When returning to site the temporary bracing may have moved causing walls and roof structures to no longer be correctly aligned or level.
Steel beams, columns lintels etc.
It is important that if the frame is to be exposed to the elements for some time that all structural steel is protected against corrosion. This can be achieved by ensuring the steel is provided with suitable coatings. These are specified in either the NCC or in accordance with relevant Australian Standards AS/NZS 4791, AS/NZS 4534, AS 1397 and AS 1214.
All metal used in structural timber connections shall be provided with corrosion protection appropriate for the particular conditions of use and taking into consideration weather exposure, timber treatment, moisture and presence of salt.
Prior to recommencement of work
Where the frame has been sitting for some time and exposed to the elements either partially or fully it is recommended that the structure be inspected by a suitability qualified person before re-commencing work.
For further information HIA members can contact HIA’s Building Services team on 1300 650 620 or email firstname.lastname@example.org