MATERIALS: SUB FRAME TIMBERS
The material used for the sub frame is as important as the deck boards. The timber in this case was a north European white wood (Spruce). It was pressure treated but we couldn’t establish who by and to what level. Without this information we were unable to establish if the sub frame was fit for purpose. We recommended that the client go back to the supplier and ask for confirmation of the standard of treatment in writing.
This is an issue we see time and time again. Pressure treatment is tailored to the end use of the timber, the key standard of relevance being BS 8417: (Preservation of wood – Code of practice). Joists intended for internal building use, which can easily be mistaken for timber decking joists, are not suitable for outdoor use. If used, the service life of the deck structure is unlikely to meet expectations.
As from January 2020, the TDCA recommend that deck joists or any timbers providing exterior structural support should be preservative pressure treated to Use Class 4 - regardless of whether they are in ground contact or not. This is to ensure the enhanced durability and safety of the deck substructure and is in line with BS 8417 (Preservation of Wood: Code of Practice) which is to adopt this change in due course this year. See our blog article here.
The joist timbers were stamped with a CE mark. It stated that they were structurally graded to C24 which is good. Paperwork that accompanies a CE marked product – a declaration of performance or DoP – will state where the timber can be used.