If the blackbutt screen is required by regulations to prevent embers from accumulating under the floor, the screen would have to be a lapped cladding profile with sub-floor ventilation openings protected with mesh. Fixings would be to suit the cladding profile. Alternatively, 70 x 30mm timbers could be spaced apart with mesh protecting the gaps. The mesh must be corrosion-resistant and have a maximum aperture of 2mm. These requirements only apply if the house floor is less than 400mm above finished ground level. If the screen is not required to prevent embers, ie. the floor is more than 400mm above ground, or protected in a different way, a size of 70 x 30mm in kiln-dried blackbutt should safely span up to 2.1m. For best results use 4.2m lengths over 3 supports.
Australian Standard 1684 Part 2, Residential timber-framed construction, contains the following requirement: Boards over 85mm cover width shall be fixed with a minimum of two face nails at each joist....nails shall be skewed slightly to the vertical in opposing directions....the mechanical fixing shall be supplemented by a minimum 6mm bead of polyurethane or polymer flooring adhesive to the top surface of the joist (Clause 5.5.2.2). The problem with departing from this requirement is that if anything goes wrong, it will be pointed out that you haven't complied with the Australian Standard. Our Technical Design Guide #09 on flooring installation goes into the subject in more detail and may be helpful. A copy can be downloaded from our website free of charge via this link https://www.woodsolutions.com.au/publications.
Sending an SOS to the Wood Solutions team. Last June we had a custom spotted gum door made. By end of summer it had split at multiple ship-lap joins. We designed the door to have an internal steel frame (to eliminate cupping / warping etc). We used Hurfords (Coulee profile) Spotted gum cladding as internal & external face. The cladding sandwiched the steel frame & whiteboard core. The spotted gum was ran through a thicknesser down to 14mm to achieve the required overall door thickness of <50mm. The Spotted gum was glued using a joinery glue that set hard as a cats head (no flexibility). To achieve the contemporary look we wanted the profile to extend as a continuous line from bottom to top (not be interruped with horizontal top & bottom rails). The Spotted gum was oiled all sides & allowed to acclimatize for 6+ weeks. See photo's where the door has split. My question is... would using a flexible glue be recommended (or is this not advised with door construction)? if you would recommend a flexible glue - what make and model? Was running the door thickness to 14mm too thin for external application even though it was continuously glued? Are you able to assist with guidance how this construction method can be improved to mitigate this happening again? If you can't assist, can you please connect me with a custom door specialist that can help. We ultimately need to re-build the door to achieve the same aesthetic - top to bottom shiplap profile in spotted gum.
From your photo it looks as if the cladding has shrunk and split off part of the lap. Glue won't stop timber from moving in response to moisture changes. It just allows stress to build up in the timber until it's relieved by a split or by the joint opening up. So the short answer is that the timber has to be allowed to move. As long as the timber isn't restrained, a lapped joint permits movement without detracting from overall appearance, unlike other joints where gaps caused by movement are more obvious. We assume the cladding isn't fixed to the steel. That can create problems since the two materials react in opposite ways when exposed to heat, ie. steel expands and timber shrinks. A flexible glue would seem a better option than one that sets rock hard, although we haven't researched different types of glue. No doubt adhesive companies could advise on a suitable choice. Alternatively, if practical, nailing allows timber a bit of wriggle room and the nails can be made a design feature - copper nails etc. as shown here https://www.thewoodworks.com.au/shop/hardware/decorative-nails. If the door is in a particularly exposed location you might need to consider a more dimensionally stable timber such as western red cedar.
