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Question

My question stems from the webinar on decking held today, which was excellent btw and thank you for it. I'm looking at the issues of installing a new external timber deck over an existing compressed sheet deck. The CFC has a membrane over it and slopes 50 mm. The boards would be 90x19 with 8 mm gaps. The deck has a wall at the top of the slope and on one side. I would like to install the decking on cut battens from 45-90 mm deep laid over the CFC and fixed down the slope with the boards across the slope. Is this feasible in terms of providing adequate ventilation between the boards and the CFC?

Woodsolutions Answer +

CFC isn't a material that absorbs water readily, and with a fall of 50mm presumably rain will drain away without pooling underneath.  Consequently the decking shouldn't be unduly exposed to moisture on the underside. We note that the fixing battens will run down the slope, allowing rain to run off without obstruction. Nevertheless, pre-sealing the decking all round before installation would be a worthwhile precaution. Regarding ventilation, it's important not to totally enclose the deck around the perimeter. Plinth boards are sometimes installed as a finishing touch, and look neat but restrict movement of air under the deck. If practical, you could consider mounting the fixing battens on packers to improve air flow, given that there are walls at the top of the slope, and on one side, that will inhibit a cross-flow of air. The deck will actually only be open on two sides. If achieving adequate ventilation seems too much of a challenge perhaps you could consider a sail cloth awning over the deck to reduce exposure to rain.

Sub-floor ventilation

Answered on 14-09-2021
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

Question

I need to run 140 x 19 hardwood decking at right angles over a 250 PFC steel edge beam. The web is 8mm thick and face 15mm with the web on the outside. I can fix the decking to timber on the inside of the PFC but then have 90 mm to cover the PFC plus an amount to overhang the edge beam.
1. How much should the decking overhang the edge beam?
2. Would it be ok to have the end deck screws fixed 10 mm on the timber inside the PFC and have the decking run 90mm over the PFC, plus an overhang of say 20 mm, i.e. the deck screws will be about 120 mm before the end of the decking or should I somehow fix to the PFC ?
Thank you, Alan

Woodsolutions Answer +

Kiln-dried hardwood is fairly stable but it is a bit risky to have 120mm unrestrained - there could be some movement with wetting and drying cycles. Is it possible to run a timber member on the outside of the PFC steel, so the screws are on the outside rather than the inside? There's no problem if the decking overhangs by 20mm plus 10mm edge distance on the fixing member.

Decking

Answered on 14-09-2021
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

Question

I am designing an outdoor seat. What is the best way to fix 135 x 32mm BLackbutt to a powder coated steel frame? I would like to run the blackbutt on its side 32mm side & plan for a spacing of 15mm between boards.

Woodsolutions Answer +

The simplest way to fix the timber would be to screw down through the timber into the steel frame. This is shown in a helpful YouTube video here https://www.youtube.com/watch?v=jRm-VBSGIsY. However, it won't work with battens that are 135 mm deep, since the bolts will be too long. If you need a depth of 135 mm for visual effect, some kind of bracket system will be needed. Otherwise if you could use smaller battens, say 90 x 35, the system shown in the video will work. Once the bolts are recessed into the top of the batten, as shown, the hole can be filled with a piece of matching timber, produced with a 'plug cutter' drill bit. This achieves a neat finish and prevents water collecting in the recess. 

Fixing timber to steel

Answered on 01-09-2021
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

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