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
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