Weekend remodeler - Composite material and Moisture Question

by Mark Heard
(Buffalo NY)

Basically i want to build a larger deck over a 6" high concrete slab, using PT 2 X 6 as joists. Most important question is the deck material. I'd love to move up from 5/4 PT and use composite - however I have not been able to get definitive advice if composite this low to the ground is doable. I have heard there would be a potential moisture problem? Any advice would be great.

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Aug 28, 2010
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Not enough air flow
by: Editor-Rich Bergman

If your joists are sitting directly on the slab then you will be creating closed cavities between each joist and I would think this would surely cause a problem as there would be zero air flow.

Perhaps you could notch the bottoms of the joists or drill holes every so often say 2" diameter with a hole saw in the middle of the width of the joist. Since it is compression loading only it would not weaken the structure above.

That might mitigate the situation a little bit but the thing is to get an answer from the manufacturer about minimum air flow etc to avoid excessive expansion from heat and reduce chance of bacteria and mould setting in because of the restricted air flow.

Aug 22, 2010
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composite moisture prob
by: Mark

Oh to clarify for any other folks that can offer comments - the miosture concern is due to the fact I would place the joists directly on the slab surface so you can see air would not be able to circulate freely under the decking if THIS causes a potential problem. Thanks - Mark

Aug 21, 2010
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composite moisture problem
by: Anonymous

Rich - thanks for your input. Yes your advice is exactly what my local lumber yard also referenced about the buckling problem with in adequate airflow for composite. I'll run this past the help desk onb a few mfg sites to see what they say. hate to make a very expensive mistake. Mark

Aug 21, 2010
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Composite decking and minimum air flow
by: Editor - Rich Bergman

I wish I could give you a definitive answer also but I do recall someone from Trex commenting in the forum about the recommended space under the deck for their composite decking.

6" may well be ok but if I recall somone wanted to basically have the deck and frame right on top of the grade which meant practically no air flow. The issue with composite and more so if it has a higher content of plastic is that plastics make the boards expand. But they expand a fair amount along their length which is different than wood.

Wood will tend to expand more across its width and only about 1-2% along its length. So when composite material with high plastic content gets too hot then ends push against each other and you can get a lot of pressure and even buckling in a worst case scenario. That is why the instructions usually say to leave adequate space at butt joints between ends of boards.

As far as moisture being a problem at 6" above grade I can't say for sure as 6" should still leave it dry. Best to go right to the manufacturer's site to verify that.

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