Been wanting to ask this group about this for a while & Zahs post got me moving on it.
Has anyone done anything not prescribed by the mfg of the composite decks like making lap joints? When I replaced the front porch deck I didn’t want gaps. Here in the SE pine needles get caught in them & it gets to be a real pain trying to keep it looking clean. I’ve read & reread (not saying I didn’t miss something) at the mfg web sites regarding installation & also where they brag about how stable the material is. But in every installation they call for a gap between deck boards. Why mandate a gap if it’s so stable? Finally found one little statement regarding the movement of the joists that the deck was attached to. Hmmm… not much movement lengthwise even in PT joists in my opinion. And widthwise… the movement will only be affecting one screw through each board in most cases. So I ran an experiment and rabbetted the edges of the boards (alternately) and installed them kind of like T&G but with rabbets instead. Each board got one screw at each joist near the edge holding down the adjacent rabbetted board (except for the starter row). The size of the of the deck is 7’ x 4’ (with the boards 7’ long running perpendicular to the joists) so kind of limited but in the year since I began the experiment I’ve not seen any issues and am happy enough to now go forward on the larger (& more expensive) back deck with the same process. The temp range it’s seen ranges from 15f to 100f with little protection from the rain. It also is about 2’ above grade so it gets decent venting. Also there’s been no mold/fungus growth so far that is common on PT around here. <!—-><!—-><!—->
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I know that this voids any type of warranty the mfg may offer but I usually don’t keep enough of the paperwork to maintain the warrantees anyway such as original receipts and the tags on the boards so I’m not worried about that
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Has anyone else done anything like this? <!—-> <!—->
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John
BTW, other than it being very heavy the comp decking is fun to work with.
Replies
Very interesting experiment.
In the Trex use I have done, I always gapped to spec, because, when I consulted with the manufacturer, they always told me the "growth" of the material was not dependant on annular rings, like wood, but that the plastic composite grew evenly throughout the piece.
Quality repairs for your home.
AaronR Construction
Vancouver, Canada
I've interpreted the gap suggestion by the manufacture as cautionary sage advice and not related to their statement about the product stability. Regardless of the material used in it's construction, when that deck is exposed to direct sun, the temp of the material (dependent somewhat upon color) is going to rise significantly. One hundred forty degrees would not be out of the question. With that temp rise, your going to get expansion and if there is no way to relieve that stress, some buckling has got to take place. In your first deck, that's only four feet in width, that expansion may not be significant enough to cause problems but it's still occurring. If you then assume that you can do the same thing on a deck that's much wider, say 12 feet or more , you had better make some allowances for that expansion or your going to experience some additional costs for repair down the road. Not to mention the labor and frustration involved. It's not the one screw per board that's the problem, it's physics and it still applies regardless of how you lap the boards. If it expands and it will, you need some space for that expansion to take place. The deck, or the wall next to it, is going to move or the deck will develop a wavy surface.
When they installed the sidewalk in front of the houses on my block, they didn't install any expansion joints that I can see. For the last six years, the sidewalk in front of my neighbor's house lifts as much as eight inches on hot summer days. Two sections, that probably weigh a couple thousand pounds each, will lift completly off the ground. The city, who installed the walk now wants my neighbor to fix the trip hazard that's developed and do it at his expense.
Thanks Aaron & Woodway,<!----><!----><!---->
This is exactly what I was looking for, somebody to knock the blinders off my tunnel vision.
Not sure why I overlooked this ‘til now but today I took a scrap of the decking I had left that was sitting in the sun and probably around 100f (outside temp was 90f and I think the material may have been actually hotter than 100f) and took a measurement then froze it in the deep freeze with a thermometer I know to be reliable to a temp of 0f (This was after a 9 hour cold soak After only 4 hours it had already shrunk 0.040”.) and re-measured to find a 0.048” difference in width (about 3/64”)(measured with a vernier caliper so it’s pretty accurate). Cumulative effect over the length of the next deck is over 1.5” of expansion and guaranteeing problems… if I install when it was the coldest/narrowest. Looking back at the 4x7 and the time of year I installed I got lucky since it was installed last August in the heat of the summer (I’d rather be lucky than good) and at max expansion.
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I’m still going to go ahead on the larger deck but won’t be trying to get it done as early in the day as possible and will wait ‘til it’s good and warm before attaching. <!----><!---->
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I don’t think I’d ever do this for a customer due to the warranty issues and the additional complications of the temp of the install but I do like the look of the front deck as well as the feel of the composite on bare feet.<!----><!---->
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Cheers<!----><!---->John
John, another reason that the manufacturers require spacing is for air circulation. Without enough air movement, water that gets through the cracks will sit on top of joists and eventually rot them and/or cause mold problems.BruceT