Our deck:


Decking and old screws

Cross section of decking

Light colored decking on left was scrubbed yesterday with a stiff natural bristle brush.

We'd like to reduce the regular buildup of moss/algae and associated slipperiness of the 10+ year old composite decking, possibly a Trex product. It's situated on north side of our 2 story home in NW Oregon. We're not concerned with any warranty issues.

I'm looking for reinstallation advice regarding:

1) gap

2) relief chamfer

3) fasteners

4) tar paper caps on joists

I've no experience with this stuff, but plenty of traditional carpentry experience.

The 14'x26' deck, 14" high, was built by/for the previous homeowner between 2000 and 2007. The decking measures 5-1/8" to 5-3/16" wide and 15/16th thick. The decking is installed over conventional pressure-treated framing. The decking seems structurally sound, is not sagging, buckling or flaking.

For other aims, I've already pulled up about 1/3rd of the decking. It's ready to go back down. Pulling up the remaining will be very easy.

What gaps should I maintain for the aged decking? Currently (year-round) there are no appreciable end gaps or edge gaps. Not sure if it was installed that way or if has swelled over the years. The lack of gap is very nice, but the minimal joints pack with dirt/debris. We think this leads to extended puddling and extra moss/algae growth.

To further aid debris clearing, I'm thinking of relieving the bottom of the gaps by chamfering all edges as in the picture. Any comments?

What fasteners are recommended? The decking is fastened by special screws and nail-gun nails, one each at every joist. I removed some of the decking 3 years ago and just laid them back in place without any fasteners. They seem fine that way, so I'm not sure I'd need any fasteners at all, let alone screws.

Should I cap the tops of the joists with tarpaper? Whenever I've built decks in the past (pressure treated framing, Midwest and New England) I've always capped the joists. The tarpaper relaxes into a type of drip cap. Lumberyards carried narrow rolls for just this purpose. This deck doesn't have any. When backing out the screws, 2/3 to 3/4 strip-out in the PT joists. Their threads are rusted off. I threw them away. The photo shows the 10% that retained their threads.

Any other comments recommendations welcome.

  • 1
    Looks like trex, yes. Presumably it's done most of the swelling it will do (which is impressive - a deck at work was installed with gaps, and has swelled to completely eliminate them, and I think it snaps the occasional scew as the surface moves with temperature changes (some LOUD "crack-booms" come out of that deck.)) I'd put at least 1/8" edge gap on it when re-laying to help it drain through and dry better.
    – Ecnerwal
    May 27, 2016 at 0:06
  • Thanks. Any idea on the original dimensions of the decking? It's currently 5-1/8 to 5-3/16 wide and 15/16 thick.
    – user217623
    May 27, 2016 at 16:01

1 Answer 1


I agree with Ecnerwal entirely. But, absolutely screw the deck boards down & don't use nails, unless you like them popping up & ripping your foot or catching a shoe or high heel. Screwing the boards down is structural, for mostly just the boards...it spreads the stress instead of snapping a board from a fatboy standing between joists.

Definitely, use the tar-paper & bravo on doing it in the past...almost no-one around me does it & laughs at me when I do it or point out it's missing. Their decks last maaaybeee 15-years & mine last easily 30+.

Go with stainless steel screws! Square-Drive like the old screws is fine, but mostly Star-Head is all I find anymore. I think chamfering the bottoms will only trap more debris on top of the tar-papered joists, this will also cause water to travel to the meat area rather than just the edge. Simply hosing, brooming or putty knifing out the new gaps annually should be greatly successful.

  • Good point about fasteners spreading some of the stress, hadn't thought about that before. The unfastened decking is not making any noise when we walk on it, nor when I jump on it in the middle of a span (16" OC) to test the idea. A noise would be produced if our weight caused the decking to bow down underneath the applied pressure, and correspondingly rise up above neighboring joists, then slap back down.
    – user217623
    May 27, 2016 at 16:01
  • That's great that you aren't bowing the boards, but definitely screw them all. The Sun or high heat days may warp them or wind gusts could use them to wreak havoc.
    – Iggy
    May 27, 2016 at 23:16

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