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I have a large deck with runs 45' and longer where I'm replacing all the deck boards using premium SYP (infrastructure of deck is in great shape, so just replacing deck boards). The project is going great thus far for the shorter runs (16' and less) where I can use a single board. However I'm now in the part that spans the house and there are longer runs.

The original installation used 45 degree beveled ends that would overlap and meet at the same joist. Considering this deck is over 20 years old, I can see from direct proof that method appeared to work well 80% of the time. There was no real expansion lengthwise on the boards that stretched, ruined, or compromised this style of butting boards together. This allows using only 2 screws/nails at a single joist driving through 2 boards.

Here is a picture of some of those old overlapped boards at a single joist and how the seam has done quite well: enter image description here

Here is an example where the old overlapped boards didn't fare as well and is a knock against this method: enter image description here

Everything I read says, "use a traditional butt joint where the boards just evenly meet at a joist, and use 2 fasteners per board at an angle to secure to the single joist." I'm not a huge fan of this because overtime the small seam that will prevail allows trapped debris inside that doesn't come out easily. Even with joist tape protecting the joists, this doesn't seem to work as well. As an example, here is an example of 2 boards I replaced 2 years ago (quick, isolated repair) using a traditional butt joint at a single joist, and look at all the trapped debris: enter image description here

I did a ton of research and found very little in the way of anyone else actually using overlapping, beveled 45 degree deck board ends at a single joist. Anything I could find say, "don't do it, someone will catch their foot on one of the boards sticking up." I could sister some blocks below to the joists to allow a wider width for fastening, but this deck is 25' in the air and I can't do full sistering of the joist lengths. I could do small sections at a time, but this will get messy underneath. To clarify, this is such a big project, I'm ripping up 2-4 rows of the deck at a time and replacing (and it's going really well), so no ladders or scaffolding underneath with all the decking removed to really attack the joists to sister in full lengths.

Due to the long runs, really a 'feature board' running perpendicular isn't going to suffice either, as the adjoining deck boards would end up floating off the ends of the prvious joist and would require some blocking.

Basically the constraint here is: I need to continue to join the deck boards at a single joist. I see 2 options:

  1. 45 degree beveled ends overlapping, secured with 2 screws
  2. Traditional board to board butt joint (I read something about back cutting at 1-2 degrees, but don't fully understand that method?)

I need help understanding why the 45 degree overlapping butt joint is not a good idea, when I've showed it did hold up over decades of wear. I'm also open to doing a traditional butt joint because honestly it's easier, but I don't like the trapped debris (and we get a ton of it living in the mountains surrounded by trees that drop stuff all year long). Or is there another method I can use I haven't thought about? I'm not really enticed by a proprietary underneath fastening system being introduced as that's not what I'm using on this deck, but I'm open to listen. Thoughts please?


Outcome

Here's a picture of a finished set of boards joining with 45 degree beveled ends; I'm pretty happy with the end result. This was one of the 1st boards I did, so I improved and the nails went straight into the joist later. In my case I'll be using some 20x Rustoleum paint on the deck to match the rest, which will fill in small inconsistencies, so that works to my favor as well using this method.

enter image description here

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    If you want to fix the one that came out the side of the joist so it holds better, just drive it back out from below, pull it, and re-drill. It'll take a bit of force to get the drill bit to cut a new path from the same starting point, but it's not difficult.
    – isherwood
    Commented Apr 24, 2023 at 21:17
  • Cool thanks good tip. I went more perpendicular in later drilled holes, my 1st thought was to bite as much of both boards as possible, but the angle really wasn't needed.
    – atconway
    Commented Apr 24, 2023 at 21:30
  • Whats under the deck? Does it need to make a water proof barrier? Commented Dec 24, 2023 at 19:01

2 Answers 2

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I've done it myself with both wood and synthetics and seen it on many other decks. I don't think there's anything wrong with a slanted lap. Protruding edges are no more likely or severe than they would be with square butts. However, the job is a bit fussier because you need to be precise with your lengths and screw placement. Here are some tips.

  • For 1½" joists, cut so the long point of the lower board is 1/4" to 1/2" short of the far edge of the joist. This puts at least some full-thickness board over the joist, and not just the toe of the beveled cut. If the top board is not at least slightly resting on the joist it can sag as it shrinks.

  • Use just two (or three) screws, passing them through both boards at the center of the lap. Three would help prevent pullout as the boards shrink.

  • Pre-drill and countersink these screws. The tips of the boards are more fragile than even square board ends. You don't want to split or mash them. Only countersink as much as is needed to get the screws slightly below flush--you want to keep all the wood you can for strength. Drive the screws carefully using a clutch or soft hands.

  • Cut slightly long (~1/8" per 10'), then install the joint end first so alignment is right on. Bow the board upward in the center until the far end is in the correct position, fasten it, then release the bow. This puts a little pressure in the board to help reduce later shrinkage.

  • When cutting 45° bevels, any alignment error with respect to the saw is greatly compounded. Keep the saw square and keep the board pressed tight to the saw table. If you're finding that the joints are coming out askew, check everything for square.

One other option: Make the butt joints an aesthetic feature. You could intentionally leave a 3/16" gap and bullnose the ends to match the sides, like you often see with interior hardwood flooring having beveled ends. This gives a nice look while eliminating the need to hide joints altogether, and it's easy on feet. Randomize them or place them in a uniform staggered pattern. Debris is a slight concern, but you'll have some cleanup to do regardless in your area.

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  • This is awesome thank you, a couple of clarifications please. Bullet point 1 makes total sense so some of the full-thickness sits on the joist. If I do that though, and moving to bullet point 2, wouldn't the 'center' of the lap now be at the edge of the joist as we've allowed for the overlap to be slightly offset to allow the full board to rest on the joist? Meaning I'd need to find the center of the joist and maybe angle the screws correct? 2nd question, I've read bullet 4 like 10x & overall makes sense (pre-load purpose), but I just can't envision well the steps. Can you help clarify please?
    – atconway
    Commented Apr 19, 2023 at 15:57
  • IMO, the problem is when the butt ends get wet. With an angled butt, the acute angle is more likely to swell than the obtuse angle, causing it to "lip" up, being more of a trip hazard and more likely to chip off. With a squared butt, both ends are equally likely to swell. (Perhaps not a problem with composite material.)
    – Huesmann
    Commented Apr 20, 2023 at 13:25
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    Thanks again for the suggestions, I marked it as the answer. One minor update you may want to make to your post, is these were 45 degree beveled cuts (through the thickness) not mitered (across the face). I added a finish pic to my OP. The pre-drilling is 100% key I think to not breaking the ends of the board.
    – atconway
    Commented Apr 24, 2023 at 20:42
  • Don't start a screws vs. nails conversation ;) There are 1.1 million posts & vids on that subject. I was trying to avoid it. This deck is 25' in the air, has sway & movement, (the nails manage this well, yes I know structural screws...) & it's a choice I made. I'm using top-of-the-line 3" stainless-steel, spiral nails. This deck is not small, is taking 1.5 years to replace all the decking, I have limited time, so using a framing nailer, vs pre-drilling hundreds of holes is a huge time saver. The old nails lasted 30 years, were crap compared to what I'm using, and held up extraordinarily well
    – atconway
    Commented Apr 24, 2023 at 21:08
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I ran into this recently on a deck repair where the original wood deck boards had a 45 lap joints and quite a few had failed due to dry rot. Coastal condition with lots of moisture and the end grain absorbed water and eventually broke down. I opted to use a router on all new joints to make half lap joints that allowed screw fastening in the middle of the joist where the overlap occurred (pre-drilled countersink holes). I sealed all joint surfaces with marine grade epoxy to eliminate the water damage issue (let epoxy cure before assembly so replacing a damaged board in the future would be possible). Took longer but joints turned out flat and eliminated the sharp top edge you get with a 45 joint.

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