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I built some basic picnic table benches five or so years ago, using what I thought were the proper components - outdoor weather-resistant paint and premium deck screws. image of loosened screw I'm surprised at the extent to which the screws have come unscrewed since the bench was built. It has, to be fair, been outside the entire time, through hot summers and very snowy winters.

My main question is, what could I have done to prevent this? My subquestions are, how does this worrying-loose happen, mechanically; and are there other components or techniques that guard against this phenomenon?

Thanks for any help you can give,

AKA

edit: I initially thought this was made from pressure-treated wood, but it wasn't - it was just the highest-grade 2x4s available from the big box store, sealed and painted with outdoor paint

AKA
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    Can you clarify that the pictured screw got there on its own? (And the top board that’s rotten at the end isn’t PT.) – Aloysius Defenestrate Jun 19 '22 at 17:00
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    Also, a little more detail about the construction. How long were the screws (and how thick were the boards they were fastening). Were they screwed into long grain, or end grain? – Walnut Close Jun 19 '22 at 17:37
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    The top boards seem pretty thick, so I have a suspicion your screws just weren't long enough...... so they didn't have sufficient hold in the piece of wood they're supposed to hold to — the one underneath/past/beyond the board that the screw passes through. Just to note I had this thought before I noticed how much the screw is leaning off to one side, which strongly suggests it's not particularly long. Unless it snapped, or got bent over? – Graphus Jun 19 '22 at 17:55
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    "It has, to be fair, been outside the entire time, through hot summers and very snowy winters" Good to mention this, because it shouldn't have mattered one jot. The entire point of this type of fastener IS that they can be used on outdoor projects that are permanently exposed to weaker, and not coddled in any way. So if I'm wrong about the screw length then you had every reason to suppose your build would have held together long-term. Until the pressure-treating chemicals all leached out and the wood started to rot in fact (not at all exaggerating here). – Graphus Jun 19 '22 at 17:59
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    "how does this worrying-loose happen" Freeze-thaw cycles during winter might have started the loosening process, and once some screws are even slightly loose the process of further loosening is pretty much taken as an inevitability I think. After enough of the screws have loosened, the whole structure is free to flex, and individual components to move relative to each other, with every gust of wind continually through the year. This could hasten the loosening of other screws that up to then were holding fairly firmly. – Graphus Jun 19 '22 at 18:06
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    Another question: did you paint the bottom of the boards before assembly? – Aloysius Defenestrate Jun 19 '22 at 23:31
  • Those screws are intended to be countersunk quite a bit, as well. They'll still come loose maybe, but that means that the threads in the bottom material have rotted out. This implies you not only have a water ingress problem, but a problem with moisture collecting and not being able to drain or air out. But there is only so much you can do in temperate locations. That being said, my deck screws countersunk into PTL utility furniture are not the least amount loose after several years of Canadian climate. (But I painted every cut end with evil anti-rot chemicals.) –  Jun 20 '22 at 20:24
  • Thanks for the update with that most important detail! Probably unrelated to what happened (can't be sure if there was a contribution) the coating on the one screw in the photo obviously didn't hold up particularly well, which is something a lot of user comments report incidentally, even with otherwise highly rated screws. Now rusting of fasteners isn't a huge deal in many outdoor projects, but if this were a deck or something where safety is important and the expected lifespan is presumably more than 10 years, this could definitely be an important issue towards the end of service. – Graphus Jun 21 '22 at 13:18

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The fact that the table was made from untreated, big box store 2X stock pretty much answers your question. That wood is almost certainly what is known in the trade as SPF (meaning, spruce, pine or fir, but probably not fir), and untreated, it has very little rot resistance. Left outside, water got under the planking, kept the structural 2X underneath continuously damp, and voila, it was too unsound in a few years to hold the screws.

So, what could you have done to prevent this: First, use treated lumber, or, if you want to spend the money, a rot resistant species like cypress, white oak or redwood. Second, construct and position the table so that water is unlikely to be able settle between boards, and will run off all horizontal surfaces. Treated lumber will generally not tolerate continuous moisture exposure, unless it is rated for ground contact. Even then, it'll last better if it doesn't stay wet.

Deck screws (assuming they were long enough) are ok for this application if the table is going to stay put. Personally, though, when I've built outdoor tables, I've used galvanized 5/16" carriage bolts, bolted through the structural members. People like to pick picnic tables up by the table planks and otherwise put loads on the fastenings that deck screws aren't designed to handle. Through bolts with larger heads take the abuse better.

One possible approach to consider is to get a steel kit for the support structure of the table, and use wood for the seats and table top only. Typically these are made of tube steel, and require using through bolts. Solves the moisture entrapment problem. They are also lighter than wooden support frames. I just replaced the seat and table boards on a table I built with tubular steel supports 41 years ago. The boards were Douglass Fir, so fairly rot resistant, and we kept them sealed for the most part, but they were outside continuously for those 4 decades, and most of the wood was still usably sound, although definitely close to end of usable life.

Walnut Close
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  • Thanks, this is helpful in several ways. Guests always pick up these benches by the planks, so I imagine that is the main culprit in terms of mechanical forces pulling the screw directly out of its boards. The (not-great) design of this bench also allows water to really collect on the lower of the two boards this screw joins, hastening its rot. I didn't paint the boards before assembly, or even think of it - what a good idea! – AKA Jun 28 '22 at 17:02