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My deck has a cantilever of about 4 feet from the girder to where the stairs attach, and the stairs tend to have a bit of a spring in them when you walk up. I want to solidify it a bit by sistering some joists in that area.

I've been trying to decide what adhesive to use, if any. I think nails themselves would be a solid repair, but the extra strength from some adhesive is also a good thing.

I've found that PL 400 (or other polyurethane) adhesive is advertised as compatible with wet pressure treated wood, and as being waterproof, and I'm leaning towards that.

The deck is about 5 years old from what I understand, so the joists are certainly dry. I'm worried about using adhesive in the repair and having the shrinkage of the greener wood delaminate from the adhesive on one side of the wood. I'm also worried about the board drying unevenly with one side in contact with dry wood and the other in the open air, I think the wood may try and cup a bit, backing out my nails. Maybe I should dry the wood for a few months first? They don't sell yellawood up here in Massachusetts.

Would you use adhesive, or just nails? What kind of adhesive?

Edit: Message received on nails vs screws for exterior applications - but I'm most curious about the adhesives

  • Unless the deck is covered in some way you didn't mention, the wood is certainly damp every time it rains. By the way, PL is a brand, they make MANY adhesives for many purposes; each has a number, as I recall, though I don't recall what they are for specific types. So "PL adhesive" is a very non-specific adhesive. – Ecnerwal Sep 14 at 0:32
  • Sure, but brand new PT wood can have moisture contents up to 75% since they have solvents forced into them under pressure, and "dry" PT would drop down to 20%. Still a good bit of differential there. Also good catch on the PL adhesive, I assumed it stood for polyurethane but I was wrong, PL = "Loctite Pro Line". I've clarified above! – kavisiegel Sep 16 at 12:56
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I'd be using stainless steel deck screws or stainless steel lag screws.

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In my humble opinion, nails have little purpose in construction of a deck. There are at least five different types of screws made specifically for this application. They have torx heads and are very solid. Only specific instructions I would otherwise have is that if you really want to press the old and the new sisters together, drill a pilot hole in the side you are screwing from. That way the threads bite into the further beam and not the nearer beam, and you can put hundreds of pounds of pressure between them. It is also my experience that drilling a hole in the first beam reduces screw breakage in the future. There will always be some movement between the sisters, whether from wet/dry cycles, difference in aging of the two boards, and different pressures caused by slightly different weight loads on the sisters. Eventually this will fracture the screws, but it takes much longer if a pilot hole is drilled. Good luck

  • I learned an important distinction here. Previously I learned that nails actually have higher shear strength than screws and should always be used in construction of interior headers, but I had not considered the pull out force of nails in exterior applications. I have a bit of a personal problem with the threaded section of a screw being load bearing, the shank should be holding the weight, but it seems like all the exterior grade screw fasteners I can find are aimed at joist hanger connectors and the load would land on the threads at that rate. Will have to do some soul searching. – kavisiegel Sep 16 at 12:45
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    DId you mean a clearance hole rather than a pilot hole? – Andrew Morton Sep 16 at 13:14
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    No, I mean to pre-drill a hole that you are gonna put a screw in. If you just put in a screw, it has to cut threads in the proximal (first board the screw goes into) and the distal (second and further boards) board. If you drill a hole proximally, then the screw will slip in that board and only bind in the distal board, making compression much tighter. And it will seem to resolve the issue raised by kavisiegel of which part of the screw bears load. It is a shear load, and there is no limit to the number of screws you can put in. There are some pretty dam stout screws on the market. 0 – Steve Foutz Sep 17 at 17:47

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