What screws can I use to securely attach a 2"x4" to a 4"x4" treated lumber? The solution would need to be rust-proof.

I recently used a structure similar to the image below for a small fence, but some of the screws are completely full of rust, due to treated wood humidity and the environment. I have to change that.

I was thinking of screws but any other solution is appreciated.

enter image description here

  • titebond3 is waterproof and stronger than wood.
    – dandavis
    Sep 23 '19 at 23:34
  • That T-Butt joint on the right hand side will catch and retain rainwater and dew. Consider another way to join your wood (or use lengths that don't need joining if possible.)
    – Criggie
    Sep 24 '19 at 19:37

People use the wrong screws outside quite a lot, but thankfully there are good alternatives. Outdoor decking and fencing are commonly assembled with coated screws advertised for such. They typically come in tan or green depending on the application and can hold up for a long time without rust or staining the wood.

Stainless steel screws are another option for even more rust prevention, but they will be quite a bit more expensive. This is pretty over the top unless you're building a boat dock or similar.

For larger fasteners, hot-dipped galvanized lag bolts are good. Galvanized screws might exist, but I'm not familiar with them. Certainly you can get galvanized nails, but I think that the coated deck screws must have filled the need that galvanized screws would have because I never see them.

  • There are also some very excellent new structural screws available that require no pre-drilling and replace common lags. For ACQ or other treated lumber these are often powder coated for corrosion resistance and will be clearly labeled as ACQ compatible. The green/tan common screws are fine for simple attachment, but for structural applications it must be a structural fastener (lags or structural screws).
    – J...
    Sep 24 '19 at 12:17
  • Galvanised screws certainly exist, but the rough surface finish makes them more difficult to drive in smaller sizes, so they're relatively uncommon at the thin end.
    – Criggie
    Sep 24 '19 at 19:35

Stainless steel deck screws will not rust. I've used them many times on all types of lumber with great results.

But screwing into end grain never works well, put a piece of 2x4 into each corner and screw into the sides of it. The joint will be much stronger.

  • What do you mean? Something like this? i.stack.imgur.com/Qy69T.png
    – rbhat
    Sep 23 '19 at 20:21
  • 4
    On the inside corners
    – JACK
    Sep 23 '19 at 20:32
  • 4
    On the inside corners with the grain of the wood going horizontally like this: imgur.com/a/AZwA3UA Sep 24 '19 at 12:05
  • 1
    An alternative is to use a metal bracket in place of the extra wood on the join. It'll be more expensive as a solution, but might be worth it if aesthetics is valued over cost, as it will be far less visible.
    – Baldrickk
    Sep 24 '19 at 16:46

In addition to the previous two answers, I recommend against galvanized hardware in pressure treated wood unless the packaging specifically indicates that they are suitable for that use. Afaik only hot dipped galvanized is suitable for use with PT, nails are often electrogalvanized.

  • Hot dip galvanized nails, straight and spiral, are commonly available. Just have to look for the right ones.
    – J...
    Sep 24 '19 at 12:20
  • Electro nails are often marketed as roofing, and are typically harder to find in longer lengths for that reason. Hot dipped is always dull grey. Electro is shiny
    – Machavity
    Sep 24 '19 at 12:34
  • I might just be thinking of collated nails for nailguns. I know the loose nails that indicate use for deck or siding are usually hot dipped, and acceptable for use in PT.
    – Nate
    Sep 24 '19 at 16:47

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