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I am building an outdoor couch from kiln dried cedar 2x4s from these plans: enter image description here https://www.ana-white.com/woodworking-projects/2x4-outdoor-sofa

This is only my second adventure with furniture building, so I wanted something simple. I have everything cut/assembled up to the point that the arms are being attached to the center section (step 5), although in my case, I have already assembled the center section with its slats.

The plan calls for 3 inch deck screws to be used throughout, but I wanted a clean look with as few visible fasteners as possible so I have used pocket holes and screws at most of the joints instead. I was going to follow the plans and use screws to join the platform to the arms, but I was a little concerned about strength/durability as these are the only things holding up the platform (and anyone seated on the couch), and screws are not famous for their shear strength.

My question is this: will 2.5 inch stainless deck screws (6-8 per side) be strong enough for this application? Or would something more sturdy such as 3/8 inch lag screws or stainless carriage bolts be better?

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    I am sure with a sufficient number of screws this will be strong enough... But I would be uneasy about them working themselves loose over time — either through the racking forces applied to the bench, or via compression shrinkage. (Basically, the wood tries to expand during the humid season, but can’t, so it deforms to be denser. Then come summer, it remains more compact, and the screws are now looser.) To reinforce this, you can add some thick dowels (blind or through) with outdoor-rated glue. Or a (stainless) metal angle plate, the kind seen in construction and sometimes in IKEA tables.
    – stanch
    Commented Jul 24 at 23:13
  • (A wooden angle block also works, table aprons are often reinforced that way.)
    – stanch
    Commented Jul 24 at 23:17
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    Hi, welcome to StackExchange. "The plan calls for 3 inch deck screws to be used throughout, but I wanted a clean look with as few visible fasteners as possible so I have used pocket holes and screws at most of the joints instead" This was a mistake, as an equal number of pocket-hole screws are nowhere near as strong, initially or over time, as much longer (and potentially beefier) screws inserted the conventional way. Not that deck screws are necessarily the best option either, but the takeaway is that different and/or larger screws used the conventional way are another step up in strength.
    – Graphus
    Commented Jul 25 at 4:31
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    "screws are not famous for their shear strength" That would be some screws. Others are specifically used in situations where they're loaded almost perpendicular to their axis, suck as in a few places in a typical deck installation. Any such screws, used in sufficient number, are more than sufficient to take the load of the seating area of a bench, although there are numerous ways that could be further reinforced with little additional effort. It's worth noting that while the plan calls for no glue to be used, strength would be SIGNIFICANTLY enhanced by correct used of a suitable adhesive.
    – Graphus
    Commented Jul 25 at 4:39
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    Consider going back to the original idea which had visible screws EXCEPT sink those screws into the wood leaving enough space to hide the screws with plugs. Plug cutters are inexpensive and the plugs can be made from the same wood as the project. In fact, plugs can be made from the project scrap pieces thus you know they are identical in kind.
    – quill
    Commented Jul 25 at 13:53

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The current world of screws is a confusing mass of alternatives that is quite difficult to navigate, particularly because of numerous lookalike screws that aren't up to the standards of other screws they emulate (or deliberately try to mimic).

It's unfortunately clear that we can't blindly trust ratings and other reassurances manufacturers give for their screws1. So, if you want to pick a top-class screw it's important to look for third-party comparative tests, and luckily there are such comparisons freely available online these days.

While I would recommend either an old-school lag screw or a good quality construction screw as a generic pick for a lot of applications in woodworking and carpentry (indoor and outdoor) where strength is paramount, it's important to realise that even lesser screws can be perfectly adequate if a sufficient number are used. Three lesser screws could easily hold more than two better ones.

Some further reading:
These Are the Screws You Should Be Using on Popular Mechanics2.
What's the Difference: Deck screws on Fine Homebuilding.
Best Deck Screws for Pressure Treated Woods To Build a Perfect Deck on Woodsmith.
What type of screw is the most resistant to rust: galvanized, stainless steel, or deck screws? on the DIY stack.

If in doubt, reinforce
As mentioned in the Comments, there are additionally numerous other ways to reinforce any project like this.

One of the chief ways is of course simply to add glue. Without any further modification, if glue is used (correctly3) it will hugely increase stiffness, strength and greatly aid long-term stability.

1 A search about deck screws failing will immediately give numerous examples.

2 I would note however that some actual testing of drywall screws shows they may NOT be as brittle as commonly supposed; at least some aren't.

3 And this means prepping the surface freshly, using sufficient glue that some will squeeze out, and clamping HARD. Remember that screws can act as clamps, additional clamping may not be required.

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