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I'm rebuilding my deck. The support structure is made of 2x10 joists between steel beams. The joists are aligned 3/8" above the steel beam so that when the wood settles/dries, the beam doesn't make a bump on the composite decking.

Since my deck boards are the same width as the steel beam, I wouldn't have any space to attach it over the beam.

I'm thinking of putting 2x4, flat, on top of the joists. See ASCII art below. That would allow having a nice flat surface bridging over the steel beam. They would be held down with deck screws. Furthermore, the deck tape on top of the 2x4 would keep the water away from the joist.

 +---------+
 |2x4      |
 +--+---+--+
    |2x |  
    | 10|
    |   |
    |   |
     ...

How bad of an idea is this? Why?

(The second half of the joists, in front of the middle beam, is missing for now, but there will be joist on that side, too, towards the photograph.)

enter image description here

enter image description here

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  • Assuming the 2x4 option, have you thought how you'd terminate the edge to the right of the picture? Would you end the deck at the last joist, or try and have something over top of the beam?
    – Matthew
    Commented May 29 at 20:23
  • yes, 2x4 laying on top of the side of the beam. will be a bit lower at first, but should settle perfect. Will provide a place fo screw composite fascia on the side of the beam. some bracket will hold down the composite and secure the posts (TBD, but similar to what was there before)
    – Jeffrey
    Commented May 29 at 20:29
  • 1
    Perpendicular to the beam, parallel to the joist. Literally flat on the 2x10
    – Jeffrey
    Commented May 29 at 21:34
  • I understand now. I falsely assumed crossways orientation despite your diagram, which means my terminology "help", calling them purlins, is also faulty. Feel free to edit that back out.
    – isherwood
    Commented May 29 at 23:10
  • Purlins to attach decking to? If you just mean deck boards, how else would you do it? I have always used 1 x 6 decking.
    – Tiger Guy
    Commented May 30 at 18:07

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The only drawbacks I see, aside from cost, are more water sitting on the boards because they're wider and a slightly odd appearance from underneath. As long as the extra height doesn't mess you up with respect to access or rim skirts, I have no concerns.

You could use 2x2 except the outer 3 feet or so to save cost and reduce visual impact.

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