Question: What size H Beams are needed?

Background: I’m building a deck over our walkout basement. The basement is only 7’ tall, so the deck beams need to be as short as possible with the fewest amount of support posts (hence the steel beams). Additionally, I’d like the steel H beams (2 total) to span 20’ between 2 support posts, with an overall beam length of 24’ (extending 2’ past each post). The 2 support beams will be parallel and spaced 10’ apart. On top of the beams will be 2x10x14 joists spaced 16” on center (with a 2’ cantilever on each side of the steel beams), so overall the deck is 14’ x 24’. I’m looking for the deck to hold a minimum of 50 pounds per sqft.

  • 1
    Have you ruled out digging below the deck to increase the clearance underneath? Mar 20, 2021 at 20:47
  • Can’t dig lower... there is a cement patio already there.
    – CRUTER
    Mar 20, 2021 at 22:11

2 Answers 2


Per IRC, the minimum deck design live load is 40 psf, typical deck weight 10 psf, and deflection is limited to Span Length (L)/360. I will add another 10 psf misc. weight in the calculation below.

enter image description here

enter image description here

Note, W10 x 15 is slightly off, but considered acceptable.

  • @CRUTER I notice you have lengthen the beam to include a 2' cantilever on both ends, the calculation above may still work but need further check. However, since this is an overhead deck, on top of the loads already included in the calculation, you will also need to consider those environmental loads (wind/snow/seismic), to properly size the support column and foundation. This is obviously not a simple task you can handle through the web. I sincerely suggest to engage a structural engineer to perform the design, as the consequence can be severe. Good luck.
    – r13
    Mar 21, 2021 at 16:15
  • @Lee Sam Please note the controlling factor of this design. The code specified load is the "minimum" required for the design of the deck, it does not include the weight of the supporting structure - the beam.. The added cantilevers essentially help the case, as the design moment and deflection will be reduced rather than increased. Please keep "make no harm" in mind, when offer advices over the internet to the novices.
    – r13
    Mar 21, 2021 at 16:46
  • appreciate the help. I’m going with A50 Steel and the W10 x 15 beam because it’s much lighter than the W8 option. I understand the risks and have done a bit more research to verify. Also, want to mention that the beam is $385 for 25’ and the steel company will cut to 24’ and local delivery is only a fuel surcharge of $10. This option is cheaper and slimmer than any LVL or Glulam option i looked into.
    – CRUTER
    Mar 23, 2021 at 13:48
  • @CRUTER I am glad that you are prudent on this matter. Only one reminder, your beam, in current span, is likely governed by deflection rather than strength required for moment. For the former, A50 does not have advantage over A36, as the elastic modulus is the same for both. It might turns out cheaper if you add intermediate column to shorten the beam span. By doing so, you might be able to use a lighter beam, which will offset the added cost of the column addition. But ironically, except misc. steel, A36 beam is not always readily available these days. Check your supplier for it. Good luck.
    – r13
    Mar 23, 2021 at 16:16

Hmmm...let me see if I have this correct. You’re using 14’ long joists with a 2’ overhang. So, that means your deck is 12’ wide and half that distance (6’) will be transferred back to the house and the other 6’ plus the overhang (2’) will bear on the steel beam.

So the total load is 6’ + 2’ = 8’ x 50 psf = 400 plf on the steel beam.

The smallest steel beam I have in my steel manual is a 10” x 4 3/8” I-Beam which will carry 19,500 lbs. per foot on a 20’ span.

As an option, you could use a 4x16, or a 6x14 SPF Select Structural grade wood beam spanning 20’.

Another option would be LVL beam that is 2 plies of 1 3/4” x 11 7/8” high will support 400 plf for a 20’ span.

The steel beam will weigh about 25.4 plf , a 4x16 cut timber will weigh about 14.8 plf , 6x14 will weigh about 19.6 plf and the LVL will weigh about 11.0 plf.

As you can see, the wood beam options will be lighter and easier to manipulate. Plus, there are ready made connectors for posts, etc.

  • Thanks @Lee Sam. Ideally, I’m looking at the whole deck being less than 18” (beam + joist + Trex). The layout is a bit different than you stated. There will be 2 beams (not relying on home). The 2 beams are parallel to each other and 10’ a part. The joists on top of the beams will extend 2’ past the beam on both sides, so 14’ joists. Only looking into steel H beam options to minimize height. Thoughts?
    – CRUTER
    Mar 20, 2021 at 22:19
  • If the 50 psf is the total expected service load on the deck (deck and joist weight included), each of the beam will support an uniform load of w = 7x50 =350 plf. From here on, you can use one of the web calculators to figure out the beam size (don't forget to add the beam weight too), that can support the load, and meet deflection limit. But ultimately, you shall contact a structural engineer to perform sanity check on the suggested size. Good luck.
    – r13
    Mar 20, 2021 at 23:39
  • The thinnest dimension I can come up with is 20”. You could use a M10 x 9 steel beam which will support 7,800 lbs. per linear foot (plf) and is 10” high. It’s more beam than you need, but smallest in my book for a 20’ span. They make a 8” high beam (M8 x 6.5) and will support 5,400 plf for a 17’ span. I suspect it would be fine, but I’m not going to hand calculate it out. So, the steel beam is 10” high plus the 2x10 is 9 1/4” high plus the tree deck is 3/4” high (they call it 1” deck but is actually 3/4” high and it’s maximum span is 16”, unless you use the WearDeck for a total height of 20”.
    – Lee Sam
    Mar 21, 2021 at 1:36
  • @Lee Sam Is your calculation based on grade 50 steel (50 ksi yield)?
    – r13
    Mar 21, 2021 at 3:09
  • @r13 ...A36 steel
    – Lee Sam
    Mar 21, 2021 at 3:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.