I've been looking at various span charts for decks and I'm still not quite sure how to calculate it all when you take into account not just the joists, but the beams and posts as well.

What we currently have:

23' wide deck attached to house with ledger. All pretreated wood.

Extends 10' with 2x6 joists 16" OC.

Outside 'beam' is 2x10 with joist hangers. Beam supported by 3 4x4 posts.

I have a hunch this isn't to code...or...if it is...just barely. It's solid--but bouncy.

I'm not thrilled with the 2x6 joists, but I also am not thrilled with having to scrap the entire deck. Plus, the 2x6s do help with the headroom underneath the deck (which we plan on turning into a patio.

What I'd like to do:

1) replace the exterior 2x10 beam with an actual beam 1 or 2' in and under the joists (so the joists are cantilevered and rest upon the beam)

2) replace the 3 4x4 posts with either 3 6x6 or 4 4x6s.

My question is: how do I calculate what that new beam has to be given the existing joists and the desired post options?

  • I'm not familiar with US codes and charts, but as for the 'span' in charts - it's (in most cases) a distance between (neutral) axes of elements. Structural calculations base on theoretical models, these - in turn - need discreet input data. personal privacy advocate listed all other. Dec 1, 2015 at 11:21

2 Answers 2


First, your question

how do I calculate what that new beam has to be given the existing joists and the desired post options?

This is a bit long winded, but it is how we figure it out accurately.

Figure out a design load per square foot (PSF), we typically use 40 PSF live load and 10 PSF dead load for floors and decks. In a simple span, the beam carries the load halfway to the next support or tributary width; in this case 5'. Multiply the PSF x the tributary width to get the pounds per lineal foot (PLF) on the beam, in this case 50 PSF x 5 Ft = 250 PLF. Then, using software or load tables find a beam that can carry that load based on the span of the beam; allowable loads reduce as the span increases. In your case this requires (3) 2x10 DF #2.

I typically use Forte for beam sizing instead of load tables.

For the post sizes, calculate the tributary load which, in simple spans, is the load halfway between supports. In your case, this would be half the beam length between post multiplied by PLF. For your center post that is 250 PLF x 11.5 FT = 2875 LB. Then, using load tables or calculations find a post that can support that load based on height and bracing of the column. Take into account bearing area of the 2x10 at about 625 PSI perpendicular to the grain for a required bearing area of 4.6 Sq. In. In your case a 4x4 is plenty.

Try a column load calculator here

For multiple span beams, cantilevers, and anything outside of "simple span" things are a little different. For this reason I typically use the beam sizing program.

If this is too intense, maybe just be guided by the following.

If I read correctly, you have deck joist spanning about 10' and on one end a single 2x10 "beam" or rim joist that the joist hang into that spans approximately 11'-6" between (3) 4x4 post.

Based on this, your deck joist are structurally OK although maxed out at a 40/10 load, but are deflecting (bending) a good amount giving part of the bouncy feel. You can add 2x6 or larger joist down the middle of the existing joist to firm things up, or just live with a safe bouncy deck.

The 2x10 is definitely undersized and should be addressed. It also contributes to the bounciness. The simplest solution to this is to add (2) intermediate post mid span reducing the span of the 2x10 to below 6'. OR beef up the 2x10 by adding (2) more 2x10's to it nailing 3 16D nails per foot per beam lamination would easily transfer the load between beams.

Finally, the 4x4 post are more than adequate to support a load of 2,875lb. An 8'-0" length of 4x4 can support over 6,000 lb in wet service.

We applied over 10,000lb to a 7' 4x4 in jacking up a floor with no indication of possible buckling; we were compression limited on the member being jacked.


you need to now live load and dead load required, soil conditions, local building code requirements, allowable sizes and spacing for the joists, hanger type used, if any, materials to be used, type of pier or foundation the posts sit on. that should be a good start. it might be time to call an architect.

the deck you have if it were built here in southern Ontario would be severely deficient. it would be built here similar to as follows:

1) all pressure treated spruce, grade 2 or better

2) 2 x 8 ledger bolted through your wall veneer to the inner rim joist. probably two 1/2" bolts (with standoffs) 4" apart on 32" centers.

3) gu26 hangers at all joist/ledger intersections

4) 3 ply 2 x 10 beam at 8" point out from wall. nailed through with 3" galv. nails every 16" x 5 nails vertically

5) doubled rim joist, end nailed into joists, if guard to be bolted to it. single if guard to be mounted to deck surface.

6) beam to be supported every 8' maximum by 4 6x6 posts on 16" dia. mpa 30 or higher concrete foundations to 48" below grade. bases and tops to be connected to beam and foundations with approved galv. steel brackets.

7) lateral reinforcement to beam to be done at each post with mortise and tenon cross bracing (or bolted through) to prevent racking of the deck

that's how it should be built here. your building code will be similar, so you can get an idea what it should be an figure out if you want to try to just reinforce what you have (more, bigger beams on closer centers to start with) or just tear it out and start new

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