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Determine maximum beam span by lumber size and joist length.
With these inputs:
| Result | Value |
|---|---|
| Maximum beam span (post spacing) | 7 ft 5 in |
| Beam size | 2-ply 2×10 |
| Joist length supported | 12 ft |
| Beam may overhang the end post by | up to 1 ft 10 in |
Southern pine No. 2, 40 psf live + 10 psf dead load — the same table as our beam span chart; reduce about 5% for Douglas fir and 10% for hem-fir or SPF. Joist lengths between table rows use the next longer row. Always confirm with your local code official. For post count and footing loads, use the deck post spacing calculator.
The beam is the member everything else depends on, and the one most often undersized on a homeowner-built deck. Its size comes from two numbers working together: how far apart the posts are, and how much deck depth it carries. This sizes the beam and the posts from those two, and gives you the ply count and lumber total to buy.
It is set by the post spacing and the tributary width — half the joist span it carries. As a rough guide in Southern Yellow Pine, a doubled 2x8 spans about 6 ft between posts carrying a 12 ft deck, a doubled 2x10 about 7 ft 6 in, and a tripled 2x10 about 9 ft. Always check against the beam span table in DCA 6 or your local code, because species changes the answer.
Yes — a built-up beam of two or three plies is the normal residential construction, fastened together in a specific nailing or bolting pattern. What matters is that the plies act as one: fasteners at the spacing your code specifies, plies the same length with joints staggered over posts, and no gaps for water to sit in. Cap the top or slope the joints so it drains.
On top, on a rated post cap, whenever you can. That path puts the load straight down through the post in compression, which is what wood is best at. Side-bolting — a sandwich beam either side of the post — depends entirely on the bolts in shear and is far more sensitive to being done wrong. Some jurisdictions no longer accept it at all.
For ordinary residential framing, eight feet is the practical maximum and six is common. Going further means a deeper beam or more plies, and the extra lumber usually costs more than the footing you saved. Beam span also has to be checked against the deck depth it carries — the same beam reaches less as the deck gets deeper.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.