The beam is the most consequential member in a deck frame. Undersize a joist and the deck feels soft. Undersize the beam and the whole frame is riding on a mistake. This calculator sizes multi-ply deck beams from the loads the joists actually deliver, using the method behind IRC Table R507.5, with the wet-service and treatment adjustments that indoor span tables quietly leave out.
Open the CalculatorA deck beam span is set by four things:
Tributary load. The beam carries half the joist span on an interior beam line, plus the full cantilever if the joists overhang. Longer joists mean a shorter beam span at the same size. This is the input people get wrong most often.
Species and grade. Southern Pine, Douglas Fir-Larch, Hem-Fir, and SPF carry meaningfully different loads at the same size. The code tables split them out for a reason.
Ply count. A triple 2x10 is not fifty percent stronger than a double at the same span behavior. Plies add capacity in proportion, but the beam depth is what buys stiffness. Deeper beats wider almost every time.
Service conditions. Decks are wet service. Allowable stresses drop, and incised treated lumber drops further. A span pulled from an interior table will be unconservative outdoors.
Same problem, approached from opposite ends. Span asks how far a given beam can go between posts. Sizing asks what beam a fixed post layout requires. The calculator runs both directions: fix the beam and get the post spacing, or fix the posts and get the beam.
Joists spanning 12 feet to an interior beam, 40 psf live and 10 psf dead, Southern Pine #2, posts you would like to keep at 8 feet.
The beam carries a 6-foot tributary width, 300 pounds per lineal foot. The calculator checks bending, shear, and bearing at each candidate ply build-up under wet-service adjusted stresses and returns the smallest build-up that clears 8 feet, along with the required bearing at each post. If a double 2x10 comes up just short, it will tell you whether a triple 2x10 or a double 2x12 solves it cheaper, which is a lumber bill question as much as a structural one.
It depends on joist span, species, and ply build-up, which is why single-number answers online are unreliable. As a shape of the answer: shorter joist spans and deeper beams stretch post spacing; long joists and shallow beams compress it. Run your actual joist span, and use the code table values as the floor.
Usually, for span. Depth drives stiffness and bending capacity faster than width. But the triple may win on post bearing and hardware fit. The calculator shows both so the choice is visible.
Exterior beams need decay resistance, which in practice means preservative-treated lumber or a naturally durable species. Treated lumber that has been incised takes an additional strength reduction the calculator applies automatically.
Yes, within limits tied to the back-span. A modest cantilever is normal and often improves the layout. Past the limit, the overhang starts controlling the design.
It runs the method the tables come from and matches them at the table conditions. Where your case falls between table rows, the calculator resolves it instead of forcing you to round.
Results from this calculator are preliminary sizing guidance for comparison and planning. They are not engineering, not sealed drawings, and not a substitute for design by a licensed professional under your local code. Final member sizing, connections, and load paths must be confirmed for your specific project.
Bardowl Design produces structural layouts and material specifications that builders and lumberyards can actually work from. Clear scope, defined assumptions, documented output.
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