How to use this calculator
- Choose beam plies. Pick the built-up 2x depth and number of plies, or enter a custom actual beam width and depth.
- Enter tributary width. Use the deck width feeding load into the beam, not the whole deck unless the whole deck bears on that beam.
- Enter deck loads. Use live and dead area loads in psf or kPa from the design basis.
- Enter adjusted design values. Replace the example E, Fb and Fv values with values for the actual lumber and deck conditions.
- Check post spacing. Read the governing beam span and compare it with the entered post spacing.
How it works
This deck beam span calculator screens a built-up rectangular beam between posts or supports. It starts by converting live and dead deck area loads into beam line loads from the tributary deck width:
w_L = q_L x tributary width
w = (q_L + q_D) x tributary width
For actual beam width b and depth d, the rectangular
section properties are:
I = b x d^3 / 12
S = b x d^2 / 6
The simple-span uniform-load checks use:
M = w x L^2 / 8
V = w x L / 2
fb = M / S
fv = 1.5 x V / (b x d)
delta = 5 x w x L^4 / (384 x E x I)
The shortest back-solved span is reported as the maximum beam span for the entered assumptions. Use the deck joist span calculator first when the joist layout controls tributary width, then use deck footing concrete for quantity takeoff after post locations are known.
Worked example
Verified against the live calculator
A two-ply 2x10 deck beam carrying 6 ft of tributary
deck width at 40 psf live plus 10 psf dead has a
total beam line load of about 300 lb/ft. With example values
E = 1.2 Mpsi, Fb = 1000 psi and
Fv = 135 psi, the calculator compares bending, shear and
deflection spans, then checks utilization at the entered post spacing.
Frequently asked questions
How do you calculate deck beam span?
This screen converts deck live and dead area loads into a beam line load by multiplying by tributary deck width. It then back-solves simple-span limits for bending stress, shear stress, live-load deflection and total-load deflection.
What is tributary deck width?
Tributary width is the deck width carried by the beam. For an outside beam supporting joists from one side, it is often half the joist span. For an intermediate beam with joists from both sides, add half the span from each side.
Is this a deck beam span table?
No. Prescriptive tables include species, grade, beam configuration, post spacing, connection assumptions, code edition and local adoption. This calculator uses the E, Fb and Fv values you enter.
Does it check beam plies and fasteners?
No. The built-up width assumes plies share load. Nail, bolt or screw schedules, splices, caps, post connections and bearing must be checked separately.
Does this size deck posts or footings?
No. It screens the beam span only. Posts, footings, lateral bracing, uplift, soil bearing and connector loads are separate checks.
Can I use this for a hot tub deck?
Not by itself. Hot tubs, planters, masonry and other concentrated loads need project-specific joist, beam, post, footing and connection design.
Method & assumptions
- Simple-span rectangular deck beam with uniform line load only.
- Built-up 2x options use 1.5 in dressed ply width and dressed actual depths; custom mode uses the actual dimensions you enter.
- Beam line load is deck area load multiplied by tributary width. Verify whether the beam carries joists from one side or both sides.
- Defaults for load, E, Fb and Fv are examples only. Use adjusted values from the adopted design standard, actual species, grade and deck conditions.
- Does not check prescriptive span tables, ply fastening, splices, bearing, post caps, post buckling, bracing, ledgers, hangers, joist reactions, guard loads, stairs, hot tubs, concentrated loads, snow drift, uplift, seismic load combinations, footing size, soil bearing or permit review.
- Use deck joist span, deck board takeoff, deck footing concrete, LVL beam and section modulus for adjacent planning checks.