🧑‍🔧 Build a Deck

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Check the joists and beam for stiffness and size the footings for a backyard deck, following the load from the deck boards down to the soil.

A deck is a chain of structural checks: the joists that carry the decking, the beam that carries the joists, the posts that carry the beam, and the footings that carry the posts into the ground. The load travels down that chain, so each check needs the load that actually reaches it, not the load on the deck as a whole.

This guide runs the checks a calculator does well (how much the joists and the beam deflect, and how large the footings must be) and shows how to work out the load that passes between them. It is a sizing sanity-check, not a substitute for your building code: spans, connections, ledger attachment, guards and frost depth vary by jurisdiction, so confirm your numbers against your code, and get the permit, before you dig or buy lumber.

How a deck load reaches the ground

Every pound on a deck follows the same route. It rests on the boards, the boards hand it to the joists, the joists hand it to a ledger on the house and to a beam on the far side, the beam hands it to posts, and the posts hand it to footings that spread it over enough soil to hold it. Because each link carries a different share of the load, each link gets its own check, and the deck is only as good as the weakest one.

Codes state the load in pounds per square foot. A common residential figure is 40 lb/ft² of live load (people, furniture, snow you did not sweep off) plus about 10 lb/ft² of dead load (the structure itself). The example below uses 50 lb/ft² for everything. Where snow can pile deeper than that, your local snow load is the number to use, and every result changes with it.

The idea that turns a deck-sized problem into calculator-sized ones is tributary area: the part of the floor that a given member is responsible for. A joist answers for a strip as wide as its spacing. A beam answers for a strip reaching halfway to the next line of support on each side. A post answers for the rectangle of deck that drains toward it. Multiply that area by the load per square foot and you have the number to type in.

What to find out before you open a calculator

Most of the work in a deck check is gathering inputs, and most bad results come from a guessed input rather than from the arithmetic. Get these first.

• The deck's plan size, its height above the ground, and whether it hangs off the house on a ledger or stands free on its own posts.
• Your local design loads. The building department can tell you the live load and, in cold country, the ground snow load, and it is the snow number that most often changes the answer.
• The lumber's species and grade, because stiffness varies. Common No. 2 grade dimensional softwoods run from roughly 1,300 to 1,600 ksi, and the grade stamp plus a published design-value table gives the exact figure.
• The soil. A soil test is best; otherwise your code assigns a presumptive bearing pressure to each soil type, and the frost depth for your location tells you how deep the footings must go.
• Whether you need a permit. For an attached deck the answer is almost always yes, and the inspection is part of the value.

Steps

1. 1📏Beam Deflection
   Check the joists first. Run one joist as a simply supported beam: the span is the clear distance between the ledger and the beam, the load per foot is the joist spacing in feet times the deck's load per square foot, the modulus is the lumber's stiffness for its species and grade, and the moment of inertia is b × d³ ÷ 12 using the ACTUAL size (a "2×10" is about 1.5 × 9.25 in). Compare the deflection ratio with your code's limit (L/360 under live load is the usual figure for a floor) and move up a size, or shorten the span, if it falls short.
   🧮 Open the Beam Deflection calculator
2. 2📏Beam Deflection
   Run it again for the beam that carries the joists. Its span is the distance between posts, its load per foot is half the joist span times the deck's load per square foot (more if the joists overhang it), and its moment of inertia is the sum of the plies you fasten together, provided the fasteners make them act as one member.
   🧮 Open the Beam Deflection calculator
3. 3🧱Footing Design
   The load on a footing is not a calculator output: it is the beam's reaction at that post. For a post between two equal beam spans it is the beam's line load times the post spacing, and about a quarter more if the beam runs continuous over the post. Enter it in kips together with your site's allowable soil bearing pressure (from a soil test, or the presumptive value your code assigns to your soil type). The footing must also sit below your local frost depth, which this calculator does not know.
   🧮 Open the Footing Design calculator

Worked example: a 12 × 16 ft deck at 50 lb/ft²

The deck is 16 ft wide along the back of the house and 12 ft deep. Joists run from a ledger on the house to a beam at the outer edge, so each joist spans 12 ft, and they sit 16 in on center. The beam rests on three posts 8 ft apart. The lumber is No. 2 grade with a modulus of 1,400 ksi, and the design load is 50 lb/ft².

Deflection is judged against span ÷ 360, a common limit for floors. We apply it to the full 50 lb/ft², which is stricter than codes that measure live load alone, and that choice matters in the first step.

What it adds up to

The framing that came out is 2×10 joists at 16 in on center, a beam of three 2×10s on posts 8 ft apart, and footings of about 18 in square under the middle post (2.20 ft² needed, 2.25 ft² supplied). The calculators handled the two places where deck arithmetic gets fiddly, stiffness and footing area. Everything that holds the deck to the house and keeps people on it, such as the ledger attachment, the post-to-beam connectors, the stairs and guards, and the frost depth, comes from your code rather than from a calculator.

It also shows how sensitive the answer is. In snow country a design load of 60 lb/ft² instead of 50 raises every load by a fifth. The 2×10 joist would sag 0.269 in (span ÷ 534) and still pass with room to spare, but the middle footing would have to grow from 2.20 to 2.64 ft², about 19.5 in square. Re-run the chain with your own loads before you buy anything.

Notes & limitations

• The tributary-area method assumes each post carries a simple share of the deck load, which is reasonable for a plain rectangular deck and less so for an irregular shape or a heavy feature (a hot tub, built-in seating) concentrated in one area.
• The prescriptive span and footing tables in most residential codes already build a lot of this arithmetic (and the strength and connection checks this recipe leaves out) into one lookup. This guide is most useful when you are outside those tables, using unusual lumber or spans, or when you want to see the numbers underneath them.

Mistakes that quietly ruin deck numbers

These are the errors that make a deck calculation look fine on screen and be wrong on site.

• Typing nominal sizes. A 2×10 typed as 2 in × 10 in has I = 166.7 in⁴, which is 1.68 times the true 98.9 in⁴, so the joist appears to sag 0.133 in instead of the real 0.225 in.
• Giving the beam the whole joist span instead of half. That doubles its line load. The reverse slip, forgetting that overhanging joists load the beam with more than half their span, understates it.
• Guessing the modulus. Sag is inversely proportional to E, so a 10 percent error in stiffness is a 10 percent error in deflection. Use the design value for your species and grade.
• Ignoring snow. Where the ground snow load is higher than 40 lb/ft², it can govern everything from the joists to the footings.
• Burying footings above the frost line. Frost heave lifts a shallow footing each winter, and on an attached deck the ledger and the house fight that movement.
• Treating deflection as a strength check. A member can be stiff enough and still be overstressed in bending or shear, which is why the span tables check all of them together.

When to call the building department or an engineer

An attached deck, and any deck high enough to need a guard (commonly 30 inches above grade, though the threshold is local), normally needs a permit. Treat the inspection as a second pair of eyes on the parts that hurt people when they fail, not as paperwork.

The connection to the house is the part no sizing calculator can see. Ledger attachment failures are one of the best-documented causes of deck collapses, and whether a ledger holds depends on the house's rim joist, the flashing behind it and the fasteners, so follow your code's prescriptive ledger details exactly or have an engineer design it.

Bring in an engineer when the deck is more than a story up, sits over a walkout basement or slope where footings cannot reach undisturbed soil, carries a hot tub or planters, or has an odd shape or long overhangs. This guide is a sizing check on paper, not a stamped design.

Frequently asked questions