Do you add snow load to roof live load?

No. IRC Table R301.5 states that snow load shall not be assumed to act concurrently with live load. You take the larger of the two and add it to the dead load. Adding both overstates the design load substantially.

What dead load should I use for a wood floor?

Ten psf is conventional for light wood framing with a single finish layer. Fifteen to twenty is more realistic with tile, stone, a second ceiling or a heavy roof covering. It is an assumption rather than a lookup and it is the term most often taken too low.

What is the live load for a residential floor?

IRC Table R301.5 gives 40 psf for most habitable rooms and 30 psf for sleeping rooms. Decks are 40 psf and exterior balconies 60 psf in the base code, though at least one state amends the deck figure.

How do I find my ground snow load?

From the map in the code for your county, or from your building department. In mountainous regions it is case-specific and determined by the building official. There is no formula, and no calculator can derive it from your dimensions.

What is tributary width?

The strip of floor or roof that drains to one member: half the distance to the next member on each side. For a beam it is half the span of the joists landing on it from each side, which is the figure people most often double by mistake.

If my jurisdiction requires 60 psf, can I still use the code span tables?

No. The IRC deck and floor span tables are computed at 40 psf and do not scale. When Washington amended decks to 60 psf its code council had to publish replacement tables, with joist spans falling about 11% and ledger bolt spacing tightening about 38%.

What is the end reaction and why does it matter?

Half the total load on a simply supported member, and it is what the post, wall or footing beneath has to carry. It is the number most often lost on the way down to the ground, and a load path that stops is among the most common serious framing defects.

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Structural Load Calculator

IRC Table R301.5, and the rule that snow and live load are not added

The mistake in load calculation is almost never arithmetic. It is adding snow to roof live load, which the code specifically says not to do, or taking a dead load off the top of your head. This works the combination the way the code does, and says plainly which two numbers you have to get from your jurisdiction rather than from a calculator.

Design load

Member

Span

Tributary width

Member spacing

Dead load (10 psf light framing)

psf

Live load (20 psf roof, 40 psf floor)

psf

Ground snow load (from your county, 0 if none)

psf

Snow and roof live load are compared, not added. Snow load and wind speed are jurisdiction values this site will not guess.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

What the member carries, and where each part comes from

Worked example

A roof rafter spanning 14 ft, 8 ft tributary, 10 psf dead, 20 psf roof live, 30 psf ground snow.

The combination. Snow at 30 beats roof live at 20, so the governing load is 10 + 30 = 40 psf, not 60.
Why not 60? Table R301.5 footnote: snow shall not be assumed to act concurrently with live load. You take the larger, not the sum.
Uniform load. 40 psf across 8 ft tributary = 320 plf.
Reactions. 2,240 lb at each end.
Per member. A rafter at 16 in on centre carries 747 lb.

Adding them would have given 60 psf and a 50% overstatement, which sounds conservative until it drives you to a member size or a foundation you did not need.

The formula

A comparison, not a sum, and then tributary area:

w_roof = D + max(L_r, S) w_floor = D + L W = w × trib R = WL/2

D: dead load, psf. The structure and its finishes
L_r, S: roof live load and snow. The LARGER is used, never the sum
L: floor live load, psf, from Table R301.5
trib: tributary width: half the span of what frames in on each side
R: end reaction, lb. What the post, wall or footing below carries

The max in the first term is the whole point. It is there because a roof does not carry a maintenance crew and a blizzard on the same afternoon, and treating it as a sum inflates every downstream number.

Two numbers you cannot get from a calculator

Ground snow load and ultimate design wind speed are read off maps in the code, by county, and in mountainous regions they are case-specific and determined by the building official. There is no formula. Any calculator that produces a snow load from your dimensions is inventing it.

This site treats them the way it treats frost depth: as jurisdiction values the reader supplies. That is why the snow field here is an input with no default that pretends to be right.

The third value that is not a lookup is dead load. Ten psf is the conventional figure for light wood framing with a single finish layer, and it rises to fifteen or twenty with tile, a second ceiling, or a heavy roof covering. It is an engineering assumption and it is the term most often taken too low.

What is a lookup is live load, and Table R301.5 gives it: 40 psf for most habitable rooms, 30 for sleeping rooms, 40 for decks and 60 for exterior balconies in the base code. That last row is amended by at least one state, and when Washington moved decks to 60 psf its own code council had to publish replacement span tables, because the R507 tables are computed at 40 psf and do not scale.

Tributary area, and the mistake in it

A member carries the load that drains to it, and that area runs half way to the next member on each side. For joists at 16 in on centre, each joist carries a 16 in wide strip.

For a beam, the tributary width is half the span of the joists landing on it from each side. A beam with 14 ft joists on one side and nothing on the other carries 7 ft of tributary, not 14.

The error people make is using the full joist span rather than half of it, which doubles the beam load. It is the same error as the one on the deck beam page in reverse: there the table wants the full joist span because the table already does the halving for you. Read what the table is asking for.

Where these numbers actually go

The load per square foot picks the row of a span table. The load per linear foot and the moment size an engineered member. The end reaction sizes the post, and then the footing, and it is the number that most often gets dropped on the way down.

A 2,240 lb reaction has to land on something that carries 2,240 lb all the way to soil. On a deck that is the post and the footing. In a house it is a post, a girder, sometimes a lally column, and eventually a pad.

Load paths that stop are the most common serious defect in residential framing, and they are invisible once the finishes go on.

Frequently asked questions

Do you add snow load to roof live load?: No. IRC Table R301.5 states that snow load shall not be assumed to act concurrently with live load. You take the larger of the two and add it to the dead load. Adding both overstates the design load substantially.
What dead load should I use for a wood floor?: Ten psf is conventional for light wood framing with a single finish layer. Fifteen to twenty is more realistic with tile, stone, a second ceiling or a heavy roof covering. It is an assumption rather than a lookup and it is the term most often taken too low.
What is the live load for a residential floor?: IRC Table R301.5 gives 40 psf for most habitable rooms and 30 psf for sleeping rooms. Decks are 40 psf and exterior balconies 60 psf in the base code, though at least one state amends the deck figure.
How do I find my ground snow load?: From the map in the code for your county, or from your building department. In mountainous regions it is case-specific and determined by the building official. There is no formula, and no calculator can derive it from your dimensions.
What is tributary width?: The strip of floor or roof that drains to one member: half the distance to the next member on each side. For a beam it is half the span of the joists landing on it from each side, which is the figure people most often double by mistake.
If my jurisdiction requires 60 psf, can I still use the code span tables?: No. The IRC deck and floor span tables are computed at 40 psf and do not scale. When Washington amended decks to 60 psf its code council had to publish replacement tables, with joist spans falling about 11% and ledger bolt spacing tightening about 38%.
What is the end reaction and why does it matter?: Half the total load on a simply supported member, and it is what the post, wall or footing beneath has to carry. It is the number most often lost on the way down to the ground, and a load path that stops is among the most common serious framing defects.

Check these numbers yourself

Washington State Building Code Interpretation No. 16-11Washington State Building Code Council. The 60 psf deck amendment and the replacement span tables it forced, quantified: joist spans down 11%, ledger bolt spacing down 38%.
IRC R301.5 live load and Table R301.5UpCodes. Minimum uniformly distributed live loads and the footnote that snow shall not be assumed to act concurrently with live load. Confirm the edition it serves you.
IRC Table R301.7, allowable deflection of structural membersInternational Code Council BCAC document reproducing the deflection table with its footnotes.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Snow load and wind speed are jurisdiction values. They come from maps by county and in mountainous regions from the building official. Dead load is an engineering assumption. Live loads in Table R301.5 are amended by some states, and the span tables do not scale when they are. This page computes loads; it does not size members.