Guide · 6 min read
Attic storage vs an attic floor: which span table applies?
"Can my attic hold storage?" and "can I put a floor in my attic?" sound like the same question, and they are not. The first is about ceiling joists doing what the ceiling joist table already assumes they might do: carry a ceiling below and a limited amount of stored material above. The second is about turning ceiling joists into floor joists, which moves them into a different span table with a heavier design load and a stricter deflection limit - and, more importantly, into territory a table alone cannot approve.
This site holds exactly the two tables that make the distinction concrete: ceiling joists at 20 psf live load and L/240, and the 30 psf, L/360 floor table used for sleeping areas and attic floors. This guide reads the same lumber out of both, so the gap shows up in published numbers, and then is blunt about everything the numbers do not decide. Allowable spans depend on species, grade, spacing, live and dead load, and the deflection limit, and whether an attic may be floored at all is a question for your building department under the locally adopted code edition - not for this page.
Two questions, two tables
The ceiling joist table reproduced here - IRC Table R802.5.1(2) - is written for attics with limited storage: 20 psf of live load, 10 psf dead, deflection checked at L/240, covering Douglas Fir-Larch and Hem-Fir. That is the table for the attic you climb into a few times a year, with boxes on boards and nobody living up there. It already assumes some storage; that is what separates it from the lighter table for attics without storage, which this site does not reproduce.
An attic used as a room is not that. A habitable attic floor is designed like any other sleeping-area floor: 30 psf of live load at L/360, IRC Table R502.3.1(1), the same table that covers bedrooms. Heavier assumed load, and a deflection limit half again as strict, because floors carry people and finishes rather than boxes.
So the honest version of the two questions is: "were my ceiling joists sized from the 20 psf table, and is my use still inside that table's idea of limited storage?" and "do these members, at their actual size, species, grade and spacing, appear in the 30 psf floor table at my span?" Those are answerable. "Can I convert my attic?" involves much more than either.
What the limited-storage table publishes
Here is a Douglas Fir-Larch No. 2 ceiling joist across the four published spacings. A 2x8 reaches 19 ft 1 in at 12 in on center, 16 ft 6 in at 16 in, 15 ft 1 in at 19.2 in, and 13 ft 6 in at 24 in on center. Those are generous numbers precisely because the table asks so little: 20 psf of occasional storage load and a deflection limit loose enough that nobody standing below should notice the flex a stored load produces.
Read the assumption in the other direction and it becomes a warning. A ceiling joist sized to exactly one of those cells has no published headroom for anything beyond what the table assumed. The span table does not say how many boxes that is - it publishes an allowable span under an assumed uniform load, and nothing on this site converts that into a safe weight for your particular attic. Piling a library into the middle of a ceiling that clears its own table on paper is exactly the kind of judgment call the table was never designed to make.
The same lumber as an attic floor
Now read identical lumber out of the 30 psf sleeping-area floor table. The gap is not subtle, and it is the whole point:
At 16 in on center, the same Douglas Fir-Larch No. 2 2x8 that reaches 16 ft 6 in as a ceiling joist reaches only 14 ft 2 in as a floor joist - 2 ft 4 in shorter. The 2x10 drops from 20 ft 2 in to 17 ft 5 in. Every cell moves the same direction, because the floor table assumes half again the live load and tolerates a third less deflection.
That difference is the trap in decking over an attic. Ceiling joists that span, say, 16 feet were fine by their own table as a 2x8 at 16 in on center - and are nearly two feet beyond the floor table's limit for the identical member the day the space starts being used as a room. Nothing visibly changes, no work is even required; the members are simply outside the table that now applies to what they are carrying.
| Size | Ceiling joist - 20 psf, L/240 | Attic floor joist - 30 psf, L/360 |
|---|---|---|
| 2x6 | 13 ft 0 in | 10 ft 9 in |
| 2x8 | 16 ft 6 in | 14 ft 2 in |
| 2x10 | 20 ft 2 in | 17 ft 5 in |
What neither table can see
Even the honest table comparison above assumes facts most attics will not surrender without inspection. The tables know nothing about your actual framing, and the list of things they cannot see is long:
One item on that list deserves its own paragraph, because getting it wrong is the most dangerous mistake this topic offers. If your roof is framed with trusses, the horizontal member at the bottom is a truss bottom chord, not a ceiling joist. It is one element of an engineered assembly, no span table on this site or anywhere else applies to it, and it must not be cut, drilled, spliced or loaded beyond its design - decking over truss bottom chords for storage is a modification of an engineered structure. Truss questions go to the truss manufacturer and a design professional, full stop.
- The actual species and grade of the existing joists - without a legible grade stamp there is no row to read.
- Their condition: splits, notches, bored holes, water damage and previous remodels all take real capacity the table assumes is still there.
- The ceiling finish hanging underneath, which was part of the original dead load assumption and cracks first when deflection grows.
- Bearing and connections at each end, and any walls or struts the joists pick up along their length.
- Access, headroom, stairs, egress and insulation - habitable-space requirements that have nothing to do with spans.
- Whether the "joists" are actually the bottom chords of engineered trusses, which no span table covers.
What actually decides it
For light, occasional storage in a stick-framed attic, the useful move is to find out what you have - measure the joist size, span and spacing, find a grade stamp - and read your members against the ceiling joist table. If they are comfortably inside their cell at 20 psf with limited storage, you know what the framing was asked to do; if they are at or beyond it, you know that too, before anything heavy goes up there. Ask your building department how they interpret limited attic storage rather than inventing your own definition.
For anything resembling a floor - decking, regular access, a room, anyone sleeping up there - the decision belongs to a design professional and your building department. An engineer can evaluate the existing members against the 30 psf table's assumptions, design reinforcement or new framing where they fall short, and deal with everything in the list above that a table cannot see; this site's data does not cover sistering, reinforcement or any other strengthening method. The permit process exists precisely because this change of use is invisible until it fails.
Use the tables the way they are used in that conversation: as shared vocabulary. Knowing that your 2x8 ceiling joists at 16 in on center are published at 16 ft 6 in for limited storage but that a floor at the same spacing is held to 14 ft 2 in does not answer "can I convert my attic" - it lets you ask the question precisely, of the people whose job is to answer it.
Frequently asked questions
Can I store boxes in my attic?
If your attic is stick-framed and the joists were sized for limited attic storage - the 20 psf ceiling joist table - then light, occasional storage is what that table already assumes. Whether your joists actually meet it depends on their species, grade, size, spacing and span, which you can check against the ceiling joist table on this site. No table converts its assumed uniform load into a safe number of boxes, so keep loads light and spread out, and ask your building department where limited storage ends.
How much weight can my attic hold per square foot?
This site cannot tell you, and it would be dangerous to pretend otherwise. The span tables publish allowable spans for members designed under an assumed uniform load - 20 psf for limited-storage ceilings, 30 psf for attic floors - but your actual capacity depends on the real members, their condition, their span and everything attached to them. Getting a number for your specific attic is an evaluation for a licensed design professional, not a table lookup.
Can I put plywood down on my ceiling joists and make a floor?
Laying decking is easy, which is exactly the problem: it changes nothing structurally while inviting floor-scale loads onto members sized as ceiling joists. The floor table assumes 30 psf at L/360 where the limited-storage ceiling table assumes 20 psf at L/240, and the same member is published more than two feet shorter as a floor at common sizes. Treat decking over an attic as a change of use that needs a design professional and a permit, not as a weekend surface upgrade.
My attic has trusses. Can I use the ceiling joist table for the flat part?
No. A truss bottom chord is not a ceiling joist - it is one member of an engineered assembly whose forces depend on the whole truss, and no lumber span table applies to it. It also must not be cut, drilled or modified, and storage loads it was not designed for are a modification. Anything involving truss attics goes through the truss manufacturer's documentation and a design professional.
What size joists do I need for an attic floor?
Size an attic floor from the 30 psf sleeping-area floor table, not the ceiling joist table - at 16 in on center, Douglas Fir-Larch No. 2 is published at 14 ft 2 in for a 2x8 and 17 ft 5 in for a 2x10. But for a conversion the sizing question comes last: whether the existing structure, bearing, access and egress support a habitable attic at all is a question for a design professional and your building department under your locally adopted code edition.