Ceiling joist span calculator: joist size and max span for attics with and without storage

Only over a short span. With limited storage the table gives a No.2 2x6 at 16 in on centre 13′ 0″ in Douglas fir-larch, 12′ 10″ in spruce-pine-fir, 12′ 8″ in hem-fir and 12′ 0″ in Southern pine. A 2x6 over a 14 ft bedroom is past its table for storage in every species, though it clears the no-storage table by more than two feet. The rating is set by the attic's clear height, not by how light the boxes are, so check the attic type before deciding a light load is fine.

Between 20′ 8″ and 7′ 5″ depending on table, species, grade and spacing; No.2 at 16 in runs 16′ 6″ to 17′ 8″ with no storage and 12′ 0″ to 13′ 0″ with storage. The full picture for every 2x size is in how far can a 2x span.

The longest 2x4 cell is 13′ 2″, select structural Douglas fir-larch at 12 in with no storage, and the shortest 5′ 1″, No.3 Southern pine at 24 in with storage. At 24 in on centre with no storage a No.2 2x4 spans 9′ 10″ in Douglas fir-larch, 9′ 5″ in spruce-pine-fir, 9′ 3″ in Southern pine and 9′ 2″ in hem-fir; with storage those drop to 7′ 3″, 7′ 2″, 6′ 7″ and 7′ 1″. A 2x4 tie spanning 16 ft with drywall on it is far outside the table.

At 16 in on centre with limited storage, a No.2 2x8 spans 16′ 6″ in Douglas fir-larch, 16′ 3″ in spruce-pine-fir, 16′ 0″ in hem-fir and 15′ 3″ in Southern pine; a No.2 2x10 spans 20′ 2″, 19′ 10″, 19′ 7″ and 18′ 1″ in the same order. So the forum rule that a 2x8 meets code for limited storage is right up to about 16 ft, and only in three of the four species at 16 ft exactly. The longest storage span in the table is 25′ 5″, a select structural spruce-pine-fir 2x10 at 12 in.

Clear height. Note b to Table R301.5 makes an attic without storage one where the clear height between joists and rafters is not more than 42 in; note g makes limited storage one where it is 42 in or more. Trusses use the same 42 in by 24 in rectangle between the webs. Without storage the joists carry 10 psf live and 5 psf dead; with storage, 20 and 10. The definitions do not mention what you intend to store, so a tall attic is a storage attic even if it stays empty.

The limited storage table, if the attic has the height. Plywood does not change the attic type; the clear height already did, and note g applies the 20 psf wherever the access opening, joist slope and insulation depth conditions are met. Laying a floor in an attic under 42 in tall does not turn it into a storage attic on paper, but it also does not make the 10 psf table safe for boxes: the code has no category for a low attic used for storage, and the joists were sized for an empty one.

No. A habitable attic, and any attic served by fixed stairs, is rated at 30 psf in Table R301.5, and Section R502.3.1 sizes its joists from the floor tables, the same as a bedroom floor. The calculator stops and points you to the floor joist span calculator when you choose habitable. The floor tables are also at L/360 rather than L/240.

No. A ceiling joist bears on the walls and carries ceiling and attic load, and this table sizes it. A rafter tie is a 2x4 minimum at 24 in maximum in the bottom third of the attic that resists roof thrust where no parallel ceiling joist does; it is not sized for a ceiling. A collar tie is a 1x4 minimum at 4 ft in the upper third, for uplift at the ridge. Where ceiling joists run parallel to the rafters in the bottom third of their height they serve as the tie as well, nailed per Table R802.5.2(1).

Nothing directly: the tables stop at 24 in on centre and there is no span for 48 in. Ties at 48 in are common in older garages built without a ceiling, where each 2x4 or 2x6 is a rafter tie, not a ceiling joist. Adding drywall or insulation to them puts a ceiling load on members that have no table. The prescriptive answer is to add joists between them so the spacing is 24 in or less and read the size from the chart, or to have the assembly designed.

Yes; 24 in is one of the four tabulated spacings and reads from its own column, which prints roughly two feet less span than 16 in for the same joist. Drywall, insulation and the trade-offs are in joist spacing and size.

Neither changes what the table says. A strongback nailed across the joists spreads the load between them and stiffens the ceiling, but it bears on nothing, so the span in the table is unchanged. Hanging joists from the rafters moves ceiling load onto rafters sized without it. First run the joists through max-span mode at the attic rating actually in use; if the printed span is shorter than the real one, the fix is a support that shortens the span, a bearing wall or a beam on posts, or a deeper joist, and that is engineering, not a lookup.

Yes, or butt over it. Section R802.5.2.1 requires ends of ceiling joists to be lapped not less than 3 in or butted over bearing partitions or beams and toenailed to the bearing member. Where the joists provide the continuous tie across the building, the lap is nailed with the count from Table R802.5.2(1) and butted joists get a connection of equivalent capacity; where they do not, the lap is nailed per Table R602.3(1). Each end also needs 1½ in of bearing on wood or 3 in on masonry, Section R802.6.

That is note a: the span exceeds 26 ft and the code does not print it. It appears in 39 cells, 36 of them in the no-storage table, all in the 2x8 and 2x10 rows at select structural, No.1 and No.2. The calculator returns a not-tabulated warning for those cells rather than a number. The table also notes that lumber longer than 20 ft may not be available, so a printed 24 or 25 ft span is a supply question before it is a code one.

Because the load per joist is the same. A joist at 24 in carries 2 ft of ceiling at 15 psf, 30 lb per foot of joist; a joist at 12 in carries 1 ft at 30 psf, also 30 lb per foot. With the same L/240 limit the tables print the same span, and all 64 cells match. It is a handy check when reading the printed page, and it shows that halving the spacing exactly recovers the span that storage takes away.

Douglas fir-larch, in every No.2 cell where all four species are printed and in 107 of the 116 four-species cells overall. The exceptions are Southern pine No.1 2x6 and 2x8 cells and select structural 2x10 at 24 in, where Southern pine is slightly longer. At No.2, Southern pine is the shortest or tied in all 30 four-species cells and never out-spans spruce-pine-fir, a consequence of its 2013 design value revision. No.1 and No.2 spruce-pine-fir print the same span in every cell.

Not less than 1½ in on wood or metal and not less than 3 in on masonry or concrete, Section R802.6. On masonry the bearing is either direct or on a 2x sill plate providing 48 sq in of bearing area. The span in the table is measured between those bearing faces, so a joist that is 1½ in short of full bearing at each end is also spanning 3 in more than you measured between the walls, which is worth checking on a cell that is close to its limit.