Floor Joist

What it's for

Framing a floor with sawn lumber joists. You give it the floor size, the joist size and the spacing, and it gives you the joist count, the number to order, the joist length, subfloor sheets, blocking, a framing plan and a material list. It checks the joist's span against the IRC floor joist span table and flags a joist that is too small for the span.

Step by step

This example uses the numbers the screen opens with.

Floor Joist inputs: FLOOR LENGTH 20' 0", FLOOR WIDTH (OUTSIDE OF RIM) 12' 0", ONE END ON A GIRDER off, JOIST SIZE 2x10 among 2x6, 2x8, 2x10 and 2x12, SPACING 16" o.c., FRAMING SPECIES, GRADE and DEAD LOAD all Not picked, then RIM JOIST, BRIDGING and SUBFLOOR on, with JOIST HANGERS and WASTE FACTOR below the fold. The answer bar reads 16 joists · 18 ordered, an amber Within IRC 2021 R502.3.1(2) — SPF No.2 · 10 psf dead, and Joists cut 11' 9" · 9 sheets · 15 blocking. — Steps 1 to 6 and the first of the switches in step 7. The span line is amber until the species, grade and dead load are picked.

FLOOR LENGTH — the side the joists are laid out along. The spacing steps off down this side. The example is 20' 0". It takes 1' 0" to 100' 0"; anything else is refused, not rounded.
FLOOR WIDTH (OUTSIDE OF RIM) — the direction the joists run, measured over the outside faces of the rim joists: the whole box you frame, the same box the drawing dimensions. The joists are cut to fit between the two 1-1/2" rims, so each is 3" shorter than the width — 11' 9" in the example. With RIM JOIST off, nothing stands outside the joist ends and they run the full width. The span check reads the span the code's table means — face to face of the supports — so with rims it is the width less the two rims (the joist's own length between them), and with no rim the full width. The example is 12' 0". It takes 1' 0" to 100' 0": a floor narrower than 1' is refused, not read as 1'.
ONE END ON A GIRDER — for one side of a floor broken over a girder or a bearing wall. Measure FLOOR WIDTH (OUTSIDE OF RIM) from the outside of the rim to the girder's centre line. The joists then run 1-1/2" past the centre so the two sides lap 3" over the girder, as the code requires (IRC R502.6.1), with three 10d nails at each lap. The list buys one rim, hangers at the rim end only and the lap nails, and the drawing shows the girder. Size the girder itself in Beam Sizing. Run the other side the same way.
JOIST SIZE — 2x6 to 2x12.
SPACING — on center.
FRAMING SPECIES, GRADE and DEAD LOAD pick the row of the code's span table the check reads. Take the species and grade from the stamp on your lumber: Douglas fir-larch, hem-fir, southern pine or spruce-pine-fir (SPF), and Select Structural, No.1, No.2 or No.3. Most framing lumber is No.2. Use the 20 psf dead load for a heavy floor, such as tile on a mortar bed. A note under the row then says the code caps a floor's dead load at 10 psf in Seismic Design Categories D0–D2 (IRC R301.2.2.2), so there a heavier floor needs an engineered design. Each one starts at Not picked. While anything is not picked, the check reads SPF No.2 at 10 psf dead and the span line is amber.
Four switches add rows to the material list. RIM JOIST is the joist on edge across the joist ends. BRIDGING is the blocking between joists at mid-span. SUBFLOOR is the sheathing. JOIST HANGERS are the hangers, two per joist, for joists hung between the rims or off a beam or ledger. A floor whose joists sit on sill plates or walls needs none: turn it off, and the list drops them and the drawing's detail shows the joist bearing on the plate (at least 1-1/2", IRC R502.6) with the rim nailed to its end.
WASTE FACTOR — the extra you buy over the measured quantity.
Read the answer: 16 joists · 18 ordered, with an amber span line under it.

Reading the results

Floor Joist scrolled to the results. The answer bar keeps 16 joists · 18 ordered and the amber span line. Above RESULTS are the RIM JOIST, BRIDGING, SUBFLOOR and JOIST HANGERS switches, all on, and WASTE FACTOR 10%. RESULTS reads Joists cut 11' 9" · 9 sheets · 15 blocking, then the amber Within IRC 2021 R502.3.1(2) — SPF No.2 · 10 psf dead, the BASIS & WHAT WAS ASSUMED row, an outlined CHECK IT IN I-JOIST HOLE GUIDE button and the link Check before you cut · Measure twice. The top of the FLOOR FRAMING drawing starts under them, its joists at their spacing, the 12' 0" width dimension and the BLOCKING and JOIST labels. — Steps 8, then the lines under the headline, the amber span check, the basis row and the I-Joist carry button.

The FLOOR FRAMING plan: 16 JOISTS IN THE LAYOUT · 18 ORDERED, SPF NO.2 · 10 PSF DEAD — VERIFY, the plan dimensioned 12' 0" by 20' 0" with the first joist 16" in, BLOCKING, JOIST and RIM JOIST labelled, and the title block FLOOR FRAMING 2x10 @ 16" OC. Under it MATERIALS: 2x10 joists — 12' stock — cut @ 11' 9", 18 pcs; 2x10 blocking — cut @ 1' 2-1/2" — clear bay at 16" OC, 15 pcs; Rim joist (2x10), 44 lin ft; Subfloor sheathing 19/32" T&G — 4x8, 9 sheets; 8d common nails, 453 nails. — The framing plan and the top of the material list.

The headline has two counts. The first is the joists in the layout. The second is the number to order, with the waste factor in it.
The line under it gives the length the joists are cut to (between the rims), the subfloor sheets and the blocking pieces.
The span check. Within IRC 2021 R502.3.1(2) — SPF No.2 · 10 psf dead means the joist size is within the table at this spacing, for the species, grade and dead load named after the dash. The line is amber until you have picked all three, because until then it is the app's assumption, not your lumber. Southern pine No.2 spans less than SPF (a 2x10 at 16" o.c. allows 14' 0" against SPF's 15' 5"), and so does a 20 psf dead load. Once all three are picked, the line turns green. The drawing and the cut sheet name the same basis, and the cut sheet lists your picks. A SPAN EXCEEDS MAX line gives the longest span that joist size allows at that spacing, for the same basis. Go up a size in JOIST SIZE, tighten SPACING, or shorten the span with a beam or a bearing wall under the floor and enter the shorter span as FLOOR WIDTH (OUTSIDE OF RIM).
BASIS & WHAT WAS ASSUMED — tap it. It names the table, and the species, grade, floor loads and deflection limit the table was read for. It also lists what the span check does not cover: bearing at the ends (1-1/2" on wood or metal, 3" on masonry), notches and holes, cantilevers, joists under bearing partitions and point loads. It notes that another common species spans a little less. It also gives the length the blocking is cut to, and how the joist cut comes off the width. Check the species and grade against the stamp on your lumber. It ends with IRC 2021 — verify local code and a sentence that states and municipalities amend the code: check the adopted edition and local amendments with your building department.
BASIS also says what is not checked: the loads are gravity only, so where IRC R301.2.1.1 requires wind design, or in Seismic Design Category D0 to E (R301.2.2; townhouses from C), the added rules are not applied.
The drawing is the floor in plan: every joist in the count at its spacing, the last one flush with the far end, the rim along the joist ends, the blocking line, and the two counts under it. The width is dimensioned over the rims, and the joists are drawn between them. Pinch to zoom, drag to pan, double-tap to reset. The expand button opens it full screen. See Reading the drawing.
MATERIALS — joists at the order count with the stock length to buy and the length to cut (the cut list shows the same cut) — lumber is stocked to 24', so a longer joist says it is past stock lumber instead of naming a length nobody sells, blocking, the rim joist in lineal feet at the joist size (the two bands across the joist ends; the two end joists close the box and are in the joist count), subfloor sheets and hangers (two per joist, except the end joists, which sit on the end walls; hardware, so the exact count with no waste). The subfloor row names the panel your spacing needs: 19/32" tongue-and-groove up to 19.2" o.c. and 23/32" at 24" (the code's single-floor rows, IRC Table R503.2.1.1(1)). Two nail rows follow the code's fastening schedule (IRC Table R602.3(1)): 8d common for the subfloor (6" apart at the panel ends, 12" in the field) and the blocking (two at each end), and 16d common, three through the rim into each joist end. A switch you turned off takes its rows out.

Under the list, CUT SHEET, the list icon at its right end, CUT LIST and SEND LIST stay dim until you change at least one input. SAVE stays lit, but while every input is still a default a tap only answers Enter your job's numbers first and saves nothing. CUT SHEET makes a PDF, the list icon at its right end shares the material list only, SAVE keeps the result in History, CUT LIST puts the list on the Lock Screen and SEND LIST sends a checkable list to another phone.

Floor Joist, and the cut sheet, cut list, send list and History buttons, are part of Pro, a one-time purchase. See Free and Pro.

Common mistakes

Entering the joist length as the width. The width is over the rims. If you measured the opening between the rims, add the two rims back on.
Swapping length and width. The joists run across FLOOR WIDTH (OUTSIDE OF RIM). If yours run the long way, enter the long side as the width. The drawing shows which way they run.
Entering the whole building width over a center beam. The span that gets checked is FLOOR WIDTH (OUTSIDE OF RIM). With a beam or bearing wall under the floor, run each side as its own floor with ONE END ON A GIRDER on, the width measured to the girder's centre line. With the switch off, each side is framed as a closed box with a rim at the girder, which is not how a lapped floor is built.
Ordering the first number. The first count is the layout. The second has your waste in it.
Taking the span line as the last word. The table is for one species, grade and loading, and a uniform floor load on a simple span. A notch or hole past the code's limits, a cantilever, a bearing wall above or a point load (a post, a tub, a safe) needs its own check. Open BASIS & WHAT WAS ASSUMED and compare. For I-joists use I-Joist Hole Guide.

Trying I-joists

CHECK IT IN I-JOIST HOLE GUIDE, under the results, opens the I-Joist Hole Guide with this floor's clear span (face to face of the rims, the span its tables read — not the outside-of-rim width), its spacing and its dead load filled in. The series, depth and deflection stay as you left them there. Back returns to the floor.