HELP & GUIDES

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 12' 0", JOIST SIZE 2x10, SPACING 16 inches OC, RIM JOIST, BRIDGING, SUBFLOOR and JOIST HANGERS on, WASTE FACTOR 10%, and the result 16 joists, 18 ordered 1 2 3 4 5
The whole form fits on one screen.
  1. FLOOR LENGTH — the side the joists are laid out along. The spacing steps off down this side. The example is 20' 0".
  2. FLOOR WIDTH — the direction the joists run. This is the joist length and the span that gets checked, so enter the distance the joists actually span. The example is 12' 0".
  3. JOIST SIZE — 2x6 to 2x12.
  4. SPACING — on center.
  5. 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.
  6. WASTE FACTOR — the extra you buy over the measured quantity.
  7. Read the answer: 16 joists · 18 ordered, with a green span line under it.

Reading the results

Floor Joist results: 16 joists, 18 ordered, length 12' 0", 9 sheets, 15 blocking, a green line reading Within IRC R502.3.1(2) max span, the BASIS & WHAT WAS ASSUMED row, the FLOOR FRAMING plan drawing, and the MATERIALS list 1 3 4 5 6
Results, the framing plan and the material list.
  1. 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.
  2. The line under it gives the joist length, the subfloor sheets and the blocking pieces.
  3. The span check. Green means the joist size is within the table at this spacing. A SPAN EXCEEDS MAX line gives the longest span that joist size allows at that spacing. 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.
  4. 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 notes that another common species spans a little less. It also gives the length the blocking is cut to. Check the species and grade against the stamp on your lumber.
  5. The drawing is the floor in plan: joists at their spacing, the rim, the blocking line, and the two counts under it. Pinch to zoom, drag to pan, double-tap to reset. The expand button opens it full screen. See Reading the drawing.
  6. MATERIALS — joists at the order count, blocking, rim joist in lineal feet, subfloor sheets and hangers. A switch you turned off takes its row 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