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Beam Sizing

What it's for

A first size for a beam under a uniform load. You give it the span, the width of floor or roof the beam carries, what holds it up, the kind of load and the material, and it gives you a beam size, the load per foot, the jack and king studs under each end in a wall (or the bearing it needs on a post), a load drawing and a material list. This is preliminary sizing, not a code check. The screen says so on every result and on the drawing. Have the size confirmed from span tables or by an engineer before you order. If the beam carries a point load, no span table covers it: use an engineer.

Step by step

This example uses the numbers the screen opens with.

Beam Sizing inputs: SPAN 10' 0", TRIBUTARY WIDTH 8' 0", LOAD TYPE Floor (a menu), SUPPORT In a wall (the other choice is On posts), WALL HEIGHT 8' 0", WIND / FRAMING ANCHOR ≤ 115 mph Exp B (the other is Higher / anchored), MATERIAL Dimensional, FRAMING SPECIES Not picked, DEAD LOAD Not picked and INCLUDE FLITCH PLATE off. The answer bar reads (3) 2x10 with an amber line, Preliminary size — SPF No.2 · 40 psf live · 10 psf dead · simple span, uniform load, then 400 PLF total load. 1 2 3 4 5 6 7 8 9
The form and the answer.
  1. SPAN — the clear distance between the beam's supports. The beam itself is longer, because it bears on the jack studs (or posts) at each end. The drawing gives both lengths. The example is 10' 0".
  2. TRIBUTARY WIDTH — the width of floor or roof whose load lands on this beam. It is usually half the joist span on each side of the beam. It is not the beam's length. The example is 8' 0".
  3. LOAD TYPE — Floor, Roof, Deck or Exterior Wall, picked from a menu. Each one applies its own load per square foot, and the drawing prints the figure that was used. Exterior Wall is a flat 70 psf: the app's own allowance, not a code table, and BASIS says so — for a header in an exterior bearing wall use Header Sizing, which reads the code's table. Deck is a flat 60 psf with no wet-service adjustment; the Deck calculator sizes deck beams from IRC Table R507.5.
  4. SUPPORT — In a wall (jack and king studs under each end) or On posts. A Deck beam starts on posts, every other load in a wall. In a wall the beam is at most three plies of 2x (4-1/2"), since four plies (6") are wider than a 2x6 wall; the jack studs come from bearing (the end reaction on the beam's width at Hem-fir's 405 psi across the grain), never fewer than one per 6' of span. On posts there are no studs: the result gives the bearing each end needs on the post, 1-1/2" at least (IRC R502.6). The post, its cap and its footing are not sized here.
  5. WALL HEIGHT — in a wall only. Sets the length of the jack and king stud stock on the cut list. It does not change the beam size.
  6. WIND / FRAMING ANCHOR — sets the king studs, the full-height studs beside the jacks, from the IRC table that counts them by header span and wind. Leave ≤ 115 mph Exp B where the design wind speed is 115 mph or less, the site is Exposure B (built-up suburbs, woods) and jack studs carry the beam. Pick Higher / anchored for more wind, an Exposure C site (open country, scattered trees and buildings), or a beam hung on metal framing anchors instead of jacks. From 140 mph in Exposure B, 130 mph in Exposure C, or on any Exposure D site (open water, flats) the table gives no count, and the kings have to be designed. Your building department gives the wind speed and exposure. It does not change the beam size. In a wall only.
  7. MATERIAL — Dimensional gives a built-up lumber beam. LVL gives an LVL beam. Glulam and Steel are always engineered: the headline says so and there is no material list; a line under it gives a rough tier (a depth, no weight) as a starting point for the engineer.
  8. What the size is worked out for. FRAMING SPECIES (Dimensional only) is the lumber on your stamp: Douglas fir-larch, hem-fir, southern pine or spruce-pine-fir, No.2 grade. Each has its own strength from the wood design tables. DEAD LOAD (Floor and Roof) is the weight of the structure itself: 10, 15 or 20 psf. Over the code's seismic cap — 10 psf on a floor, 15 on a roof (IRC R301.2.2.2, Seismic Design Categories D0–D2) — a note under the row says so. A Floor adds the 40 psf live load to it. On a Roof, GROUND SNOW is your county's ground snow: 20, 30, 50 or 70 psf. The roof is sized for the greater of 20 psf and your snow, plus the dead load, following the code's roof-load rule (IRC R301.6). Snow drifts against a taller wall are not included. Over 70 psf gives a red line: the beam must be engineered. Each one starts at Not picked. Until it is picked, the size is worked out for SPF, 10 psf dead on a floor or 15 on a roof, and 20 psf on a roof, and the line under the size is amber.
  9. INCLUDE FLITCH PLATE shows with Dimensional only. Turn it on to sandwich steel plates between the plies. Two more rows appear: FLITCH PLATES, the number of plates, and FLITCH PLATE THICKNESS. The plates now carry load in the sizing, so a plated beam can be smaller: each plate, 1/2" shallower than the lumber, takes its share by stiffness, and the 1/2" bolts at 16" apart are checked for moving that share into the plate (double shear across the grain, NDS 12.3). Every plate needs a ply each side of it, so four plates need five plies — more than the list has, and the beam is then engineered. The results add the plate size, and the load each bolt carries against what it can carry.
  10. Read the answer: (3) 2x10.

A valley from Hip & Valley

In Hip & Valley's Valley mode, SIZE THE VALLEY IN BEAM SIZING opens this screen with the valley already filled in. The example is a 20' wing on a 28' main roof.

Beam Sizing opened from a valley. The answer bar reads (2) 2x12 with an amber line Set the ground snow for this roof — sized at the 20 psf roof live load until you do (IRC R301.6), then 176 PLF total load. SPAN ≈ 14' 1-11/16", TRIBUTARY WIDTH ≈ 5' 0-3/16", LOAD TYPE Roof, SUPPORT On posts (In a wall beside it), MATERIAL Dimensional, FRAMING SPECIES Not picked, DEAD LOAD Not picked, GROUND SNOW Not picked with the same amber line under it, and INCLUDE FLITCH PLATE off. There is no WALL HEIGHT or WIND row. 1 2 3
The carried valley: the form (1 to 3) and the amber ground-snow line above it.
  1. SPAN and TRIBUTARY WIDTH arrive with ≈ in front of them: the valley's span in plan (≈ 14' 1-11/16") and a tributary width that loads it as evenly as its jacks do at the worst point (≈ 5' 0-3/16"). Retype either one and the beam is an ordinary one again, with the same load at each end.
  2. LOAD TYPE is Roof and SUPPORT is On posts: a valley bears on the wall plates at the corner and on a post at the ridge, never in a wall. The wall height and wind rows go away with it.
  3. GROUND SNOW comes from Roof Rafter when you have picked one there. When Roof Rafter has none, it stays Not picked, and an amber line leads the answer; the same words repeat in grey under the row: Set the ground snow for this roof — sized at the 20 psf roof live load until you do (IRC R301.6). Pick your county's snow and the line clears. Your species and material stay as you left them.
The results of the carried valley. The answer bar keeps (2) 2x12 and the amber Set the ground snow for this roof — sized a… line. RESULTS reads 176 PLF total load, Depth 11-1/4", On posts: 1-1/2" bearing min each end, Valley reactions: 1,517 lb at the plate end · 933 lb on the ridge-end post, LVL equivalent available, Baseline estimate uses tributary width and common residential loading. and Confirm bearing, fasteners, and species before ordering., then two amber lines: Set the ground snow for this roof — sized at the 20 psf roof live load until you do (IRC R301.6) and Preliminary size — SPF No.2 · 20 psf roof live · 15 psf dead · simple span, the valley's jack load (its peak moment as a uniform load). Then the BASIS & WHAT WAS ASSUMED row, the link Check before you cut · Measure twice and the top of the drawing, titled VALLEY JACKS — PEAK-MOMENT EQUIV. (≈ 5' 0-3/16"), w 176 PLF (35 PSF × ≈ 5' 0-3/16") · W 2450 LB. 4 5
The valley's results: its two reactions (4) and the amber basis line (5).
The valley's drawing and material list. The drawing, titled VALLEY JACKS — PEAK-MOMENT EQUIV. (≈ 5' 0-3/16") with w 176 PLF (35 PSF × ≈ 5' 0-3/16") · W 2450 LB, shows the load arrows on the beam, a ≈ 14' 1-11/16" SPAN and a ≈ 14' 4-11/16" BEAM, and the tributary plan marked ≈ 5' 0-3/16" TRIB. Under it R 1517 PLATE · 933 RIDGE · ON POSTS · 1-1/2" BRG MIN and PRELIM — SPF NO.2 / L/360 — VERIFY PER ENGINEER, over the strip BEAM — (2) 2X12, ROOF, NOT TO SCALE. MATERIALS lists Beam (2) 2x12 — cut @ ≈ 14' 4-11/16" · buy 2 × 16', 2 pcs and 20d common nails 18 pcs. Under it CUT SHEET with a list icon, SAVE, CUT LIST and SEND LIST, all lit. 6
The drawing prints both reactions (6), the plate end first.
  1. A valley's jacks load it more at the plate end than at the ridge, so the result gives its two reactions: Valley reactions: 1,517 lb at the plate end · 933 lb on the ridge-end post for this 20' wing on a 28' main roof at 35 psf. The ridge-end post takes 38% of the total, 933 of 2,450 lb. The post bearing is sized for the plate end.
  2. The line under the size says what it was worked out for: the roof live load (20 psf here), 15 psf dead, and the valley's jack load as a uniform load that gives the same peak moment. It stays amber until the species, dead load and ground snow are picked.
  3. The drawing is titled VALLEY JACKS — PEAK-MOMENT EQUIV. and prints both reactions beside the bearing, R 1517 PLATE · 933 RIDGE, instead of one figure for each end.

Reading the results

Beam Sizing scrolled to its results. Above them DEAD LOAD Not picked and INCLUDE FLITCH PLATE off; the answer bar keeps (3) 2x10 and the amber Preliminary size line. RESULTS reads 400 PLF total load, Depth 9-1/4", 2 jack / 2 king per side, LVL equivalent available, the notes Baseline estimate uses tributary width and common residential loading. and Confirm bearing, fasteners, and species before ordering., then the amber line Preliminary size — SPF No.2 · 40 psf live · 10 psf dead · simple span, uniform load, the BASIS & WHAT WAS ASSUMED row and the link Check before you cut · Measure twice. The top of the beam drawing follows: TRIBUTARY — ½ WAY TO NEXT BEARING EA SIDE (4' 0"), w 400 PLF (50 PSF × 8' 0") · W 4000 LB, the load arrows on the beam, the 10' 0" SPAN and the tributary plan marked 8' 0" TRIB. 2 3 4
The lines under the headline, the amber check and the basis row.
The rest of Beam Sizing: the load drawing with w 400 PLF (50 PSF × 8' 0") · W 4000 LB, a 10' 0" SPAN and a 10' 6" BEAM, R 2000 LB EA END · 2 JACK + 2 KING · 3" BRG, a PRELIM line, and the title block BEAM — (3) 2X10, FLOOR, NOT TO SCALE. MATERIALS lists Beam (3) 2x10 — cut @ 10' 6" · buy 3 × 12' 3 pcs, Jack studs 4 pcs, King studs 4 pcs and 20d common nails 28 pcs. Under it a dimmed CUT SHEET with a list icon, a lit SAVE, and dimmed CUT LIST and SEND LIST over the line Enter your own measurements to export, start a cut list or send one. 6 7 8
The load drawing, the material list and the buttons.
  1. The headline is the beam: the number of plies in brackets, then the size.
  2. The lines under it. PLF total load is the load on each foot of beam: the load per square foot times the tributary width. Depth is the actual depth of the beam, in inches. Then the jack and king studs under each end, or on posts the bearing each end needs. A line such as LVL equivalent available tells you another material would also work. An engineered beam shows none of these: no depth, no studs, no alternate. Two standing notes finish the list. They remind you the load is a common residential figure, and to confirm bearing, fasteners and species.
  3. The check. The line under the answer starts amber. It names what the size was worked out for: Preliminary size — SPF No.2 · 40 psf live · 10 psf dead · simple span, uniform load on a floor. It stays amber until you have picked the species, dead load and (on a roof) ground snow. It turns green once they are picked, because the size is then worked out for your lumber and your loads. On a Roof it names the snow too. Deck and Exterior Wall use one flat load each, so their line stays amber. The cut sheet and the drawing carry the same basis. A red line shows when something is wrong. GROUND SNOW OVER 70 PSF — THIS BEAM MUST BE ENGINEERED (IRC R301.2.3) means the code sends that roof to an engineer, so nothing is sized. NO STOCK SIZE CARRIES THIS means nothing in the list carries that span and load, and the beam has to be engineered. Shorten SPAN with a post, reduce TRIBUTARY WIDTH, or change MATERIAL. Glulam and Steel always show a red line saying those sizes are rough and must be engineered.
  4. BASIS & WHAT WAS ASSUMED — tap it. It gives the species and grade the lumber sizes assume and their design values, notes that another common species is weaker, gives the LVL values and how an LVL's bounce is checked (the span ÷ 360 under the live load, ÷ 240 under the whole load), says how the beam's buy length is worked out, names the column of IRC Table R602.7.5 the king studs were read from (the span and your wind choice), and ends with the preliminary sizing statement. Check the species against what your yard stocks.
  5. BASIS also says what is not checked: the loads are gravity only (the wind column on Table R602.7.5 is the one wind input), 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.
  6. The drawing shows the load on the beam, the span and the beam length, the reaction at each end with the studs and bearing under it (a valley's two: R … PLATE · … RIDGE), and the tributary width in plan. The PRELIM line is on every drawing. It is a reminder, not a failure. Pinch to zoom, drag to pan, double-tap to reset. The expand button opens it full screen. See Reading the drawing.
  7. MATERIALS — the beam plies, each cut to the span plus its bearing at both ends and bought on even-foot stock (for example Beam (3) 2x10 — cut @ 10' 6" · buy 3 × 12', 3 pcs); the nails that make the plies one beam (20d at 32" top and bottom of each ply, plus two at each end, IRC Table R602.3(1) item 28; for LVL the maker's multi-ply nailing); the jack and king studs for both ends in a wall; and with flitch plates the plates, the 1/2" bolts with their length, nuts and washers. When the beam has to be engineered there is no list.
  8. 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 pieces on the Lock Screen with the beam length and the stud stock length, and SEND LIST sends a checkable list to another phone.
Beam Sizing, 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