Bar Joist Takeoff Guide (K and LH Series)
Bar joists (open-web steel joists) are taken off by count and designation, not by scaling a section out of a shape database. A designation like 24K7 tells you the depth and chord series, but the weight per foot comes from the SJI load tables — it's not encoded in the name the way W24x76 is. Your takeoff needs three things per joist mark: the count, the length (bearing to bearing), and the SJI approximate weight so you can carry tonnage for the buyout and the erection estimate.
That's the core difference from rolled steel. A W-shape is something your shop fabricates; a joist is something you buy from Vulcraft, New Millennium, or Canam, designed by their engineers to the loads on the drawings. You still have to count every one of them, because the joist buyout is usually 15-30% of a warehouse steel package, and a missed row of bridging or a miscounted bay comes straight out of margin.
If you're bidding warehouse and industrial work regularly, this pairs with our warehouse and industrial steel estimating guide — joists and deck are the two packages that dominate those roofs.
How do you read a joist designation like 24K7 or 32LH10?
The format is: depth (inches) + series letter(s) + chord section number.
| Designation | Depth | Series | Section number | Approx. weight |
|---|---|---|---|---|
| 24K7 | 24" | K (open web) | 7 | ~9.7 lbs/ft |
| 16K4 | 16" | K | 4 | ~7.0 lbs/ft |
| 30K12 | 30" | K | 12 | ~17.6 lbs/ft |
| 32LH10 | 32" | LH (longspan) | 10 | ~21 lbs/ft |
| 48LH15 | 48" | LH | 15 | ~40 lbs/ft |
Three things to know about the section number:
- Higher number = heavier chords = more capacity. A 24K9 carries more load than a 24K7 at the same depth and weighs more per foot.
- It is not a weight. 24K7 does not weigh 7 lbs/ft (it's closer to 9.7). The number is an index into the SJI chord table. Estimators who treat it as pounds-per-foot under-carry tonnage by 20-40% on every joist line.
- The same section number at different depths weighs differently. An 18K7 and a 24K7 have similar chords but different web material.
You'll also see KCS joists (e.g., 24KCS4) — K-series depths with constant shear capacity along the full length, spec'd where mechanical units or piping hang between panel points. Count them separately from plain K joists; they're priced differently by the joist manufacturer.
Joist girders use a different format: 44G8N9K means 44" deep girder, 8 joist spaces along its length, 9 kips delivered at each panel point. The girder weight isn't in the designation at all — use the manufacturer's table or carry a rule-of-thumb number (below) until the joist supplier quotes it.
What's the difference between K-series, LH, DLH, and joist girders?
| Series | Depths | Typical spans | Where you see it |
|---|---|---|---|
| K | 8"-30" | 8-60 ft | Standard warehouse/retail roofs, bays up to 50 ft |
| KCS | 10"-30" | 8-60 ft | Same depths as K, constant-shear for point loads (RTUs) |
| LH | 18"-48" | 25-96 ft | Longer roof spans, gymnasiums, big-box clear spans |
| DLH | 52"-120" | 60-240 ft | Very long clear spans — arenas, aircraft hangars, distribution centers |
| Joist girder (G) | 20"-120" | 20-100 ft | Primary framing carrying joists, in place of a rolled girder |
For takeoff purposes the practical differences are:
- K joists are commodities. Standard bays, standard seats (2.5" deep), fast quotes. Count, length, done.
- LH/DLH need bearing attention. Seats are 5" (LH) or 7.5" (DLH) deep, and long spans often mean sloped chords, special camber, or bolted splices shipped in two pieces. Flag anything over about 60 ft for the joist supplier.
- Joist girders replace beams in your rolled-steel scope. When the plan shows a joist girder line where you'd expect a W30, that tonnage moves out of your fab scope and into the buyout. Get the scope split right before you price it — this is one of the most common warehouse bid busts.
For a refresher on how joists fit alongside the rolled shapes on the same plan, see understanding steel member types.
How much does a bar joist weigh per foot?
Exact weights come from the SJI tables or the manufacturer's quote, but for carrying tonnage at bid time these ranges hold up:
| Series | Weight rule of thumb | Notes |
|---|---|---|
| K-series | 5-12 lbs/ft (light sections), up to ~18 lbs/ft at 30K12 | Most warehouse roofs land 7-10 lbs/ft |
| KCS | Add ~10-20% over the equivalent K | Constant-shear webs are heavier |
| LH | 15-40 lbs/ft | Scales with depth and section number |
| DLH | 25-100+ lbs/ft | Get manufacturer weights; too much spread to guess |
| Joist girders | 40-100+ lbs/ft | Roughly comparable to the rolled girder they replace, minus 30-40% |
A sanity check that catches most designation misreads: a typical 40 ft x 50 ft warehouse bay with 24K joists at 5 ft on center carries roughly 0.8-1.2 tons of joists. If your takeoff shows a bay at 3 tons of joists, you've probably read section numbers as weights or double-counted a typical bay.
Bridging adds real weight too: horizontal bridging is usually 1-1/4" x 1-1/4" angles at about 1 lb/ft, run in continuous rows across the full bay width, both top and bottom chord on many jobs. Three rows across a 200 ft building width, top and bottom, is 1,200 lineal feet of angle per bay line — it adds up.
How do you count joists and bridging from a roof framing plan?
Joists on a roof framing plan are single lines between girder lines, usually with a spacing callout instead of every piece labeled. The count workflow:
- Find the spacing callout per bay — "11 SP @ 5'-0"" or "JOISTS @ 6'-3" O.C." The piece count is spaces + 1 at the start of a run, then spaces per bay after (don't double-count the joist shared at a column line — check whether the plan draws one joist or two at each girder).
- Confirm the mark applies to the whole bay. Plans commonly label one joist "24K7 TYP." and rely on you to count the rest. That's normal for joists — unlike rolled members, "typical" callouts are how joist plans are drawn, so this is the one place count-by-callout is legitimate.
- Catch the exceptions inside typical bays. KCS substitutions at RTU locations, doubled joists at folding partitions or heavy screenwall posts, short infill joists at stair and hatch openings. These are the marks buried in plan notes and detail bubbles.
- Count bridging rows separately. Bridging shows as dashed lines across the joists. Rows are set by span and chord size per SJI (roughly: 1 row under 20 ft spans, 2 rows to ~30 ft, 3 rows to ~40 ft, 4-5 rows on longer LH spans). Multiply rows x bay width x number of bay lines, and note whether it's horizontal (angles) or diagonal (X-bridging, which roughly doubles the lineal footage).
- Pick up the joist-adjacent steel that stays in your scope: bearing seats' anchor bolts or welds, tie-joist bolted connections at column lines (OSHA requires bolted tie joists at columns), edge angles, and headers at openings.
We walk a full plan through this process, bay by bay, in the warehouse steel takeoff example.
Who supplies bar joists — and what does the fabricator actually bid?
Joists are designed, fabricated, and usually delivered by a joist manufacturer — not your shop. The structural fabricator's bid typically includes:
- The joist and bridging buyout — you carry the manufacturer's quoted price (or your tonnage x a $/lb allowance, commonly $1.20-1.80/lb delivered for K-series, more for LH/DLH and girders) plus markup.
- Erection, if you're erecting — joists set fast (a good crew sets 60-100 K-joists a day) but bridging and decking pace the job.
- Interface steel in your fab scope — embed plates, seat stiffeners at girder bearings, tie-joist bolts, edge angles.
What you're not doing is detailing or fabricating the joists. But you're still on the hook for the count: the joist manufacturer quotes off your quantities at bid time (they only do their own takeoff after award), and the GC holds you to your number. This is also why joists frequently ride in the same buyout package as metal deck — same suppliers, same erection sequence — so run both takeoffs together; our metal deck takeoff guide covers the deck half.
One practical note on software: most takeoff tools built around shape databases ignore joist designations entirely because 24K7 isn't in AISC's shapes table. SteelFlo detects and boxes joist marks on the framing plan the same way it boxes W-shapes — every occurrence gets a bounding box you verify on screen — so your joist count comes from the same reviewed takeoff as your rolled steel instead of a separate manual pass.
Are bar joists cheaper than rolled beams for the same span?
For roof loads, almost always — that's why they exist. A worked comparison on a 40 ft roof span at 5 ft spacing, typical warehouse loading:
| Bar joist (24K9) | Rolled beam (W16x31) | |
|---|---|---|
| Weight | ~12 lbs/ft → 480 lbs | 31 lbs/ft → 1,240 lbs |
| Material/supply cost | ~$1.50/lb delivered → ~$720 | ~$0.55/lb mill + fab ~$2.75/lb all-in → ~$3,400 |
| Fabrication | None (manufacturer) | Cope, holes, paint in your shop |
| Erection | Fast, light picks | Heavier picks, more crane time |
Roughly speaking, a joist delivers the same span for 40-60% of the weight and a fraction of the in-place cost, because the open web puts steel only where the stress is. Rolled beams win when there are heavy concentrated loads, vibration or deflection limits (office floors), future flexibility requirements, or fire-rating assemblies that favor solid sections — which is why you see joists on the roof and W-shapes on the floors of the same building.
For the estimator, the comparison matters at scope-review time: when a value-engineering round swaps rolled purlins for joists (or vice versa), your fab tonnage, buyout dollars, and erection hours all move in different directions. Re-run the takeoff; don't ratio the old number.
Counting a few hundred joist marks, bridging rows, and the rolled steel around them is exactly the kind of repetitive plan-reading that eats an estimating day. SteelFlo reads your PDF framing plans, puts a bounding box on every steel label it finds — joist marks included — and gives you a reviewable count with 95-99% accuracy after human verification, backed by 4,500+ sections across 6 standards for the rolled-steel side. Start with 3 free takeoffs and run your next warehouse roof through it.