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Project tools

Joist Span Checker

Check proposed joist sections against simplified span values for C16/C24 at common centres.

Related guidesSpecies data
Slide the span first. Tap grade, spacing, depth until PASS appears—everything is illustrative vs generic span tables excerpted internally.

Quick start

  1. Read your proposed clear span joist-to-joist in mm along unsupported direction.
  2. Choose grade badge (C16 cheaper, C24 reaches further for same spacing).
  3. Centres = neighbouring joists gap—400 frequent domestic floors, 600 sometimes lofts/roofs simplified.
  4. Depth buttons map to common merchant sections—the tool compares maximum tabulated span excerpt vs your slider.

What each thing means

Span slider
Unsupported length between bearing faces. Bearings assumed adequate (wall plate, hanger, masonry pocket). Includes no cantilever logic.
C16 / C24
Strength classes from EN 338. Higher number generally permits longer spans at same spacing if sizes match stocking.
400 / 600mm centres
Spacing along bearer determines load share per joist — wider centres shorten allowable span excerpt.
47×147-style depths
Nominal CLS style sections mirrored from simplified reference slice—never every supplier SKU.

Tap combinations—no Submit button.

Result

PASS

Max safe span: 3700mm

Your span

3200mm

Margin

500mm

Utilisation

86%

Section

47×147

Get better results

  • Heavy wet trades ( tiling, aquariums ) warrant structural engineer—even PASS here might be nonsense.
  • Check bearing length minima (~35–50 mm masonry practical rules of thumb)—not modeled.
  • Creep & vibration comfort often govern before absolute rupture — longer spans might feel springy PASS still.
  • Strapping, hangers, restraint straps are separate—not implied by PASS.
  • If anything structural for Building Control, commissioning calc beats this preview.

Reading the numbers

PASS means span ≤ excerpted allowable snapshot; margin shows leftover capacity before theoretical cap within this toy table—not global safety factor stack. FAIL means lengthen section, tighten spacing, or pick stronger grade—not legal advice.

Not a substitute for structural sign-off

Ultra-simplified illustrative subset—not TRADA, SCI, Approved Document A replacements, excludes point loads, holes, notches, composite action, diaphragm bracing verification. Mandatory professional design where law or insurer demand.