Framing & structure
CLT floor panel span calculator
Estimate the maximum single span of a PRG 320 CLT floor panel from bending, shear, deflection and vibration, and the thinnest layup for your span.
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How far can a CLT floor panel span?
A CLT span calculator estimates the maximum span of a cross-laminated timber floor panel and which check governs it. Choose a PRG 320 layup, enter the superimposed dead load, live load and panel density, and it returns the span limits for bending, shear, live-load and long-term deflection and vibration, the governing maximum span, and whether your design span works.
For typical residential and office floors, vibration usually governs rather than strength. With 20 psf of finishes and 40 psf live load, a 5-ply E1 panel (6⅞ in) could reach 32.8 ft in bending and 19.8 ft in long-term deflection, but the vibration criterion limits it to about 17.4 ft (5.3 m). A 3-ply panel reaches about 12.5 ft and a 7-ply about 21.8 ft.
The calculator uses the ANSI/APA PRG 320-2019 reference values for basic CLT grades and a single simply supported span with uniform load. Proprietary layups, continuous spans, point loads, openings, toppings that add stiffness, and fire and acoustic design are not covered. Confirm with the manufacturer's certified values and a structural engineer.
How to use the CLT floor panel span calculator
- Choose Imperial or Metric. Imperial is the default, and your choice carries across the free tools.
- Pick the CLT layup: grade E1, E2, E3, V1 or V2 with 3, 5 or 7 plies of 1⅜ in (35 mm) laminations. Enter the Design span you want to check.
- Enter the Superimposed dead load for finishes, toppings, ceilings and services, and the Live load. Panel self-weight is added from the CLT density.
- Keep or change the Live-load deflection limit (L/…), the Long-term deflection limit (L/…) and the Creep factor Kcr.
- Read the maximum span and the criterion that governs, the span for each check, and the thinnest layup in the same grade that reaches your design span. Compare every layup in the table and download the PDF or CSV.
Worked example
Imperial: An E1 5-ply (6⅞ in) panel with 20 psf superimposed dead and 40 psf live load reaches 32.8 ft in bending and 19.8 ft in long-term deflection, but vibration limits it to 17.4 ft.
Metric: An E1 5-ply (175 mm) panel with 1.0 kPa superimposed dead and 2.0 kPa live load reaches 9.84 m in bending and 5.98 m in long-term deflection, but vibration limits it to 5.32 m.
PRG 320 CLT layups and design values
ANSI/APA PRG 320 defines basic CLT grades by the lumber in the layers. E grades use machine stress-rated lumber in the longitudinal layers; V grades use visually graded lumber. Table A2 gives ASD reference values per foot of width in the major strength direction: the moment capacity (FbS)eff, the bending stiffness (EI)eff, the shear rigidity (GA)eff and the shear capacity Vs.
Manufacturers also certify their own layups under PRG 320, often with different lamination thicknesses and values, so use their published numbers for design.
| Layup | Thickness | (FbS)eff (lbf-ft/ft) | (EI)eff (10⁶ lbf-in²/ft) | (GA)eff (10⁶ lbf/ft) | Vs (lbf/ft) |
|---|---|---|---|---|---|
| E1 3-ply | 4⅛ in | 4,525 | 115 | 0.46 | 1,490 |
| E1 5-ply | 6⅞ in | 10,400 | 440 | 0.92 | 2,480 |
| E1 7-ply | 9⅝ in | 18,375 | 1,089 | 1.4 | 3,475 |
| E2 5-ply | 6⅞ in | 8,825 | 389 | 1.1 | 3,300 |
| E3 5-ply | 6⅞ in | 6,400 | 311 | 0.69 | 1,930 |
| V1 5-ply | 6⅞ in | 4,800 | 415 | 1.1 | 3,300 |
| V2 3-ply | 4⅛ in | 2,030 | 95 | 0.46 | 1,490 |
| V2 5-ply | 6⅞ in | 4,675 | 363 | 0.91 | 2,480 |
| V2 7-ply | 9⅝ in | 8,275 | 898 | 1.4 | 3,475 |
Maximum CLT floor span by layup
These are the calculator's results for a single span with 20 psf (1.0 kPa) superimposed dead load, 40 psf (2.0 kPa) live load, 30 lb/ft³ (480 kg/m³) panel density, L/360 live-load and L/240 long-term deflection limits and Kcr = 2.0. Vibration governs every layup at this loading.
| Layup | Bending | Deflection | Vibration | Maximum span |
|---|---|---|---|---|
| E1 3-ply | 22.7 ft | 13.2 ft | 12.5 ft | 12.5 ft (3.82 m) |
| E1 5-ply | 32.8 ft | 19.8 ft | 17.4 ft | 17.4 ft (5.32 m) |
| E1 7-ply | 41.8 ft | 25.9 ft | 21.8 ft | 21.8 ft (6.65 m) |
| V2 3-ply | 15.2 ft | 12.4 ft | 11.9 ft | 11.9 ft (3.62 m) |
| V2 5-ply | 22.0 ft | 18.6 ft | 16.5 ft | 16.5 ft (5.04 m) |
| V2 7-ply | 28.1 ft | 24.3 ft | 20.7 ft | 20.7 ft (6.30 m) |
Why vibration usually governs CLT floors
CLT is strong for its weight but light, so a floor that is safe in bending can still feel lively underfoot. The US CLT Handbook gives a vibration-controlled span based on testing: L ≤ (1/12.05) × (EIapp)^0.293 ÷ (ρA)^0.122, with L in feet, EIapp in lbf-in² for a 12 in strip, ρ the specific gravity and A the strip's area in in². The limit depends only on the panel's stiffness and mass, not the live load.
Heavier live loads bring deflection into play. At 100 psf, long-term deflection governs the 3- and 5-ply panels while vibration still governs the 7-ply. The handbook method assumes a bare panel on rigid supports; panels on flexible beams, or with toppings and ceilings, need a fuller vibration design.
| Layup | 40 psf live | 50 psf live | 100 psf live |
|---|---|---|---|
| E1 3-ply | 12.5 ft · vibration | 12.5 ft · vibration | 11.1 ft · deflection |
| E1 5-ply | 17.4 ft · vibration | 17.4 ft · vibration | 17.0 ft · deflection |
| E1 7-ply | 21.8 ft · vibration | 21.8 ft · vibration | 21.8 ft · vibration |
| V2 5-ply | 16.5 ft · vibration | 16.5 ft · vibration | 16.0 ft · deflection |
Shear deformation and apparent stiffness
The cross layers of CLT are weak in rolling shear, so a CLT panel deflects more than its bending stiffness alone predicts. NDS 2018 section 10.4.1.1 accounts for this with an apparent stiffness: EIapp = EIeff ÷ (1 + Ks × EIeff ÷ (GAeff × L²)), with Ks = 11.5 for a uniformly loaded, pin-supported span. The effect is largest on short, thick panels.
For an E1 5-ply panel at 17.4 ft, EIapp is about 89% of EIeff, so deflections are about 13% larger than a bending-only calculation. The calculator uses EIapp for both deflection and vibration, and multiplies the dead-load deflection by Kcr = 2.0 for creep before comparing the long-term total with L/240.
Worked example: a 16 ft E1 5-ply floor
An apartment floor spans 16 ft between bearing walls with 20 psf of finishes and services and 40 psf live load. An E1 5-ply panel is 6⅞ in thick, so at 30 lb/ft³ it weighs 17.2 psf, for a total load of 77.2 psf.
Bending allows √(8 × 10,400 ÷ 77.2) = 32.8 ft. Long-term deflection allows 19.8 ft and live-load deflection 24.9 ft. The vibration limit is 17.4 ft, so it governs, and the 16 ft span is within it. A 3-ply panel stops at 12.5 ft, so the 5-ply is the thinnest E1 layup that works. In metric, a 175 mm E1 panel with 1.0 kPa dead and 2.0 kPa live reaches 5.32 m.
Who it is for and when to use it
Early mass timber feasibility
Test a structural grid against panel thickness before engaging a CLT supplier, and see whether a 5-ply or 7-ply floor is likely.
Comparing grades and layups
See how E and V grades and 3, 5 and 7 plies trade thickness against span, and which criterion limits each one.
Coordinating floor depth
Estimate the panel thickness to carry through sections, stair geometry and floor-to-floor heights before detailed design.
Embodied carbon studies
Use the panel thickness and span to estimate CLT volume, then compare options in the embodied carbon comparison.
Method, formulas & assumptions
Per metre (ft) of width, with self-weight = density × g × thickness: bending L = √(8 (FbS)eff ÷ w); shear L = 2 Vs ÷ w; deflection 5wL⁴ ÷ (384 EIapp) with EIapp = EIeff ÷ (1 + 11.5 EIeff ÷ (GAeff L²)) (NDS 10.4.1.1), live ≤ L/360 and Kcr × dead + live ≤ L/240; vibration L ≤ (1/12.05) × EIapp^0.293 ÷ (ρA)^0.122 in ft, lbf-in² per 12 in strip (US CLT Handbook). Layup values are ANSI/APA PRG 320-2019 Table A2 (E1, E2, E3, V1, V2; 3, 5 and 7 plies). The smallest span governs.
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Treating the bending span as the answer. For most floors, vibration or deflection governs at a much shorter span.
- Forgetting the panel's self-weight. A 5-ply panel adds about 17 psf on top of the superimposed dead load.
- Using PRG 320 basic grade values for a proprietary layup. Use the manufacturer's certified values for the product you will buy.
- Applying a bare-panel vibration limit to panels on flexible beams. Beam deflection makes the floor livelier, and a system vibration design is needed.
- Assuming a continuous span behaves like a simple span. Continuity helps deflection but changes moments, shear and vibration.
Scope & limits
Single simply supported span, uniform load, CD = 1.0, bare-panel vibration on rigid supports. PRG 320 basic-grade reference values; confirm with the manufacturer's certified values. Point loads, continuous spans, cantilevers, openings, panels on flexible beams, composite toppings, bearing, connections, diaphragms, fire and acoustics are not covered. A structural engineer governs.
Frequently asked questions
How far can a 5-ply CLT panel span?
With 20 psf of finishes and 40 psf live load, a PRG 320 E1 5-ply panel (6⅞ in) reaches about 17.4 ft (5.3 m) on a single span, limited by vibration. Bending alone would allow about 33 ft.
How far can 3-ply CLT span?
About 12.5 ft (3.8 m) for an E1 3-ply floor at 40 psf live load, governed by vibration. 3-ply panels are more common for roofs and walls than for floors.
What governs CLT floor spans?
Usually vibration, then long-term deflection. Bending and shear rarely govern typical floors because CLT is strong for its stiffness and weight.
What is the difference between E and V CLT grades?
E grades use machine stress-rated lumber in the longitudinal layers and V grades use visually graded lumber. For the same thickness, E grades generally have higher bending capacity and stiffness.
What is PRG 320?
ANSI/APA PRG 320 is the North American standard for performance-rated cross-laminated timber. It sets manufacturing and qualification requirements and lists reference design values for basic CLT grades.
Does the calculator include a concrete topping?
Only as added dead load. A topping adds mass and may add stiffness if it acts compositely, which changes the vibration response, so enter it as superimposed dead load and confirm the floor with an engineer.
Does it work in metric?
Yes. Switch to Metric for metres, kilopascals and kg/m³. The layup values are the PRG 320 US values converted exactly, so a 175 mm E1 panel reaches about 5.3 m at 1.0 kPa dead and 2.0 kPa live.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
Further reading
- Fire-rated wall assemblies explained
- STC sound ratings for walls
- All framing & structure tools
- All free architecture & construction tools
- Harth resource library
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