CLT Floor Design Calculator

CLT floor design under Eurocode 5 is more nuanced than glulam or LVL because the cross-layers don’t carry bending — they only carry shear. That single fact governs almost every interesting decision you make on a CLT floor, from layup selection to vibration tuning. This guide covers the five checks the calculator runs, the three stiffness methods it supports, and the practical situations where each one matters.

Each calculation runs five independent checks against EN 1995-1-1, in both Ultimate Limit State (ULS) and Serviceability Limit State (SLS):

• Bending capacity (fm,Rd)

  moment capacity of the longitudinal layers, per EN 1995-1-1 §6.1.6. Includes kmod, ksys, and γM factors.
• Shear capacity (fv,Rd)
  longitudinal shear strength of the panel, per EN 1995-1-1 §6.1.7.
• Rolling shear (fr,k)

  shear in the cross-layers. This is the “Achilles heel” of CLT and frequently governs short spans. Per EN 1995-1-1 §6.1.7(2) with manufacturer-specific characteristic values.
• Deflection (winst, wfin)
  both instantaneous and long-term creep deflection, applying kdef per service class.
• Vibration (f1)
  fundamental frequency, with f1 > 8 Hz typical for residential and office floors. The calculator implements the draft 2nd-generation Eurocode method including impulsive velocity response.

All five run on every calculation — you don’t have to switch modes. The verdict panel surfaces the governing check and the overall utilisation.

EN 1995-1-1 doesn’t mandate a single analytical method for CLT panels. The choice depends on the panel layup and how rigorous you need to be. SPEC supports three.

The classical approach from Annex B of EN 1995-1-1, valid for standard 3, 5 or 7-ply uniform panels. It accounts for rolling shear deformation in the cross-layers via a simplified efficiency factor (γ) and treats the panel as a single laminate with one effective bending stiffness.

Easy to verify by hand. Slightly conservative for thicker panels because it can’t model the rolling shear stiffness of each individual cross-layer separately.

An extension of the standard Gamma method that accounts for the rolling shear stiffness of every individual cross-layer rather than treating the panel as a single laminate. This produces a more refined effective stiffness and prevents the overly conservative designs that the standard Gamma method gives for thick CLT (7-ply and above).

It’s the default in the calculator for good reason — on a typical residential or office span, the difference between Gamma and Extended Gamma can be the difference between specifying a 200mm and a 240mm panel.

he most rigorous analytical approach, derived from Kreuzinger’s beam-on-elastic-foundation model. Treats the panel as a composite beam with distinct bending and shear stiffness components.

Use it for complex or asymmetric layups, panels with different grades in each layer, or any configuration where bending and shear stiffness can’t be assumed to act in parallel. Slower to evaluate but it’s the gold standard.

For mass timber floors, vibration is more often the governing serviceability limit than static deflection. The calculator runs the draft 2nd-generation Eurocode vibration method with options for stiff or flexible support modelling and performance-level targeting:

Composite action with a concrete topping significantly improves both frequency and impulsive response — the calculator handles bonded or mechanically connected toppings with user-defined thickness, modulus and connection stiffness.

Every CLT panel option in the calculator pulls characteristic values directly from the manufacturer’s European Technical Assessment (ETA), not from generic catalogue numbers. Pick a supplier and the numbers match the product:

• Binderholz BBS — ETA-06/0009. Full layup range from BBS 60 to BBS 300.
• KLH BSP — ETA-06/0138. Standard and custom layups.
• XLAM Dolomiti L5s — ETA-22/0009. Including fire performance data shared with the CLT Fire calculator.
• Plus Kalvasta, MTT, Södra, Theurl, Hasslacher, Stora Enso and Mayr-Melnhof.

For unlisted manufacturers or research configurations, custom layup entry is fully supported — define layer thicknesses, grades and orientations manually and the calculator applies the standard Eurocode framework.