Slab deflection calc for a glass floor: engineering
What 'allowable deflection' means, how SP 20 calc works, and why your house may not fit a glass floor.
A glass floor isn't "floating" — it sits on a steel frame, which sits on a concrete slab. The slab's deflection determines if the glass cracks. Calc is mandatory.
01What deflection is
A slab under load "bends" — its centre drops by mm. Per SP 20.13330 for residential, allowable deflection = L/250, where L is the span. 3 m span → 12 mm allowable.
02Why it matters for glass
Glass is brittle. If a slab deflects 12 mm and glass is on a rigid bearing — it cracks in the middle. So either: glass sits only on rigid supports, or a steel frame compensates the deflection, or laminated PVB accommodates 3–5 mm of flex.
03Formula
f = (5·q·L⁴) / (384·E·J) — for single-span uniform load. Simplified; real calcs include rebar, edge effects, skews.
04Slab types
| Type | Thickness | Allow. load | 3m deflection |
|---|---|---|---|
| Monolithic B25 (new build) | 200–250 mm | 600 kg/m² | 4–6 mm |
| Monolithic B30 | 250–300 mm | 800 kg/m² | 3–4 mm |
| Precast (Brezhnevka) | 220 mm | 300 kg/m² | 8–10 mm |
| Hollow slabs (Khrushchevka) | 220 mm | 200 kg/m² | 10–12 mm |
| Old wooden joist | varies | 100 kg/m² | 15–25 mm |
05Where it works
- Monolithic B25+ — any configuration
- Precast with steel frame compensation
- Over load-bearing walls
- Between two load-bearing walls
06Where it doesn't (without rebuild)
- Khrushchevkas / Stalinkas with hollow slabs
- Old houses with wooden joists
- 5–6 m+ spans without intermediate support
Glass floor is a structural element, not interior decor. Engineer's calc costs 32 000 ₽ and takes 5–7 days. Far cheaper than a destroyed floor a year after renovation.
07This service at REAL GLASS
If you've read this far, the topic is probably relevant to a real project. We make glass floors in-house in St. Petersburg, with a measure within 1–3 days, 1-year warranty and a fixed contract price. All numbers in this article come from our actual 2026 quotes.
For a project-specific estimate — see the service page or request a free on-site measure.
