Mezzanine Floor Calculator — Joists, Hangers & Deck

Joist size depends on the span and the imposed load. The figures below are conservative C24 examples for domestic loading only (1.5 kN/m² imposed, BS EN 1991-1-1 category A) at 400mm centres: 47×200mm up to ~3.6m, 47×225mm up to ~4.2m, 47×250mm up to ~4.8m. Most mezzanines are NOT domestic: BS EN 1991-1-1 gives offices 2.5–3.0 kN/m², retail/assembly ~4.0 kN/m², and storage (category E) up to 7.5 kN/m². At those loads the allowable spans are far shorter and a structural engineer must size the joists and the supporting beam. This calculator counts joists for the span you enter — it does not size them or check the span is safe.

Joists can be supported by joist hangers on a ledger board bolted to the wall, or by a beam-and-post frame. Ledger bolts should be at 600mm centres minimum using M12 coach bolts into masonry.

18mm structural plywood (2440×1220mm = 2.98m²/sheet, max 450mm joist centres) or 22mm P5 moisture-resistant chipboard (2400×600mm = 1.44m²/board, up to 600mm centres) are the most common. Plywood is stronger and more moisture-resistant per square metre; 22mm P5 chipboard tongues together quickly. A 25mm OSB3 T&G panel (2500×625mm = 1.56m²) is an alternative for heavier-duty decks. Loose 25mm tongue-and-groove softwood floorboards are sold per metre rather than as sheets, so size those by lineal metres of board, not sheet count.

Yes, where the drop exceeds 600mm in a dwelling (380mm in non-dwellings). Approved Document K Diagram 3.1 (the guarding-height table; there is no "Table 3.2" in AD K) sets the minimum height: in a single-family dwelling the edge of an internal floor is 900mm (the same 900mm covers stairs, landings and ramps), while external balconies and roof edges are 1100mm. In other buildings — offices, retail, factories/warehouses, storage — floor edges are 1100mm (900mm only alongside a flight). Most commercial and storage mezzanines therefore need 1100mm guarding. Balusters must be spaced so a 100mm sphere cannot pass through.

Yes. A mezzanine that creates additional floor area requires Building Regulations approval. This covers structural adequacy, fire safety, means of escape, and balustrade height. A structural engineer’s calculation is typically needed.