What size joists do I need for a mezzanine floor?

Joist size depends on the span (distance between supports). For domestic loading: 47×200mm spans up to 3.6m, 47×225mm up to 4.2m, and 47×250mm up to 4.8m at 400mm centres. Consult BS span tables for your specific loading.

How are mezzanine joists supported?

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.

What decking should I use for a mezzanine?

18mm structural plywood (C24) or 22mm P5 chipboard are the most common. Plywood is stronger and more moisture-resistant. For exposed/feature mezzanines, 25mm tongue-and-groove boards give a finished look.

Do I need balustrade for a mezzanine?

Yes. Building Regulations require a minimum 1100mm high balustrade on any open edge of a mezzanine floor. Balusters must be spaced so a 100mm sphere cannot pass through.

Does a mezzanine need building control approval?

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.

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Mezzanine Floor Calculator — Joists, Hangers & Deck

Calculate mezzanine joist size by span table, hanger count, ledger bolt spacing, deck board count and balustrade.

Mezzanine Length

Along the joist run

Mezzanine Width (span)

Joist span (wall to wall)

Joist SizeBased on span and loading

Deck Board

Price Per Joist (optional)

How We Calculate This

This calculator works out mezzanine floor components: joists, hangers, ledger fixings, deck boards and balustrade from your floor dimensions.

The formula

Joists = Floor length ÷ Joist spacing + 1

Hangers = Joists × 2 (one each end)

Deck sheets = Floor area × 1.1 ÷ Sheet area

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.