Loft Floor Strengthening Calculator — Joists, Strutting & Boarding
Calculate additional joists needed to strengthen a loft floor for habitable use, including joist sizing by span, strutting rows and boarding.
Along the joist run direction
Perpendicular to joists (the span)
Per new joist length
How We Calculate This
This calculator determines the additional joists needed to upgrade a ceiling structure to a habitable floor.
The formula
Existing Joists = floor(Room Length / Spacing) + 1
New Joists = Existing Count (one between each pair)
Boarding Sheets = ceil(Area × 1.1 / 2.88)
Joist sizes by span
- 175×50mm C16: up to 2.7m span
- 200×50mm C16: up to 3.2m span
- 225×50mm C16: up to 3.8m span
- 250×50mm C24: up to 4.5m span
Frequently Asked Questions
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Last updated: March 2026
All calculations are estimates. Verify with your supplier.