What Are Joists?
Written by James Walker · Last reviewed 16 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
Joists are the horizontal beams, usually timber, that support a floor or ceiling. They span between walls or beams and carry everything above them.
Why they come up in quotes
Three situations, and all three can change a project's cost:
Bathrooms. A tiled floor and a filled bath weigh considerably more than carpet. Existing joists may need strengthening.
Loft conversions. Existing ceiling joists are almost never adequate for a habitable floor. New floor structure is standard, not an extra.
Removing walls. If a wall is carrying joists, taking it out needs a beam and structural design.
What "notching" means
Plumbers and electricians cut notches in joists to run pipes and cables. Done in the wrong place or too deep, this weakens them significantly.
There are rules about where and how deep. If a floor becomes bouncy after plumbing work, this is a common reason.
Sistering and strengthening
Adding a new joist alongside an existing one, sistering, is a common fix, as is adding noggins between them. On anything structural, get it designed rather than guessed.
How it works
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