A common joist is a relatively light floor joist that runs across, or partly into, the heavier main beams, spanning from beam to beam or from beam to wall. Together they form the traditional main beam and joist floor found in almost every historic Dutch building; the Dutch terms are moerbalk (main beam) and kinderbalk (common joist).
The arrangement is simple: a few heavy main beams span the room in one direction, and many much lighter joists sit on them, usually running along the length of the house. The floorboards go on the joists.
How the structure works
| Element | Role |
|---|---|
| Main beam | Heavy primary beam, usually wall to wall; carries the joists |
| Common joist | Lighter member on or housed into the main beam; carries the boards |
| Floorboards | On the joists; form the walking surface |
| Corbel / brace | Shortens the effective span of the main beam at the wall and takes up moment |
Joists can bear on top of the main beam or be housed into it. Housing them keeps the floor build-up shallower, but locally weakens the main beam — which is why such notches were traditionally kept small and cut with care.
Historic significance
This floor type also carried status. Sawn timber counted as better than cleft timber, a smaller spacing between joists (so more of them) signalled wealth, and mouldings, bead edges and paintwork turned the floor into a decorative ceiling. Thin boarding or lathwork was sometimes laid between the joists to conceal the top of the floor and hold back dust.
Points to watch when altering
- Never notch casually — a notch or service hole belongs in the neutral zone (mid-depth) and certainly not close to the bearing
- Check the joist ends — where joists and beams sit in the wall, wood rot is the most common problem; probe the ends
- Feel for deflection — a bouncy floor points to too great a span, weakened members or missing bracing; have it assessed structurally
- Heritage value — in a listed building the exposed beams and joists are often exactly what has to be kept; agree the approach with the authority first
- Insulation and services — run pipes and cables alongside the joists rather than through them; a trimmer is a better answer than a large hole
Related terms
- Joists
- Main beam
- Beam
- Timber joints
- Joist anchor
- Wood rot
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