How is a floor joist supported at its ends?
Built into the wall, sitting on a wall plate, or hung in a steel hanger — the three details, what each one implies for damp and rot, and why the bearing is where floors usually fail.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 5 min
- Safety
- caution
Short answer
A joist end either sits in a pocket in the masonry, rests on a timber plate on top of a wall, or hangs in a galvanised steel hanger fixed to the wall or a beam. Built-in ends are the traditional detail and the most vulnerable to damp, because the end is buried in the wall. Hangers keep the timber out of the masonry entirely. Every hole in a hanger is meant to take a fixing.
When a floor drops, sags or goes springy near a wall, the joist itself is usually fine along its length and the problem is at the end. Knowing which of the three details you are looking at tells you immediately what is likely to have gone wrong and what a proper repair involves.
Safety
Step by step
- Recognise the built-in end.The joist runs into a pocket in the wall and bears on the masonry. Standard in older houses. The end is surrounded by wall, which means it is in the coldest and dampest position in the room and gets no air.
- Recognise the wall plate.A timber laid flat along the top of a wall or sleeper wall, with the joists sitting on it. It spreads the load and keeps the joist ends in open air, and there should be a damp proof course between the plate and the masonry.
- Recognise the hanger.A pressed galvanised steel bracket fixed to a wall or a timber, with the joist sitting in it. The joist never enters the masonry. This is the standard modern detail and the usual repair detail as well.
- Understand what bearing length means.A joist must sit on its support over an adequate length, not merely touch it. Too little bearing concentrates the load and crushes the timber or the masonry beneath. The required length is a design matter.
- Look at the end, not the middle.Probe gently with a bradawl at the end of a joist near an external wall if you can reach one. Sound timber resists; rotted timber takes the point easily. This is where wet rot starts and where it is worth checking.
- Fill every hole in a hanger.Hangers are tested with all their holes fixed, using the nails or screws the manufacturer specifies — square twist nails or approved connector screws, not whatever is in the tin. A hanger with two fixings is not the product that was tested.
- Know the repair pattern.A joist that has rotted at a built-in end is normally repaired by cutting back to sound timber and supporting the end on a hanger or a new plate rather than rebuilding it back into the damp wall. Fixing the reason the wall was damp comes first.
Tips
- If a floor is springy across the whole room rather than at one point, that is usually a span issue rather than a bearing failure, and it is a different conversation.
- The damp proof course under a wall plate is easy to omit during a repair and it is the reason the original lasted. Do not leave new timber sitting on bare masonry.
- Where a joist end is built into a wall, leaving a small air gap around it in the pocket was traditional practice for a reason — it let the end breathe.
Common mistakes
- Using the wrong fixings in a joist hanger — A hanger's rating depends on the specified fasteners. Ordinary round wire nails or drywall screws pull through or shear, and the hanger fails at the fixings rather than the steel.
- Packing a short-bearing joist with slate or timber offcuts — It concentrates the load onto a small area and moves the failure rather than fixing it. Inadequate bearing needs a proper support, not a shim.
- Notching a joist near the wall to get a pipe past — The end of a joist carries the highest shear force, and it is the worst place in the whole member to remove material. The permitted notch and hole zones are near the middle of the span, not at the ends.
If it doesn't work
The floor drops away noticeably in the last half metre before a wall
Cause: A failing joist end at its bearing — Fix: Lift a board and look at the ends. Soft, dark or crumbling timber at a built-in end means damp in the wall, and that has to be found and stopped before any timber repair is worth doing.
A creak that always comes from the same spot near a wall
Cause: Movement at the bearing, or a nail working in a shrunken joist end — Fix: Expose it and look before treating it as a squeak. A creak in one fixed place near a support is worth a look; a creak that moves around the room usually is not.
A hanger has pulled away from the wall
Cause: Wrong or insufficient fixings, or a fixing into unsound masonry — Fix: This is structural. Support the floor and get advice — the reason may be the wall rather than the hanger, and a bigger fixing into weak masonry is not the answer.
Questions people ask
Can I fit a joist hanger myself for a repair?
Fitting a hanger is straightforward; deciding that a hanger is the correct repair for a failed structural bearing is not. For a small non-structural addition it is ordinary work. For a failed floor bearing, get the repair specified.
Where can I drill a joist for a cable or a pipe?
Within defined zones along the span, with limits on hole size and spacing, and never near the ends. GuideHQ has a guide on drilling through a joist that sets out the zones; treat them as limits rather than guidance.
Why do the joists in my house run one way in one room and the other way next door?
Joists span the shorter direction between supporting walls, so the direction changes with the room shape and with which walls are load-bearing. It is also a useful clue about which walls are doing structural work.