One of the repeated timbers holding your floor up
A joist is a horizontal member, one of many, spaced at regular intervals and carrying a floor or a ceiling. Lift a floorboard in most houses and what is underneath is a run of parallel timbers at four hundred or six hundred millimetre spacings, each doing an equal share of the work. Those are the joists.
The defining feature is that there are lots of them. A single big member carrying a concentrated load is a beam. A field of similar members each taking a slice is joists, and the difference is about the job rather than the timber.
The words you will hear on site
- Span. The distance a joist covers between its supports. Not the length of the timber, which is longer, because it has to sit on something at each end.
- Bearing. How much of the joist actually rests on the wall or hanger at each end. Too little bearing is a real failure mode and an invisible one.
- Centres. The spacing between joists, measured centre to centre rather than gap to gap.
- Trimmer. The joist framing across an opening, such as a stairwell.
- Trimming joists. The full-length ones either side of that opening, carrying the trimmer and everything on it.
- Strutting or noggins. Bracing between joists that stops them twisting and shares load sideways.
- Joist hanger. The galvanised bracket used where a joist meets a wall or another timber rather than sitting into it.
Which way do they run, and why you want to know
Almost every practical question about a floor starts here: hanging something from a ceiling, putting a hatch in, lifting boards, working out whether a wall below is doing anything.
Three ways to find out without opening anything up. Floorboards run across joists, so if you can see boards, the joists are at ninety degrees to them. Fixings in the ceiling below give away the lines, either visibly on an old lath ceiling or with a detector on plasterboard. And failing both, joists usually span the shorterdirection of a room, because spanning the short way is cheaper in timber and always has been.
Solid timber, or manufactured
A traditional floor is sawn softwood, graded and stamped, and the grade is part of what decides how far it goes. Modern floors are increasingly engineered: an I-joist, which is a web with flanges top and bottom, or a metal-web joist, which is timber flanges with a steel lattice between them.
Both go further for the same depth and stay straighter. The metal-web one has a genuinely useful side effect, which is that the open web lets a plumber run a soil pipe through the floor without cutting anything at all. The trade is that a manufactured joist comes with a manufacturer's rules about what may be removed from it, and unlike solid timber there is no informal margin: what the specification says is the whole of what you get.
Strength and stiffness are not the same thing
A floor can pass every check and still feel like a trampoline. Strength is whether it fails, stiffness is how much it moves, and the second one is what you feel when a floor bounces as somebody walks past. That is why bounce is fixed by depth, closer centres or proper strutting, and not by another sheet of chipboard on top.
Sizing and cutting them
How far a joist will actually span depends on its depth, its grade, its centres and what it is carrying, and the joist span table and calculator is the page for that, including why depth is worth far more than anything else you can change.
Before anybody puts a saw or a hole through one, the notching and drilling limits tool covers where you may and where you may not, which is narrower and more specific than most people expect. And the timber quantity and cut list turns the layout into lengths to order.
Common questions
What is the difference between a joist and a beam?
Repetition, mostly. Joists are many similar members spaced at regular centres, each carrying a share of the floor. A beam is a single larger member carrying a concentrated load, often the ends of a lot of joists. So joists spread a load out and a beam collects it. The same piece of timber could be either depending on what it is doing, which is why the words describe a role rather than a product.
How do I tell which way my joists run?
Look at the boards, then at the ceiling below. Floorboards run ACROSS joists, so the boards and the joists are at right angles to each other. Downstairs, the line of nails or screws in a plasterboard ceiling gives them away, and so does a stud detector. Failing both, joists almost always span the shorter direction of a room, because that is the cheaper way to build it.
What is a trimmer?
The joist that goes around a hole. Where a floor is interrupted, by a stairwell or a chimney, the joists that would have crossed the gap are cut short and framed into a piece running the other way, which is the trimmer. The full-length joists either side of the opening carry that trimmer and are called trimming joists, and they are doing more work than their neighbours, which is why they are often doubled.
Can I cut a notch in one for a pipe?
Within limits, and the limits are narrow and specific about both depth and position along the span. It is not a judgement call and it is not the same for a notch as for a drilled hole. There is a whole tool on this site for it, because getting it wrong quietly removes a large part of the strength of a member you cannot see afterwards.
What is bounce, and is it a structural problem?
Usually not, and it is still a real complaint. A floor can be perfectly strong and still deflect enough to feel unpleasant, because strength and stiffness are different properties. That is why deeper joists, closer centres or proper strutting fix a bouncy floor while adding another layer of chipboard does almost nothing. A floor that bounces is normally telling you about its depth rather than about its safety.
Are engineered joists better than solid timber?
For long spans, generally yes, and they come with different rules. An I-joist or a metal-web joist is lighter, straighter and spans further for the same depth, and metal-web joists have the useful property of an open web you can run services through without touching anything structural. The catch is that they are far less forgiving of being cut: a manufactured joist has a specification for what may be removed and where, and improvising is not an option the way it half is with solid timber.