Floor joist design and sizing
Floor joists carry the people, furniture and finishes on a floor across to the supporting walls or beams. We size joists and check spans, deflection and notching to the Eurocodes, for clients across the UK.
What size floor joists do I need?
It depends on the span, the spacing and the loads the floor carries. We calculate the dead and imposed load, check deflection, which usually governs timber joists, and give the section, grade and centres that work. We will not quote a size without the span and loads.
Loads and the span that governs
Floor joists carry a dead load, the weight of the deck, the ceiling below, the flooring and any partitions, and an imposed load, the people and furniture the floor must support, which for a domestic floor is taken from BS EN 1991-1-1 and its UK National Annex. These loads are combined with the correct factors and shared between the joists according to their spacing, typically 400mm or 600mm centres. Closer centres share the load between more joists and allow a shallower member; wider centres put more load on each joist. We set the spacing and the section together to reach an economical floor.
For timber joists, deflection almost always governs rather than strength, because timber is relatively flexible. A joist can be plenty strong enough in bending yet still feel bouncy and crack the ceiling below, so the serviceability check, keeping deflection within span/360, usually decides the size. We also check vibration and the sense of liveliness on longer spans, since a floor that passes the numbers can still feel unpleasant to walk on. Solid timber sizes are read against the strength class, commonly C16 or C24 to BS EN 338, and we choose the grade and section that suits the span rather than defaulting to a standard size.
Engineered joists and longer spans
When a span is beyond what a sensible depth of solid timber can reach, engineered joists take over. I-joists, with timber flanges and an OSB web, are light, straight and span further than solid timber for the same depth, and their webs are made to take service holes for pipes and cables in defined zones. Metal web, or posi, joists use a steel lattice web that leaves the whole depth open for services and ductwork, which is why they suit floors with a lot of pipework or mechanical ventilation. We size these to the manufacturer's engineering data against our calculated loads and span, and specify the exact product, depth and centres.
Choosing between solid timber, I-joists and metal web joists is a balance of span, service routing, depth available and cost. A short domestic span is often cheapest in solid timber; a long open span, or a floor that must carry ducts, usually justifies engineered joists. We also check the bearing at each end and the connection where joists meet a supporting steel or timber beam, whether they sit on top, hang from joist hangers, or bear into a wall, so the reaction is properly delivered.
Notching, holes, strutting and buildability
Services are the usual cause of a floor being weakened after it is built, so we set out where joists may be notched and drilled. In solid timber the rules limit notches and holes to defined zones and sizes, notches only in the top edge within a band near the supports, holes only on the neutral axis within the middle of the span, because a notch or hole in the wrong place cuts straight through the most stressed fibres. Engineered joists have their own service rules, generous for metal web joists and defined knockout zones for I-joists, and we make these clear so the plumber and electrician do not have to guess.
Longer or deeper joists need restraint to stop them twisting or buckling sideways, so we specify strutting, solid blocking or herringbone, at the spacings the span requires, and lateral restraint straps tying the floor to the walls. The finished output is a joist layout with the section or product, grade, centres, bearings, hangers, notching and strutting, ready for the builder and Building Control. We deliver these designs across the UK, with a site visit arranged by region where an existing floor or opening needs surveying first.
What we check.
The points our calculations resolve for a project like this.
- Dead and imposed floor loads to BS EN 1991-1-1 shared by joist spacing
- Deflection to span/360 and floor vibration, which usually govern timber joists
- Choice between solid timber, I-joist and metal web joist for span and services
- Bearings, joist hangers and connections to supporting beams and walls
- Notching and drilling zones so services do not weaken the joists
- Strutting, blocking and lateral restraint straps for longer spans
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What size floor joists do I need?
It depends on the span, the spacing and the loads the floor carries. We calculate the dead and imposed load, check deflection, which usually governs timber joists, and give the section, grade and centres that work. We will not quote a size without the span and loads.
Why does my floor feel bouncy even though it holds the weight?
Because strength and stiffness are different. A joist can be strong enough in bending but still deflect and vibrate enough to feel bouncy and crack the ceiling. We design to a deflection limit and check vibration, so the floor feels solid, not just stands up.
How much does floor joist design cost?
Our fee starts for a joist calculation and layout, fixed before we begin. A large or complex floor, or engineered joists with service coordination, will affect the fee, which we agree with you up front.
Can I notch or drill my floor joists for pipes?
Only within defined zones and sizes. In solid timber, notches sit in the top edge near the supports and holes on the centre line in the middle of the span; a cut in the wrong place seriously weakens the joist. We mark the permitted zones, and engineered joists have their own, often more generous, service rules.
When should I use engineered joists instead of solid timber?
When the span is beyond a sensible depth of solid timber, or when the floor has to carry a lot of pipework and ductwork. I-joists span further for their depth, and metal web joists leave the whole depth open for services. For short simple spans, solid timber is often cheaper.
Related in structural design & elements.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
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