Timber Floor Joists: Simple UK Guide to Sizes, Spacing and Floor Design
Timber floor joists are used in almost every UK home. They support the floors we walk on and carry the load of furniture, walls and everyday use. This guide explains joist sizes, spacing, spans and common issues in simple terms.
What Timber Floor Joists Are
Simple definition
A timber floor joist is a long piece of timber that spans between walls or beams to carry the floor above.
How joists support a floor
Joists act like small beams. They carry the floorboards or chipboard, spread the load, and transfer the weight into the supporting walls or RSJs.
Standard Joist Sizes Used in UK Homes
Common depths
Most UK domestic floors use:
- 47 x 100 mm
- 47 x 150 mm
- 47 x 175 mm
- 47 x 200 mm
Deeper joists span further and reduce deflection.
When you need deeper joists
You need deeper timber when:
- The span is over 3.5–4.0 metres
- The room is wide
- You’re adding a bathroom or heavy load
- The engineer checks deflection and finds it too high
Engineered timber vs standard timber
Engineered joists (I-joists, metal-web joists) span further with less depth but cost more. They’re common in new-builds.
Joist Spacing Explained
400 mm spacing
Most UK floors use 400 mm centres. This makes the floor feel solid and keeps deflection low.
600 mm spacing
600 mm centres are sometimes used with engineered joists or thicker flooring boards.
Why spacing matters
Tighter spacing means:
- Greater strength
- Less bounce
- Smoother finish for plaster below
Typical Span Rules for Timber Joists
How far joists can span
Span depends on:
- Joist depth
- Timber grade
- Spacing
- Type of floor
- Load
What affects span
The biggest factor is joist depth. For example:
- 47 x 150 mm often spans around 3.0–3.5 m
- 47 x 200 mm can reach around 4.0–4.5 m
These are rough figures. Engineers check exact values.
Simple span examples
A 4 metre room usually needs 47 x 200 mm timber or 47 x 150 mm on a supporting steel beam halfway across.
Joists in Loft Conversions, Extensions and Existing Floors
Loft joists
Loft joists carry both the new floor and sometimes dormer loads. They often connect to new steel beams.
First-floor joists
These carry bedrooms, bathrooms and partitions. Deflection control is important so floors don’t feel bouncy.
Ground-floor joists
These rest on sleeper walls or beams. Ventilation and moisture control matter here.
How Engineers Design Joists
Load
The engineer checks self-weight, live load, and partitions.
Deflection
Floors should not feel springy. Engineers limit deflection so floors feel firm.
Bearing and support
Joists usually need 40–60 mm bearing on walls or beams.
Signs of Joist Problems in UK Homes
Sagging floors
Often caused by long spans, undersized joists or lack of support.
Creaking or bouncing
Usually due to poor spacing or old, flexible timber.
Cracks in walls or ceilings
Joist movement can crack plaster. Sometimes the joist ends are rotten.
FAQs
What is the best joist size for a 4 metre span?
Often 47 x 200 mm, but it depends on spacing and load.
Can I use 100 mm deep joists?
Only for small spans or non-habitable areas.
How much bearing do joists need?
Usually 40–60 mm on masonry or steel.
Do I need a structural engineer?
Yes if you’re altering walls, adding steel beams or changing the floor layout.
Need Help with Joist Design or Floor Upgrades?
If you’re planning new floors, loft works or a wall removal, SECalcs can design the joists, steels and support layout for Building Control.