Floor and ceiling joists look like ordinary timber, but they are graded structural components, and the letters and numbers stamped on them matter. C16 and C24 are the two grades you will meet in almost every builders' merchant, and choosing between them changes how far a joist of a given size can span. The second half of the joist story is what happens afterwards: plumbers and electricians need to get pipes and cables through them, and a careless notch or an oversize hole can turn a sound floor into a bouncy, cracked or even unsafe one. This guide explains the grades, how to think about span and size, and the accepted limits on cutting joists.
What C16 and C24 mean
Softwood used for structural purposes is graded into strength classes under the European standard BS EN 338. The "C" stands for coniferous, and the number is the characteristic bending strength of the timber in newtons per square millimetre. So C16 timber has a characteristic bending strength of 16 N/mm2, and C24 of 24 N/mm2. The classes also define stiffness: the mean modulus of elasticity is around 8 kN/mm2 for C16 and 11 kN/mm2 for C24, so C24 is both stronger and stiffer for the same cross-section.
Timber is graded by visual inspection or by machine, and then marked. Machine-graded timber is typically stamped with the strength class, the grader, the species group and the standard. C16 is the default grade for general structural softwood in the UK, widely available and cheaper. C24 is a higher grade, often supplied as a specific order, and usually costs more per metre. Both grades are normal for domestic use, but a C24 joist can span further, or be spaced further apart, than a C16 joist of identical size.
You may also see "regularised" timber, which is sawn and then planed to a consistent thickness, and "CLS" (canadian lumber standard) stock, which is planed on all four faces to fixed sizes, often 38 mm thick. Planed sizes are smaller than nominal ones, so a nominal 50 x 150 mm joist may measure 47 x 145 mm in practice, and span tables are written for the stated finished sizes. Always check that you are looking at the right column.
How to choose joist size and spacing
The size you need depends on the span (the clear distance between supports), the spacing (centres), the loading and the strength class. Domestic floors are normally designed for a typical imposed load, and rooms like bathrooms with heavy baths, or floors carrying a masonry hearth, a water tank or a partition wall, need special consideration. Span tables published by the timber industry, such as those from TRADA, and the span tables referenced in the Building Regulations guidance, list the maximum span for each size, spacing, grade and load. Do not guess; use a published table or ask a structural engineer.
- Larger depth is more effective than larger width. Bending resistance increases with the square of the depth, so a deeper joist makes a big difference, while stiffness increases with the cube of the depth.
- Closer spacing improves stiffness. Floors at 400 mm centres are stiffer and give the board more support than 600 mm centres.
- Deflection, not strength, often governs. A floor that is strong enough can still feel bouncy, and tiles or plaster ceilings may crack, so serviceability limits matter.
- Bearing length. Joists need adequate bearing on walls or hangers, commonly at least 100 mm for timber on masonry, subject to the design.
The timber joist span calculator gives an indication for your span and grade, but the result is a guide, and the final design should be confirmed against current published tables or by an engineer. Structural changes, such as removing a wall or adding a loft room, are building regulations matters in England, as set out in Approved Document A (Structure).
Why notching and drilling are risky
A joist in bending has its highest stress at the top and bottom edges, and its highest shear near the supports. Cutting a notch at the edge removes material where the stress is highest, and a notch near the support cuts into the shear-critical zone. A hole near the middle of the depth removes much less strength, because the centre line, the neutral axis, carries little bending stress. That is why the accepted rules allow holes on the centre line and restrict notches to a small depth in a set zone.
The following limits are those published by the Local Authority Building Control body (LABC) for solid timber floor and ceiling joists in domestic properties, and they match widely used guidance based on TRADA recommendations:
| Item | Notch | Hole |
|---|---|---|
| Position along the span, measured from the support | Between 0.07 and 0.25 of the span | Between 0.25 and 0.4 of the span |
| Size limit | Depth up to 0.125 x joist depth, with a maximum of 35 mm | Diameter up to 0.25 x joist depth, with a maximum of 65 mm |
| Position across the depth | On the top or bottom edge only, never both | On the centre line of the joist |
| Spacing | One notch per joist zone | Neighbouring holes at least 3 diameters apart, centre to centre |
The same guidance adds that structural beams, purlins and binders must not be notched or drilled without calculations, rafters may be birdsmouthed by no more than one-third of their depth, and the rules do not cover engineered joists, such as I-joists, which must be cut only as the manufacturer allows. Check that the joist is adequate for the span before cutting anything.
Worked example
Illustrative numbers: a 47 x 200 mm joist spanning 3.6 m between supports.
- Maximum notch depth: 0.125 x 200 = 25 mm (below the 35 mm cap).
- Notch zone: from 0.07 x 3600 = 252 mm to 0.25 x 3600 = 900 mm from the support.
- Maximum hole diameter: 0.25 x 200 = 50 mm (below the 65 mm cap).
- Hole zone: from 0.25 x 3600 = 900 mm to 0.4 x 3600 = 1440 mm from the support.
- Minimum spacing between two 22 mm holes: 3 x 22 = 66 mm, centre to centre.
So a 22 mm hole for a copper pipe is acceptable between 900 and 1440 mm from the bearing, drilled on the centre line, but a 100 mm waste pipe will not pass through this joist in any position. A 25 mm notch on the top edge to bury a 22 mm pipe would be allowed in its zone, but it is better to drill, and notches for larger pipes are not permitted. The larger the joist, the larger the permitted hole, but it is capped at 65 mm.
Tables from LABC list further sizes: for example, a span of 4000 mm gives a hole zone from 1000 to 1600 mm and a notch zone from 280 to 1000 mm, and a 250 mm deep joist allows a hole up to 62.5 mm and a notch of about 31 mm.
Running pipes and cables
- Plan the route first. Run services across joists at right angles, within the permitted zones, rather than along them. Cutting a long slot along a joist weakens it.
- Drill, don't notch, where possible. Use a hole saw or an auger bit on the centre line. Make holes a little larger than the pipe to allow for movement and to reduce noise when pipes expand.
- Cables. Cables passing through joists should be well clear of floor fixings, and where cables run in notches or within 50 mm of the surface, additional mechanical protection or other measures from the wiring regulations (BS 7671) may be required. Have the circuit designed and installed by a competent person, and see our guide to Part P electrical work for DIYers for the rules in England. Our cable size and voltage drop calculator helps check cable sizes for circuits.
- Large services. Waste pipes, soil pipes and ducts are too big for drilled holes. Reroute, box them in below the joists, or have the joists trimmed and supported with a properly designed trimmer arrangement.
- Don't forget fixings. Floorboard nails and screws near pipes and cables are a classic cause of punctures and shorts. Mark the routes on the boards.
When a joist has already been damaged
If you find joists with oversized holes, deep notches in the wrong place or large sections cut away, perhaps by an earlier plumber or when installing a bath, do not simply ignore them. Signs of trouble include sagging or bounce, cracks in the ceiling below, and splits starting from a notch. Repairs may include bolting or screwing a new "sister" joist alongside, or adding timber or steel plates, but the details depend on the cut and the load, so ask a structural engineer. A loft room or an extension that adds floor load is a particular reason to check, because the original joists may be undersized for the new use.
Regulations and where to ask
In England, structural alterations such as new loft floors and the removal of load-bearing walls need building regulations approval. Planning Portal guidance on ceilings and floors points to the relevant requirements. Fire and sound separation can also come into play if you are altering floors between flats or in a loft conversion. Scotland and Northern Ireland have their own standards, so check locally. This guide is general information, not engineering advice, and the permitted limits above apply to domestic solid-timber joists as described. For any case outside those conditions, consult a structural engineer or your local building control team.