Sistering means fixing new timber alongside an existing joist, but the correct detail depends on why you are doing it. There is no single UK rule that says every sister joist should use the same length, bolts or screw spacing.
Levelling a sound floor is not the same as strengthening it
If the existing floor is structurally sound but uneven, the aim may simply be to create a level fixing surface. SPAB notes that sound sagging floors do not automatically require structural strengthening and that less invasive levelling methods can sometimes be appropriate.
A springy floor may need stiffening rather than a local repair
Before adding timber, check the cause: joist size and span, missing strutting, poor support, movement or damaged timber can all produce different symptoms. Noggins or strutting can improve rigidity where required, but they do not turn an undersized joist into a correctly designed one.
A damaged joist needs the cause and remaining structure assessed
Historic England and SPAB both describe strengthening and partnered-timber repairs, but neither gives a universal fixing schedule. Rotten ends, severe notches, splits and overloaded joists can need different repairs. Structural capacity changes should be designed for the actual defect, span and load.
Do not copy a generic bolt pattern from the internet
The connection between the old and new timber is part of the structural repair. Where the sister is intended to restore or increase load-bearing capacity, use a repair detail appropriate to that floor rather than assuming a standard screw or bolt spacing.
See SPAB guidance on timber floor structures. If replacement timber is specified, browse structural timber and timber connectors.
