How to get it right: Bolting timbers together correctly

If you are doubling up floor joists, creating trimmers or trimming joists around stairs and chimneys they all need connecting properly so that they can act as a single unit.

Whilst timber floors and their connection details should be designed by a competent person, the ‘Eurocode 5 Span Tables (4th edition)’ published by TRADA offers good advice on typical connection details for solid timbers, such as  the sizes of nails or bolts and their location and centres. Some of the key parts of this guidance document in relation to connecting trimmers or trimming joists are summarised below:

The nail specification used is dependent on the thickness of the timbers being used:

• 3.75mm diameter x 75mm long for 38mm wide sections
• 4.00mm diameter x 90mm long for 44mm and 47mm wide sections
• 5.00mm diameter x 125mm long for 63mm, 72mm and 89mm wide sections

The minimum bolt specification to be used is:

• M12 bolts (min. 3mm x 38mm M12 washers under head and nut) for 38mm, 44mm, 47mm and 63mm wide sections
• M16 bolts (min. 4mm x 48mm M16 washers under head and nut) for 72mm and 89mm wide sections

It may be possible to use other types of fasteners for which capacities have been obtained experimentally and declared by manufacturers with third party certification e.g. dog tooth connectors.

Fixing can be either along the centreline or staggered as shown in the diagrams below:

Nails must be a minimum of 15 times their diameter from the joist end and at least 5 times their diameter from the top and bottom.

Bolts must be a minimum of 7 times their diameter or 80mm from the joist end and at least 4 times their diameter from the top and bottom.

Images courtesy of Eurocode 5 Span Tables (4th edition)’ published by TRADA

Double member trimmers and trimming joists should be fixed in line with a minimum of two nails or two bolts at 1/3 span positions. Furthermore, as a minimum, the fixing centres (S) for double member trimmers and trimming joists should be at centres of no more than twice the joist spacing (up to a maximum of 1200mm). However, required fixing centres vary according to the total length and load on the joists.

These tables (click to open in a new tab/window) have been taken from ‘Eurocode 5 Span Tables (4th edition)’ published by TRADA to illustrate how the above works and should be consulted for any other timber sizes not shown.

No notching or drilling of trimmers or trimming joists should be carried out unless proven by calculation, other than to accommodate the flanges of joist hangers.

Further reading

Eurocode 5 Span Tables 4th edition for solid timber members in floors, ceilings and roofs for dwellings, August 2014, BM Trada, ISBN: 978-1-909594-14-2, Published by BM TRADA

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^{Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.}

^{This article was reviewed and updated on 10 August 2023}