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.
Eurocode 5 offers good advice on how this should be done and covers the size of nail or bolt and their location and centres.
The nail specification used is dependent on the thickness of the joists 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. If staggered, nails must be at least 5 times the diameter of the nail apart (at least 25mm) and bolts 4 times the bolt diameter apart (M12 = 48mm, M16 = 64mm)
Nails must be started 15 times the diameter from the joist end and at least five times the diameter from the top and bottom. Bolts must be closer than seven times the diameter or 80mm from the joist end and four times the diameter from the top and bottom.
For example, if two 47mm x 195mm are bolted together, then M12 bolts should be used and positioned 48mm from the top and bottom and at least 48mm apart vertically if staggered fixings are used. The first bolt should be positioned between 80mm and 84mm (7x12) from the joist ends. Each trimmer should be fixed in line with a minimum of two nails or two bolts at 1/3 span positions and as a minimum the fixing centres (S) for double member trimmer and trimming joists should be at centres of no more than twice the joist spacing (a maximum of 1200mm). However required fixing centres vary according to the total length and weight on the joist. The extract below has been taken from Eurocode 5 to illustrate how this and should be consulted for any other timber sizes not shown. No notching or drilling of a trimmer should be carried out unless proven by calculation, other than to accommodate the flanges of joist hangers.