Maximum bolt size when sistering joist Kitchens, doors, rails, stud, tables, chairs, stair cases, garden furniture etc... Find answers and ask questions here!

[skyan]

I wish to sister three joists in my bathroom because I am going to have a cylinder installed which will contain 210 litres of water and I am going to then tile the entire bathroom. The cylinder will end up near to the center of the span so I want to ensure at this stage the floor will have little movement.

The joists are approximately 50 * 100 mm and span just over 1.7 meters. The existing three joists have had notches cut out near the center and holes drilled near the edge. I will not be running any pipes through them after I have finished.

I intend to glue and bolt a 50 * 100mm C16 timber to one side about 1.5 meters long and think it might be worth also fixing 18mm OSB3 smart ply to the other mainly because I have an offcuts which will fit perfectly across 1.7 meter span and I figured this might add some further stiffness.

My main question though is regarding the size of coach bolts to use. I already have M16 bolts but I worry these might be too big for sistering 50*100mm? I have read people recommending M8, 10 and 12 and have read that using bolts too big might weaken the joist. Is M16 too big though for a 50*100 joist ?

[stoneyboy]

skyan,
Suggest you use 10 or 12mm bolts and don't forget the square washer plates on both sides.
end

[Rob HIM]

Regardless of the bolt size you need to use toothed timber connectors between the timbers and sq washer on the outside.
Glue will eventuarly breakdown under tension, so there needs to be a proper physical connect between the new and old floor members, hence toothed timber connectors.

[skyan]

Hi,

Thank you very much for your replies. I will use M12 bolt size with square washers and toothed timber connectors.

"Regardless of the bolt size you need to use toothed timber connectors between the timbers and sq washer on the outside. "

Do you mean to use toothed timber connectors between the timbers and between the square washer and timber for example

SqWasher---TimberConnector---NewJoist---TimberConnector---OldJoist--TimberConnector--SqWasher

or just toothed timber connector between the square washer and the timber for example

SqWasher---TimberConnector---NewJoist---OldJoist--TimberConnector--SqWasher

Apologies if this sounds like a stupid question - I think you mean the second example but I have noticed that there are double sided toothed timber connectors so would just like to be sure before bolting it all together and building over it.

Also I am considering using 5 bolts per joist with one in the center and the other two either side evenly spaced. Does this sound reasonable ?

Thanks again

[skyan]

Hi,

Actually having now seen a diagram on another website I think you mean:

SqWasher---NewJoist--TimberConnector--OldJoist--SqWasher

So that is a double sided timber connector between the old and new joist with just square plate washers on the outsides.