I have a house that is 120yrs old. The floor joists on the ground floor are only 74mm x 55mm and spaced 350mm on centres. The brickwork that holds up the floor joists is pretty rough and doesn't have foundations, just laid directly on the ground. It is generally sound but the spacing is also 1.5m, so the span of the floor joists between the brick support is 1.5m.

As you can imagine there is quite a bit of bounce in the floor and I have repaired some of the joists by sistering some treated timber that I ripped down from 90mm x 45mm pieces. I have done this in the areas that needed it with stainless steel coach bolts. This is OK for the repair work but could full replacement be an option also?

I was considering putting in additional brickwork in between the current spans, but obviously this is a significantly different cost basis, not to mention not being able to do this myself, whereas the timber I can.

My question is, should I continue to sister the current joists, or consider another method? Obviously 74mm joists are not going to be the best to reduce bounce in the floor, but I can't increase the height too much without causing issues of height on door frames etc.

I was also planning on putting down two layers of 18mm ply in place of the current 22mm floor boards, glued and screwed, but not sure if I am overdoing it if I can use another method, not to mention the height issue also.

Room dimensions are 3.2m x 3.6m with the joists running along the 3.6m length.

Thanks in advance.

1 Answer 1


Your joist timbers are very small dimension (top to bottom) for a length span of 3.6m. I suggest that when you have sistered on the few additional side beams that this is barely enough to help unless you at least doubled (or more) its top to bottom dimension.

You would probably be better off adding additional support at right angles to these puny joists that spanned the 3.2m width of the room. You could do this at two places across the room to roughly break up the spacing to thirds across the length of the joists. The ends of these beams can rest on posts by the side walls. You could adjust the position as necessary so that any doors could still operate OK. The height of such beams would be a key factor at stiffening up the floor above. Obviously the higher the better but if you were able to make a beam for this with some a couple of 45mm thick members with plywood sandwiched in between it could be quite sturdy even if only 170mm in beam height.

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