Is this a case for joist sistering?

25 Apr 2008
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United Kingdom
Hi. I own a 1930s semi-detached house in Greater London. There is about two feet of crawl space under the house.

I have noticed that the floorboards under a certain part of the house (at the base of the stairs) are creaking really loudly, almost sounding like they're banging on something hollow when trod upon.

To find the source, I got into the crawl space and asked the partner to walk across the floor and then to gently bounce up and down whilst I was closely observing the joists from underneath. There was a little bit of give in a few of the joists whilst the partner was bouncing (i.e., they flexed slightly downwards but sprung back up). This give was not substantial but it was definitely not all in my mind.

I inspected where the joists rest upon the plates which are on top of the sleeper walls and it doesn't appear that there are any gaps or looseness there. Plus, the creaking noise is definitely coming from the centre of the floor, not along the edge where the joist meets the wall.

I would like to repair this, obviously, to strengthen the joists. I have read up on the process of sistering and think that this might be a solution. I would make sure to get new joists the same size as those already there, would put a sister-joist on either side of the existing joist and would make sure to get coach bolts and attach them at at least three points along the new pieces. I would also make sure to use a washer (or several washers) broad enough so as not to damage the new sister-joists. Unfortunately, there is not enough room for the new sister-joists to sit upon the plates at the edges. The existing joists are pretty tightly crammed in there as it is.

Can anyone offer any advice? Does this sound like a case for sistering? If I attach the new sister-joists to the old joists but don't rest them on anything else (like the plate on the wall), will that be too heavy for the old joists?

Many thanks for making it through this and for your help.

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