My kitchen floor sits on joints that are mostly OK. One wobbles though as I walk over it near a wall. It moves 0.5cm (0.25"). I lifted a floor board and had a look. You can see the crumbled mortar here:

joist on crumbling mortar

Question: Do I replace the mortar after jacking up the joist a little? Or do I get a hardwood wedge and tap it in - as the joist goes out the other side and supports a narrow hallway?

The house was built in 1973 in the UK, if that's important. Lastly, it is my poor photography that makes the joists look non-vertical, not poor construction.

Here's the adjacent one that is fine for now:

joist on mortar that is OK

The sub floor area is a crawl space - but only just - not for anyone with a belly. The black is a bitcuhmen seal over the earth underneath.


I had a similar problem.

  1. support joist

  2. rake out old loose motar

  3. fill with new mortar - force to full depth

  4. allow to cure

  5. remove support

Worked fine for me.

  • I agree, though a properly-fitted pressure-treated wooden shim would do as well as mortar. Ideally it has very little taper, providing a large bearing surface. Drive it in until reasonably snug and be done. – isherwood Apr 23 at 12:59
  • So, you need at least one or two more, for the sides. I took the time to reset the odd loose brick as well. – Solar Mike Apr 23 at 13:02
  • Wouldn't hurt, but not critical. A joist under load isn't going anywhere. – isherwood Apr 23 at 13:06
  • Helped stop the squeaking of the floor once finished. Worth doing though as the floor came up nice once waxed. – Solar Mike Apr 23 at 13:08
  • I'd buy standard mortar in the just-add-water mixing tubs, right, as this joist isn't supporting a ton. And for shims, pressure-treated soft/white wood is OK? If I'm trimming it do I lose the pn'tressure-treated aspect? How about hard-wood? The house never ever gets rising water or damp under there, so not having it pressure treated could be OK. – HomeOwner33 Apr 23 at 15:55

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