Sinking floor

[Tony Eva]

(Long post - sorry)

This year I had a double-storey extension done on my house. It was started in April, finished in August.

Part of the extension includes an en-suite shower upstairs, and in October I got round to tiling the floor and installing the shower.

However, since then, it has become obvious that the upstairs floor is sinking. By looking at the level of the floor tiles against the skirting boards (screwed to the wall) I can see that the floor in the corner of the en-suite has dropped by about 6-7mm so far. While this is not a huge problem with the floor (the grouting is still intact), unfortunately the shower tray has also dropped by a few mm and the waterproof seal along the edges is starting to crack and break as a result.

As far as I can tell, the settling is not happening to the same degree across the whole of the extension's first floor. I haven't got an easy way to tell elsewhere (no flooring yet!), but I am reasonably sure that any other settling is only around a couple of mm, if at all.

The floor's construction is pretty standard - 2x10 (or 12? not sure) wooden joists, laid on 40cm centres, one end bedded into the inner blockwork leaf of the new wall, the other end suspended on joist hangers from the existing wall. The end that's dropping is the one bedded into the new wall. The flooring surface is sheets of T+G flooring grade chipboard, screwed to the joists at approx 30xm intervals.

Inspecting the ceiling on the ground floor below (plasterboard screwed to the underside of the joists) shows no signs of the ceiling dropping or any other movement.

So as far as I can tell, this floor movement can only be explained by some of the joists actually shrinking by about 2-3% on their vertical axis. What do others think? Is this normal, and if so, how much shrinkage can I expect? How long will it last - i.e. how long should I wait before starting to repair the tiling and shower tray waterproofing?

[Rick Dipper]

The joists should nto shrink as they dry, cause they should have been kiln dried before installing.

What I think is that the packer put under the joist to hold them level has either come out, or crushed. I would put a straight edge on the ceiling below, to see if its no longer straight.

Rick

[BigWallop]

It might just be the timbers shrinking in the cold weather Tony. The small spaces are appearing normally for a new build, that's why builders come back after months not weeks to do any snagging work on a new build. Small gaps could appear in plasterboard joints over the next few months. Skirting boards may appear to have lifted off the floor by a few millimetres. Chipboard flooring may show signs of opening up at the joints by a couple of millimetres. These are all perfectly normal on something as new and heavy as an extension.

If you'd said that the skirting board had fallen off, or that it had risen up the wall by a few centimetres, then I would have worried about your build. But for a couple of millimetres of movement over such a short period of time after completion, it's nothing un-normal about it.

[Tony Eva]

OK, thanks for that. I suppose what I also need to know is how long I can expect this to continue for? There's no point in my doing a full repair job on the shower if it's yet to get significantly worse.

[Tony Eva]

The ceiling below is still 100% level with no signs of movement.

The other reply suggests that joist shrinkage *is* normal, though I must admit I am surprised by the extent (6-7mm over 25-30cm) of it.

[Michael Mcneil]

The other explanation is that the wall is rising. If it is a stud wall I imagine it has. Have a look in the loft to try an see if the trusses have been pushed up at the spot.

I have no idea how that might be overcome, if it is the problem.

[BigWallop]

It is normal Tony. Most new builds move and settle for years after the main build completion. I live in a tenement that's over one hundred years old, and it's still moving slightly here and there. The difference in temperatures inside and out can also create shrinking and expanding of the building, so if the timbers were installed in summer hot weather they will shrink slightly in the colder winter months. It's a natural process for everything and everybody. If the building were moving by huge centimetre amounts at a time, then you start to worry, but a few millimetres here and there is natural.

The shower should have a base seal which allows for millimetre movement at any time. The weight of a body in the shower will cause the base to move anytime it's used, and this should always be allowed for in the type of sealant used. A skirting should always allow for movement against the floor to stop it forcing when the temperatures change in and around the house, so this to should be allowed for when fitted.

Believe me, it's nothing to worry about unless the movement is huge amounts at a time.

[John Rumm]

There are a few clues in there. The fact the the floor is moving relative to the skirting would suggest it is *not* the whole wall that is moving since the joists are presumably being supported by the same wall. So that does lend weight to the shrinkage theory. Also the fact that the joists are in hangers at one end might also go some way to explain why you are not seeing the same (or as much) movement at that end. Depending on the hanger design it may well have a row of nails down both sides for most of the depth of the joist, this would tend to restrict the shrinkage a little, and also maintain the centre of the joist in its original position, causing the shrinkage to be shared partly above and partly below. The other end however the joist is simply sat on a wall plate and hence all shrinkage will appear as a loss of height of the joist.

You may also see some variation as the centre of the span sags a little as it will over time. If the builder was careful then he will have placed all the joists with their natural camber rising, so they settle towards level.

While in theory the timber is kiln dried, I know from recent experience that is only part of the story! What happens to it after it leaves the lumber yard is another matter. Many builders merchants seem to keep constructional timber outside in the rain. (I know I had some bits of

4x2" delivered that were so wet you could almost wring them out!)

Did you start to notice more movement after the weather got colder and the heating came on?

[Tony Bryer]

suggests 1.5-2% shrinkage across the grain for a 5% change in moisture content, air-dried timber will have a MC of 18-20% and in a centrally heated house this may drop to 10%, so what you're seeing is not unexpected. Traditional joinery practice when making panelled doors or panelling a room is to put the panels into grooves with no glue so that they can move with the changing humidity.

[Tony Eva]

We were lucky this year - over the period that the joists were on site (delivery to roof completed) there was no rain. What happened to them in the yard is another story, of course - April was *very* wet.

I couldn't really say... I didn't pay any attention to floor movement until after I'd tiled the floor around Sep-Oct, which was about the time the heating came on.

However another post suggests that I can expect around 3-4% shrinkage as the joists reach their stable moisture content, with a smaller seasonal fluctuation after that. So I might well have seen the worst of it by now.

Thanks for all the responses.