Bowing joists - how to level a sagging l&p ceiling?

2 Sep 2007
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United Kingdom

I have just bought my first house which is a 1930s terrace. We initially thought that the old, cracked plaster in the bedroom was sagging away from the joists, so we secured with big washers by screwing up through the existing L&P, with the intention of putting plasterboard underneath. This didn't help as it turns out that the joists have bowed - you can see this if you go up in the loft.

Having read a previous posting ( // ) I have some idea of how fix the problem but I still have a few questions:

1) A couple of people above suggested putting in a false ceiling - does this mean just fixing new plasterboard to the existing joists by screwing up through the L&P?
2) If we were to go ahead and overboard it, would this disguise the bow in the ceiling?
3) I am guessing the best option is to pull down the old plaster and re-board, having fixed level timbers to the sides of the bowed joists?
4) If I go ahead with this, will removing the old, heavy L&P cause the joists to recover (I am guessing not but just thought I'd ask).

If Ell still looks at this forum - what did you end up doing?

Many thanks in advance to anyone who replies ;)
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i had a smallish bedroom with old lathe and plaster that was all cracked and rubbish... so i took it all down, plasterboarded, and got someone to plaster, now its all nice and shiny new :)

takin old plaster down is a messy job and its hard work doing the ceiling, we did it ourselves... got dust everywhere, but hey! it hoovers...

dust in the room is more difficult to move... i have bagless hoover with hepa filter and it clogs it really bad so i had to keep cleanin... i found it helped when i put a layer of tissue round the filter... keeps the grit from gettin trapped and clingin to filter. id recommend you sweep what you can up before hoovering. plaster is heavy, so you will need a skip or several trips to the tip.

although time-consuming i found it quite easy to screw plasterboard to the ceiling (cordless drill), i was even up a step ladder jugglin on my own for a lot of it. plasterboard is cheap, labour isnt, so you could save quite a lot by doing it yourself. the plasterer i got in did dot n dab plasterboard onto the walls, was going to do it myself but wasnt sure if i could do it right.
Thanks for the tips diykat. i'll get the hoover on standy as our loft is very dusty!

Can anyone suggest what to do about my bowing joists though?
better to buy an Aquavac and some of the big paper bags (this saves the filter cartridge clogging) or hire an industrial vac. Hoover out as much dirt as you can before breaking the ceiling down. Tape up the gaps round the door and window as it can easily drift round the house in draughts. Wear a mask, hat and goggles. Take your overalls off and bag them before walking out of the room.

If you roof timbers are sagging noticeably:

Have a look up top to see why and remove anything heavy.

See if they need to be replaced (they won't ever go straight again).

However it is fairly straightforward to either lay new timbers beside them with battens to bring them down (you will have to take the ceiling down), or to nail new timbers to their sides, slightly deeper so that they project below the lowest point of the bow, and fasten your plasterboard to that.

Take the sag as a warning not to try to store stuff in the loft, or turn it into a games room, without laying new joists across the wallplates.
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There's nothing heavy in the loft so i guess its just the weight of the plaster? The joists dont look to be in bad shape but you can see that the free end, which protrudes beyond the central load bearing wall, is poking up above the level of the joists on the other side of the house (hard to explain but you might know what i mean!). If you walk accross the joists in the loft (which span a gap of 15ft) you can see the ceiling moving from below... is this something i should be worried about?
is the ceiling more than 7foot 6high :?: you could bring a new one down to that and it`ll look good ;) leave the old one up there, and save the mess
Pull down the old ceiling,level the joists through and re-sheet with 12mm plasterboard.If you put a false ceiling below an old lath and plaster ceiling that might not be in the best of nick,you cannot see any problem that might occur in the future.If any part of the old ceiling goes,your false one will go with it.So for me it's take down,level through and re-board etc.then forget about.

There is another way, take it down and use adjustable ceiling joists hangers over the wallplate and find the lowest point of the bowed ceiling for the new level with new joists and screw to the side of the ex-joists
Thanks for the ideas people. I just need to clarify exactly what you mean...

masona - this sounds more or less like what i was planning to do already. i am guessing the wallplate is something running around the supporting walls, which the joists are resting on or hanging from. Is this right?

Also, what dimension timber should I use for the 'new' joists (2"x4")?

roughcaster - when you say "level the joists through", how would I go about this?
Hi again Irv,
To level the joists through would be done exactly as Masona said.Find the lowest point of your ceiling,and either using adjustable ceiling joist hangers, set new joists into them, leveled through across the ceiling,or
fix new timber to the original joists,again leveled through either side of the lowest point.Either way would bring all the joists down to the same level.I think the adjustable hangers would be best option.Less hassle,but still a fair bit of work,you'll manage.

IrvWorth said:
i am guessing the wallplate is something running around the supporting walls, which the joists are resting on or hanging from. Is this right?
Yes ;)
or sometimes one of these. this pic shows both the wall plate and the hanger :LOL:

I think i've seen some of those those down at B&Q!
Thanks everyone :D
OK, I've pulled down the old lathe and plaster and yes, is was dusty. Vacuuming the loft first did help - filled about 5 bags - and we put polythene on the bedroom floor to make the clean-up easier, and sprayed a bit of water to keep the dust under control. Sawing through the lathes first (from above) also helped.

Now that i can see the joists i'm not sure how to proceed. They are 2x4", spaced at about 14", spanning a 13' gap, resting directly on the supporting front and middle walls of the house. The saggiest joist is about 1" below the horizontal and 1/2" below its neighbour, which seems like a big jump. Someone suggested that i should just try boarding over with 12mm and see whether it is noticable, before creating lots more work for myself - does anyone think this is a good idea?

Two of the joists have warped so that their outside ends are twisted about 25 degrees from the vertical, meaning i can't fix new timers to the sides. I only want to drop the new joists by about 2" max but I havent managed to find any adjustable hangers that are suitable - can anyone suggest what to use?

I have thought of 2 ways round this: 1) lay new timbers flat alongside the old ones, packing out with a strip of plasterboard or wood below, or 2) notch the ends of the new joists so that they rest on the walls, sitting a bit lower than the old ones. Would the second method weaken the joists?

Incidentally, can anyone tell me what timber to use - i am guessing kiln dried, c16 47x100mm??? Or should i use something thicker in case they sag again?

Also, should i be concerned about building regs here - i am not planning to board the loft, just need a ceiling in my bedroom?!

Sorry about all these extra questions, it seemed quite simple till i'd knocked it all down!
either method will work. You could even use both depending on the straightness of the old timbers.

you can also nail timbers to the sides of the old ones, with their lower face lower than the lowest point of the lowest one, and all others leveled up with that.

Deeper timbers will be stronger if you want to use it as a floor and walk about up there

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