Levelling sagging bedroom floor - sister joists or packing?

Discussion in 'Carpenters' Talk' started by sharpjimbo, Apr 27, 2022.

  1. sharpjimbo

    sharpjimbo New Member

    Hi all,

    I'll be sorting out the sagging bedroom floor in our Victorian terrace next week - before I do, I wanted to ask, which is the best approach:

    1). Attach level sister joists?
    Or...
    2). Put hardboard packing (or equivalent) on top of the affected joists, and obviously under the floorboards?

    I'm not sure if one approach is better than the other in certain situations, so looking for a bit of guidance. Here's more more information about the room and the sagging.

    Further information
    1. The bedroom is approx 5.3m x 3.5m.
    2. The sagging appears to be roughly above the living room bay window. covers a span of about 7 out of 13 joists I'd say.
    3. At its worst, there appears to be a gap of about 3cm from the top of the joist to the string level that I've made.
    4. Original 170 year old joists, original pine floorboards nailed directly into them.
    Questions
    1. I'm concerned that adding loads of heavy wood sistering the joists, to what appears to be an already pretty old/knackered floor/ceiling would be too much weight and would be a bad idea? I'd be using the following wood (https://www.wickes.co.uk/Wickes-Treated-Kiln-Dried-C16-Timber---45-x-95-x-2400mm/p/166403) and attaching with wood glue and cartridge bolts.
    2. The floor currently has some pretty rubbish noggins in running down the centre that don't really look like they're doing anything. I was planning on installing a herringbone strut down the centre, but would some decent noggins be just as good (noggins made from the same wood as I'd use for the sistering).
    3. What's the best way to check how level of the floor is from one end of the room to the other? I've used both a piece of string attached to joists at either end of the room, and also turning a long floorboard on its thin side and resting it on the joist. Do I need to rent a laser level for a couple of days while I sort this? Any and all suggestions are welcome.
    [​IMG]
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    Thanks,

    James.
     
  2. MRY

    MRY Screwfix Select

    Bearing in mind that apart from the sagginess everything looks in reasonable condition, do you need to do anything about it?
     
  3. Mr Rusty

    Mr Rusty Screwfix Select

    Victorian house? there is a very strong possibility it's been like that for >120 years - is there any other indication of structural stress? If it's over a bay the ends of the joists must be supported by the bay lintel? Possibly the bay lintel has sagged? woodworm? - you'll know because there will be other cracking.

    If it all looks sound, then a sister joist will work to level up the floor, however, if there are other issues they need sorting first.
     
  4. sharpjimbo

    sharpjimbo New Member

    Thanks for the replies!

    Yes, that's right, the joists are supported by the bay lintel, I've actually made a separate thread on that in the link below where I was asking whether it would be necessary to fix the joist to window lintel joint with some heavy brackets - link here:

    https://community.screwfix.com/threads/joist-bracket-screw-or-nails.251949/

    General consensus was that likely wasn't needed, but, you can't buy experience, and in lacking that, so I plan to fit them with some equally heavy duty screws that I've ordered from Simpson Strong-Tie. It doesn't look like the lintel has sagged as the joists closer to the lintel are level(er), it only as you come, say, a meter and a half away down the joists, away from the window lintel, does the bow become more pronounced.

    To answer your other question, yes, there's evidence of historic woodworm in the original floorboards. I've had someone come out to assess and mostly appears to be in the floorboard edges, with only a small amount in the joists. There's no cracking in the joists, just a couple of pin holes here and there on the top face of the joists where you'd nail the floorboard into them. Speaking to our neighbours, everyone's had issues with woodworm (we're in Bristol, quite common apparently), and I don't think may have started replacing all the joists in their properties. Tapping the joists, they don't seem hollow like they've been chewed through to the point that they've lost their structural integrity, but then, I'm not structural engineer, nor carpenter, so I guess I'm just optimistically assuming.

    Separately, the plumber that's historically installed the radiators has notched the joists to lay the pipework. Assuming that this pretty standard, but I understand that this can affect the integrity of the joist too.

    I guess my question is, based on the above info, would adding sister joist be the better thing to do, or should I just consider adding packing (albeit a fair amount of it in places) on top of the joists. Appreciate that adding sister joists will level the floor and add latereraly strength to the existing joists, but as above that I'll be adding more weight to the existing joists, what with the questions around the joist joints above the bay window.

    As previous, any and all advice would be very welcome.
     
  5. sharpjimbo

    sharpjimbo New Member

    I get it, and I kind of agree - but, while I'm replacing some of the floorboards, I figured that I might as well make it right. Now I've noticed it, I kinda can't unsee it.
     
  6. sharpjimbo

    sharpjimbo New Member

    Haha, unfortunately it's just Totterdown in Bristol.

    [​IMG]
     
  7. Mark Griffiths

    Mark Griffiths Active Member

    I had a few warped joists in my house when we bought it, this was due to water damage. I replaced the affected joists with new and all is now good.

    It took some doing to work out which were warped and how badly, but I did it all using a laser level approx 100mm up from the joist level and then measured each joist at 30cm intervals and mapped the floor level. It is way overkill but gave me a good indication of how bad each joist was and I could work out the best repair method.

    My laser is just a cheapy from B&Q & cost about £30 a few years ago.

    Mine had warped up and down which made lift slightly more complicated, as yours are all down you could just measure and shim the top of the necessary joists and replace the floor.
     

    Attached Files:

  8. Mr Rusty

    Mr Rusty Screwfix Select

    So they have actually bowed rather than dropped. In which case, yes you could sister them to level the top surface.

    To check for woodworm, get a core-plug drill bit and drill a hole in the middle of the joist and see what it looks like - solid and you're OK, woodworm in the middle of the timber, and it has to be replaced or substantially reinforced.

    If you have criss-cross noggins (herringbone struts) that look like they are so lightweight they are doing nothing - do not remove them. They brace the joists together and resist bounce.
     
  9. sharpjimbo

    sharpjimbo New Member

    I would say they they've bowed, rather than dropped, and I guess I'll be reinforcing the lintel joint anyway with those heavy brackets. Is sistering the preferred method, over say, packing under each floorboard? Is this because it's generally quicker and you know that joist will be totally level across the whole length?

    bowed = fine to sister
    dropped = not fine to sister

    Is my understanding correct?

    What kind of diameter core-plug drill bit do I want to be getting to check this. Obviously want to be taking enough material out to have a look, but not too much. Would something like this do the trick?

    https://www.screwfix.com/p/erbauer-...zBZmYFBqI6VDeh0TYEhoCp3QQAvD_BwE&gclsrc=aw.ds
     
  10. Mr Rusty

    Mr Rusty Screwfix Select

    It's not so much that dropped is not fine to sister - it's that you need to sort the cause first. Yes sistering is easier to get a flat surface than trying to cut little fillets. I would use coach screws (hex head) driven in with an impact driver to fasten together.

    That plug cutter is fine. A hole in the middle of the joist height will tell you what you need to know.

    Your brackets are unnecessary, and might conceivably create "creak points" as they flex slightly with normal movement.
     
  11. sharpjimbo

    sharpjimbo New Member

    Thanks, I got the plug cutter and cut some plugs - see below. In each case the bottom of the plug in the picture is the outside of the joists.

    The top three were from three different joists towards the middle of the room, and the bottom one is from a joist at the end of the room where I'm planning on putting the gigantic wardrobe. I picked a part of this joist to drill into where the top looked particularly bad - see image below (you can see where I've taken the plug from.

    As you can see, there's some pin holes in this plug, however, I'm not sure how many holes is too many? The affected plug still feels strong, and I can't break it with my fingers. As there's previously been some woodworm in the middle of the joist, should I consider sistering it?

    [​IMG]

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  12. Mr Rusty

    Mr Rusty Screwfix Select

    Personally, I don't think that is too bad - even the bad one isn't that bad. I would give it all a good spray with woodworm killer (suitable for DIY spraying), sister the joists which need levelling with some 4 x 2 or 5 x 2 fixed with coach screws every 200mm or so (they don't need to be sistered full depth, or be let into the brickwork provided ends of the original joists are sound), make sure any herringbone struts are in place, and re-board it.
     
  13. sharpjimbo

    sharpjimbo New Member

    Perfect, thanks for your help Mr Rusty!

    Probably a stupid question, but as I'm altering the structure of the joists etc (e.g. sistering the joists, replacing most of the herringbone strut with something that's fit for purpose), am I supposed to be telling the council so someone can inspect for building regulations? It was my understanding that anything structural would have to be signed-off by someone with a clipboard?
     
  14. Mr Rusty

    Mr Rusty Screwfix Select

    If it helps, your joists are very similar to how mine were - a little bit of worm but not really enough to require ripping everything out. I did what I suggested above.

    It's an intermediate floor, so just a repair. I didn't , but I am not sure if technically this might require notification. Once completed, who will know anyway?
     
  15. sharpjimbo

    sharpjimbo New Member

    One last question - I've bought some of the wood in the link to make new herringbone struts (as, upon closer inspection, 50% of the existing noggins are loose and aren't really doing a thing).

    https://www.wickes.co.uk/Wickes-Saw...TKKuuITdOISrP0GZBsRoCr44QAvD_BwE&gclsrc=aw.ds

    I've installed a couple, but before I do the rest of the floor, I just wanted to check that this wood is appropriate. Bought it on the basis that it was kiln dried, but label says it's softwood, so just wanted to check.
     
  16. Mr Rusty

    Mr Rusty Screwfix Select

    Y it's fine. Virtually all wood is kiln dried, and virtually all construction timber is softwood.

    The purpose of the herringbones is to help support the joist in compression, so from the top of a joist the struts go diagonally down to the base of the adjoining - the idea being that as a joist is loaded it tries to bend downwards and the herringbones in compression transfer some of the load to adjacent joists - it helps to minimise bounce in a floor. http://www.ultimatehandyman.co.uk/how-to/woodworking/herringbone-strutting
     
  17. stevie22

    stevie22 Screwfix Select

    Solid blocks would be a whole lot easier than herringbone especially as retrofit.
     
  18. Mr Rusty

    Mr Rusty Screwfix Select

    I think the answer to that is "it depends". If all the joists are nice and upright/parallel and at the same centres, then possibly, but for a solid to do the same job it has to be a tight snug fit between the joists top and bottom which means each one has to be very accurately cut to fit. With herringbones you can cut them to length, fix the bottom, push the top down tight and nail the top, and they individually don't have to be cut quite so accurately. Easier round services too...
     
  19. Rosso

    Rosso Screwfix Select

    Given that you already have woodworm in your property, it would be better to use treated timber both for your sister joists and for the herringbone bracing.
     
  20. sharpjimbo

    sharpjimbo New Member

    Alright, final final question, promise - I'm probably just being paranoid about this, but would be good to hear from people will actual experience...

    Back on the subject of the mrs' gigantic wardrobe, it's going to be sitting on the back wall of the bedroom, with the joists running parallel to the length of the wardrobe (see pictures below). This means that the wardrobe is really only going to be sitting directly on top of two joists (i.e., the one under the skirting board and the next closest one). The first three joists in the first picture are obviously located in the wall in the left hand side, but under the cupboard door on the right, they're overlapping with some other joists that go on to the bathroom, and I'm assuming that they're resting on the arch in the hallway (see second picture below). The forth joist in is actually resting on top of the hallway wall.

    The wardrobe's dimensions are 200x58x236 cm and it weighs approximately 300 kg with all of the mrs' stupid clothes in it.

    My question is, am I going to be okay putting the wardrobe in this location without sistering them with some decent plywood or something like that to make them stronger? Is it less desirable to have the length of the wardrobe running the length of the joist rather than across it? If needed, I'm happy to entertain any ideas of improving the strength of this area where the wardrobe is going to go (such as sistering, adding cross bracing between the joist to attach to the joist that's resting on top of the wall.) Just a little concerned as the second joist in was the one with the light woodworm as discussed above.

    Last picture is the hallway that the wardrobe is going to be sitting over.

    Also, retrofitting the herringbone was an absolute pain in the *** as the joists are all slightly different heights and none of them are particularly straight. We're getting there though, slowly.

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