Rotten floor joists

Discussion in 'Builders' Talk' started by Burnz0, Dec 5, 2020.

  1. Burnz0

    Burnz0 Member

    Hi All. I’m currently in the process of repairing/replacing some floor joists that were rotten at the ends. I’ve chopped them back to well past the rot (& woodworm I think) and have got new timber ordered. I’m just wondering if there’s any advantage of keeping some overlap or should I take them back to the sleeper wall and fit the new ones next to them? I’m not sure if there’s any extra strength from leaving it doubled up for part of the span or if it would cause some other issue? I can’t find it now but I read somewhere that it should be a max of 10cm overlap & elsewhere it seems to suggest that the bigger the overlap the better.
    I’ve ordered enough timber to take them right back to the sleeper wall but the existing ones are in pretty good condition so if there’s a benefit to keeping them it seems a waste to cut them back
     
  2. Jord86

    Jord86 Screwfix Select

    The bigger the overlap, the stronger the area of the joists that are doubled together near a bearing point, so leave them as they are unless the old joists will kick up into the flooring due to them being twisted, out of level, line etc.
     
  3. Burnz0

    Burnz0 Member

    Thanks. Should I let the new timber overlap on the other side of the sleeper wall too? I’ve got long enough lengths but it will just mean taking up more boards!
    I was planning on overlapping the wall by about 15cm (1 or 2 boards back) then the span will be doubled up by about 2/3 or more. & I’ll put several screws or bolts into keep them joined together properly
     
  4. Burnz0

    Burnz0 Member

  5. Severntrent

    Severntrent Screwfix Select

    Basically do what ever you want and whatever is easiest, unless the size of joists are under designed it doesn't matter one way or another.
     
  6. Burnz0

    Burnz0 Member

    Ah, okay, I’m probably overthinking it!
    They’re correct sized as per the tables I’ve seen so I think cutting them back a lot more will be easier as it gives me more space if nothing else. Trying to screw or bolt them together seems like a faff given that they’re only about 30cm apart so no room to get in between!
     
  7. ginger tuffs

    ginger tuffs Screwfix Select

    why asked then do your own way
     
  8. Burnz0

    Burnz0 Member

    Huh? I asked & was told it didn’t really matter so do it the way that’s easiest
     
  9. stevie22

    stevie22 Screwfix Select

    If you have a sleeper wall to go back to then this is your best approach and what I would do if 'twas mine.

    Overlap past the support does have a very minor advantage in that the contact pressure that's allowed can be increased but this is rarely an issue.

    If you want to just lap the joists away from a support then the joint needs to be thought about properly and the greater the lap the better.
     
  10. Burnz0

    Burnz0 Member

    Thanks. I’m go back to the sleeper wall with a small overlap on either side. Do I need something specific for fixing them together or just attach them to the wall plate & a few screws between them?
    The sleeper wall doesn’t have any air gaps so the only through circulation is between the joist & now I’m going to be blocking that up with an extra joist width. I’m not confident that I could add some holes to them as even though it’s quite solid the lime mortar is a bit crumbly and I think it would be a bad idea to disturb it too much. The wall plate is completely sound though and there’s no dpc so it seems it be ventilated enough. I’m also clearing out a load of rubbish that was left under there so freeing up even more airspace
     
  11. Mr Rusty

    Mr Rusty Screwfix Select

    If you just rely on the sleeper wall for 100% support, you also have to rely on the sleeper wall footings being decent. If you create a lap joint over the sleeper, you will transfer some forces to the ends and some to the sleeper. Personally I would overlap as far as is sensible and either bolt through or use coach screws (I'd prob use the latter). If you read the blog of this young couple, you will see their latest is having to dig out for new sleeper walls because they had settled and din't have decent footings https://www.whathavewedunoon.co.uk/
     
  12. Severntrent

    Severntrent Screwfix Select

    But if you haven't had any issues thus far with your sleeper walls supporting your floor there is no reason why you would in the future
     
  13. Mr Rusty

    Mr Rusty Screwfix Select

    That may not be the case. A joist between wall plates will have most forces supported at the wall plates. The sleepers will take a very small proportion of the load which will just be from what would otherwise be the flex across the span. If you break the joist and support the cut ends at the sleeper, then 100% of the floor load sits on the sleeper at the joint, so it is taking more pressure than it was before. Maybe it's OK, or maybe it isn't
     
  14. Burnz0

    Burnz0 Member

    All but 1 or 2 of the ends were completely rotten & weren’t even touching the wall plate so the weight must have already been mainly on the sleeper wall? I’ve now cut them back to about 50cm away so will overlap by the same on the other side giving 100cm in total so hopefully that’ll be fine!
    My main headache is trying to fix them together as it’ll be hard to get access between them so this seems more manageable - maybe 2 lots of bolts either side compared to having to do several if I’d overlapped by more
     
  15. Burnz0

    Burnz0 Member

    And thanks All for the advice!
     
    Jord86 likes this.
  16. Jord86

    Jord86 Screwfix Select


    Just screw them together with large screws and if you're inclined to a couple of washers but it doesn't matter really. Impact drivers are smaller than cordless drills to get between joist spaces. It will be easier for you and your tools if you drill pilot holes first rather than just try to drive a screw straight in.
     
  17. Severntrent

    Severntrent Screwfix Select

    Sorry but disagree, if sleeper walls are put in then the span for joist tables are taken as the distance from the sleeper walls and as such it must be assumed that the sleeper walls are taking the full load of the proportion of the floor that is acting on them. i.e. to take it to the extreme, assume we have a room 100m wide with 20 sleeper walls , the sleeper walls would take 5m run of floor loading and the side walls wall plates would be taking 2.5m run of floor loading not 50m of floor loading.
     
    Jord86 likes this.
  18. Mr Rusty

    Mr Rusty Screwfix Select

    IMO it depends whether the sleepers are intended to be fully structural as you suggest, or whether they are there just to take the bounce out of the floor. If the latter, the joist is sized to the full span, and the sleepers take very little load. Think of a bow supported at each end and teh string represents the joist. If the upward force from the spring of the joist matches the downward load there is zero residual force on the sleeper. So, the sleeper only takes the excess force against the natural spring of the joist.

    20201206_085838 (1).jpg
     
  19. Jord86

    Jord86 Screwfix Select

    Sleeper walls are structural, being as they break up the longer span of a room into bays where smaller section timbers can be used, no point building them just to take the bounce out otherwise it would have been far quicker just to nail a 3x2 support leg to each joist. Rusty you do realise Severntrent is a Structural Engineer don't you? :D
     
  20. stevie22

    stevie22 Screwfix Select

    If you follow your logic then the sleeper wall could carry all the load if the joist was curved the other way and was hogging over the sleeper wall.

    Severntrent is correct.
     

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