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General: Advice for strengthening floor joists

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    • CommentAuthordelprado
    • CommentTimeFeb 28th 2017
     
    I believe my joists are undersized by modern standards. They have a fair bit of spring. I am going to add some noggins where individual ones have been removed, and add them altogether in those rooms without them entirely.

    However, I'd like to go further and do something to the joists. As I want to preserve ceiling height I will apply something to the side. One idea was to just run another 6 x 2, but I wonder whether using structural ply along each one instead. That would be cheaper I think and more pleasant to do. Any thoughts?
  2.  
    Why not just look at the span tables?
    • CommentAuthordelprado
    • CommentTimeFeb 28th 2017
     
    I suppose I won't necessary follow the recommendation but defo want to remove spring. Is there a good table I can see?

    Thanks
    • CommentAuthordelprado
    • CommentTimeFeb 28th 2017
     
    It looks like my 6 x 2 should only span about 3m. My span is 4m. The span tables do not talk about sistering, and I cannot find guidance as to what sistering achieves if you are not running the extra joist to a support.
    •  
      CommentAuthordjh
    • CommentTimeFeb 28th 2017
     
    Google throws up lots.
    • CommentAuthorCWatters
    • CommentTimeFeb 28th 2017
     
    If you sister up take them as close to the wall as you can and bolt/nail together close to the wall.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 28th 2017
     
    A bit of spring is half the charm of an old building - so much more friendly to walk about on. Never dangerous.
    • CommentAuthordelprado
    • CommentTimeFeb 28th 2017
     
    If old buildings are safe (I don't disgeee) what is behind thee increased size of joists needed now? Wood
    Not as good?
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 28th 2017 edited
     
    Brittle modern materials can't stand deflection without visible distress
    • CommentAuthorArtiglio
    • CommentTimeFeb 28th 2017
     
    Two runs of well fitted noggins or herringbones , then wedged , will make a huge difference if this has not already been done.
    • CommentAuthorHollyBush
    • CommentTimeMar 1st 2017
     
    if you don't want to pay a structural engineer for an answer - why not try bolting your 6x2 to the middle 3m - this is similar to a situation my engineer recommended for me.

    The logic is that the largest bending moments will be at the centre of the spans. I think you'll find that removes the spring altogether - if not just unbolt and try something else. Just cost you a few bolts
  12.  
    http://www.johnbridge.com/vbulletin/deflecto.pl with a 6x2 and 4m [edit: corrected this from 3m] span gives:

    "For joists that are SYP or Douglas Fir, in good condition, 5.5 inches tall, 1.5 inches wide, 16 inches on center, and 13.5 feet long between supports, the deflection calculated is 1.565 inches.

    This translates to a deflection of L / 104.

    Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood."

    Sistering this with a 6x2 will halve the deflection.

    Paul in Montreal.
    • CommentAuthordelprado
    • CommentTimeMar 1st 2017
     
    When I mentioned 3m span above that is the max the span table said a 6 c 2 should span. My actual span is 4m

    Holly bush thanks for your thoughts - what do you mean bolting to the middle? Sorry I can't follow
  14.  
    Posted By: delpradoWhen I mentioned 3m span above that is the max the span table said a 6 c 2 should span. My actual span is 4m


    Ooops, I had actually used 4m (13.5 feet).


    Posted By: delpradoHolly bush thanks for your thoughts - what do you mean bolting to the middle? Sorry I can't follow


    I think he means just sister a 3m piece, centered on the middle of your 4m span - you don't need to go all the way to the ends to stiffen the joist (though it won't hurt). I did something similar for a 20 foot span with 12 foot sisters centered and then two 4 foot sisters at the ends on the opposite side of the original joist all the way to the supports. Made a big difference, but was more for leveling old sagging joists than stiffness per se.

    Paul in Montreal.
    • CommentAuthorHollyBush
    • CommentTimeMar 1st 2017
     
    Exactly what Paul says - whatever length of timber you have, put it in the middle, see how you go.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 2nd 2017
     
    You could try using some decent G-Clamps to sister the joists together. No need for holes, bolts and nuts/washers.
    Then when you are happy with the size of timber needed, you can do the job properly with bolts and adhesive.
    • CommentAuthordelprado
    • CommentTimeMar 2nd 2017
     
    Thanks everyone. Is there any validity to the idea that by doubling up, say 3m of the 4m span, then, without the natural flex, you are putting more not less strain on the ends?
  18.  
    Posted By: delpradoThanks everyone. Is there any validity to the idea that by doubling up, say 3m of the 4m span, then, without the natural flex, you are putting more not less strain on the ends?


    No as the mass on the ends stays the same, assuming the same dead and live loads on your floor.

    Paul in Montreal.
    • CommentAuthorDarylP
    • CommentTimeMar 2nd 2017
     
    no... think where beams fracture with imposed loads from above?
    • CommentAuthordelprado
    • CommentTimeMar 3rd 2017
     
    Thanks. So where I have one set of struts and where I need two, shall I remove the old struts, sister the joist, then add the two struts on afterward.

    What type of nail should I use as I understand I shouldnt be using screws? I dont want to buy a nail gun as they look so expensive, yet I need to be careful in one of the rooms as its just been plastered below

    Thanks
  21.  
    If you can get a drill in the space between the joists then bolts can be used and create less vibration than nails (less vibration means less falling debris and cracks at the ceiling/wall junction). Also when nailing you need enough space between the joists to usefully swing a hammer, which sometimes can be a problem.
    • CommentAuthorDarylP
    • CommentTimeMar 3rd 2017
     
    Glue(PU "bubble glue") and screw, with dog-tooth connectors, clamp first...:bigsmile:
    • CommentAuthorHollyBush
    • CommentTimeMar 3rd 2017
     
    delprado,

    Just want to get a few assumptions out:

    1 - is this a floor, where you have access to top and bottom of the joist, as well as both sides of the joist?
    2 - Are the current joists 6x2
    3 - is the total unsupported length 4m
    4 - this is a domestic room?
    5 - do the joists already have any notching or holes in? (if not good)

    If so, I would bolt (with dog tooth). you will be safe making the holes for the bolts - and bolting together an additional piece of timber - do two and see what difference it makes, if you are happy, do the rest.

    Nailing is tricky with a hammer in confined spaces and makes a lot of movement/damage. Screwing also difficult in confined spaces and may move over time.
  24.  
    You can get structural wood screws (e.g.http://www.fastenmaster.com/products/lok-line.html - there are lots of other makes, (or see screwfix)) which are easy if you have access to an impact screwdriver
    • CommentAuthorHollyBush
    • CommentTimeMar 3rd 2017
     
    Posted By: Peter_in_HungaryYou can get structural wood screws (e.g.http://www.fastenmaster.com/products/lok-line.html" rel="nofollow" >http://www.fastenmaster.com/products/lok-line.html- there are lots of other makes, (or see screwfix)) which are easy if you have access to an impact screwdriver


    thanks for those - everyday is a school day :smile:
    •  
      CommentAuthordjh
    • CommentTimeMar 3rd 2017
     
    Posted By: Peter_in_HungaryYou can get structural wood screws ... which are easy if you have access to an impact screwdriver

    FWIW, my carpenters sometimes had to resort to an 'electric' driver (i.e. a big mains-powered Hilti drill) rather than their normal battery-operated Makita kit for some of the bigger structural screws we used.
    • CommentAuthordelprado
    • CommentTimeMar 3rd 2017
     
    Thats interesting djh and surprising. I have driven 8 inch timberlok into c24 with an 18v impact driver dead easy

    Holybush - thats correct, thank you for setting out so clearly. On your final question - some do, yeah. Most of the time not in excess of a copper pipe
    •  
      CommentAuthorfostertom
    • CommentTimeMar 5th 2017
     
    Posted By: Paul in Montreal
    Posted By: delpradoThanks everyone. Is there any validity to the idea that by doubling up, say 3m of the 4m span, then, without the natural flex, you are putting more not less strain on the ends?


    No as the mass on the ends stays the same, assuming the same dead and live loads on your floor.
    Surely, both both the bending stress and the shear are concentrated into the unreinforced portions. Bending stress no problem because that's at minimum towards ends, so can afford to be increased - but shear is already at maximum towards the ends and you're increasing that.

    Yes/no?
  29.  
    Posted By: fostertomSurely, both both the bending stress and the shear are concentrated into the unreinforced portions. Bending stress no problem because that's at minimum towards ends, so can afford to be increased - but shear is already at maximum towards the ends and you're increasing that.

    Yes/no?


    IMO No - well only a little bit. Shear is surely a function of the total load and whilst the beam is at a fixed angel the shear remains the same. The little bit of increase will be due to the addition (weight) of the sister beams
    •  
      CommentAuthorfostertom
    • CommentTimeMar 5th 2017
     
    Yeah that's right about shear - no extra at ends except what's added by the reinforcement and anything else.

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