Sistering Joists

23 Dec 2012
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United Kingdom
I have a springy floor - I've decided to take up the chipboard floors and sister some of the joists and add some noggins.

As found (photo from below):
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As above (repairing a previous drilled pipe job :( )
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Thinking about the sistering, is it better to do B, C, D or E?
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A) is the plain joist (2x9", 4.5m) with holes drill for central heating and cabling

B) is the joist, with a 3x2" or 4x2" sistering towards the bottom edge. Since the central heating supply and return run up the middle I can't use a whole length joist, it'll have to be cut at the centre

C) Sistering at the top edge, no full cuts required be perhaps a notch as the central heating piping is only a few inches from the top edge

D) A mixture of both, but only on one side rather than both

E) A more fuller beam, but with slots formed to slide the new joist down over the services

I will be using M10 150mm coach bolts with nyloc nuts and most likely "timber connector/dog tooth washers" clamping the two joist surfaces together. Unfortunately these seem to come in M12 size and I've already purchased 100 off M10 fittings. I think I need to buy a large bag of really big washers too to get the fixings tight enough without crushing the timber.

As I understand it, the sistering is least important at the ends and most important in the central 1/3, so it seems best to avoid any cuts in the middle, but options B, D and E all have central-ish cut outs.

I also understand it's better to have the reinforcement at the bottom rather than top so weight for weight B is better than C.

How far apart should my fixings be? - I assume 18" to 2'.
Is is beneficial to glue and bolt?
I wasn't going to sister all the joists, perhaps just every 3rd or 4th.

After the sistering is complete, I'll use more metalwork to keep the noggins square and true. The noggins will be offset from one gap to the next. I'll try to make these 9" too, so they're same depth as the joist.

All this work is to be done exclusively from above so I don't have to interfere with the ceiling below (plus the floor is being replaced anyways)
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If you want to do it properly, none of the above.

Remove the pipes, sister the joists with new, full size joists and then drill and refit the pipes.

None of your options are going to add significant strength.
How about a compound approach F??

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It's not just the central heating pipes but also cabling that's in the way and it's not an option to make the entire under-floor area accessible at the same time. (This is because I'm having the carpet rolled back by the fitters and doing 1/2 the room at a time. The carpet is large and has a seam through it that cannot be roughly handled because it cannot be re-made. The carpet is large and too expensive to replace).

I recognise what you say is best, but I have to make a compromise because of limitations. The joists are correctly sized and do not appear to sag to the eye (perhaps by measurement they do), but I think would benefit from beefing up and "boxing" in with noggins to add stiffness.

If you put a join in the sister, you've taken out a lot of its strength.

Noggins will help more than poor sistering.
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I've read up and talked to some people about stresses and strains in the joists and now I understand why the bit in the middle is so important, particularly at the top and bottom edges.... exactly the wrong place to put central heating pipes....

I'm going to move them to "outboard" of one of the cable holes and re-do in copper while I'm at it.

The I can do some "proper" sistering and a generous sprinkling of noggings.

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There is no feasible way to re-thread the cables which unfortunately fall on both sides of the room but at least not along the centre line

I think some Unviersal frame anchoring and a nail in in hole with the noggings directly end on end should be the way to go.

Why not forget adding joists and bolt a metal plate along the length of each joist eg 5" x 0.25"
Why not forget adding joists and bolt a metal plate along the length of each joist eg 5" x 0.25"
Bonkers! Have you any idea how difficult a task this would be? The weight of the steel, drilling the steel, allowing for services (pipes, cables etc), manoeuvering the steel etc etc. Crazy idea.

As stated you would gain a lot more from full depth, mid-span or 1/3rd span bracing. Get them nice and tight and fix them well. It serves to stiffen a floor up no end.

You could also screw ply sheathing to the underside before fitting the plasterboard but this would be a faff for a marginal gain.
So I started the job last week but the paid work is getting in the way of progressing any quicker; but it is going well. I've moved the fixed furniture, pulled up 1/2 the room of chipboard, including cutting it away from underneath stud walls, replaced a cable that was chaffed re-routed the plumbing and pulled out the old pipes.

I'm left scratching my head over how best to finish/re-finish the cement "in-fill" that was provided to flatten off what used to be an external wall but is now cut off at floor level. The brickwork either side of the joists isn't doing a huge amount of work but there is clearly some movement in joists relative to bricks as when the concrete was "poured" it was at floor level and now is proud.

a) I have an idea to chip out all the old cement, (fairly easy to do with my air compressor and chisel), build some shuttering, re-pour new cement, then leave it 5mm lower than the new floor surface. Let it set, lay a sheet of 5mm ply, stick it down with some Gripfill and that should leave a decent enough surface.

b) Or just "grind" back a thin skim of the cement, do the same with the ply and go ahead a just re-enforce where there are a few missing half-bricks and the "slots" either side of the joists with some other rigid gap filling material that isn't cement - perhaps timber?

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In either event joists will have added noggings towards middle and either end so that should deal with any tilting/twisting of each individual joist.


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