Suspended Timber Floor

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I am due to start the ground floor suspended timber floor structure in the next 2 weeks.

External walls will be wall plates with the joists hung from these and sat on a sleeper wall in the middle of the room (circa 4m span) with an intermediate support wall at 2m.

I have a few questions before i start if anyone can help;

1) Are these acceptable for hanging the joist from the wall plates <Link>
2) I am using 8" joists so are the tops just cut off the hangers after installation?
3) Should joists be installed at 400mm centers?
4) What centers should the wall plate be bolted to the external wall?
5) Ar eM10 bolts sufficient or should i use M12?
6) Should the joists be nailed together where they meet in the middle of the room sat on the supporting wall?
7) where should noggins be installed over the 4m span - at mid span support wall and end span support wall?

 

[jks7492]

Assume you mean a timber bearer fixed to the face of the wall rather than a wallplate? (wallplate is a timber laid on the top of a wall)
If so then -
1. Yes those hangers will be fine.
2. Fix hangers to timber bearer, bend hanger over top edge of timber and trim excess before fixing to wall.
3. Yes joists at 400mm ctrs - although 8" joists are a bit overkill for a 4m span with a midspan support?
4/5. Fix the bearer to the wall with M12 bolts @ 600mm ctrs.
6. Yes i'd skew nail them down onto a wallplate laid over the supporting wall.
7. Noggins at external walls and at mid span wall.

 

[padstar]

Is there a recommended overhang for the joist on the mid wall where the 2 joists meet? Or should I just cut them so there is no overhang past the mid wall plate?

 

[conny]

No harm in letting them overhang by a few inches.

 

[Deleted member 174758]

Better to overhang as much as they do and nail/screw the tails. You'll need to get some solid strutting in any case, so the ends will need to be trimmed for those.

A couple of minor points:

If it hasn't been pointed out, joist hangers should be nailed, not screwed, using sherardised twist nails. Every hole must be filled. I was taught never to cut hangers unless completely unavoidable - just wrap over and nail on

The best way to deal with ledgers (referred to as bearers/wall plates here) I've found is to mark off from the end and pre-fix all the joist hangers to the ledgers, using an offcut of joist to get your alignment, bevfore fixing to the wall. If the wall they are fixed to is external they ideally should be pressure treated timber with a layer of DPM behind them for additional protection.

If the masonry you are fixing the ledgers to is ropey, use resin anchors instead of Rawlbolts

It is often easier to temporarily fix a piece of 3 x 2 CLS to the wall with brown plugs and 5 x 90mm (or so) screws (2 to 4 no) and get that levelled up, then use that to support the ledger, rather than trying to get the ledger on the wall without any aids. The CLS is pulled off after the resin has set or the Rawlbolts have been tightened

Joist spacing should always match the sub floor material, but also take into account the ceiling material below (if there is one), so if your sub floor is going to be something T & G chipboard, which comes in 2400 x 600mm (metric) size, or T & G larch plywood, which comes in 2400 x 1200mm (metric) size, then 400mm (metric) is correct - however, if your sub-flooring is something like 18mm square edged plywood sheet, which comes in 8 x 4ft (2440 x 1220mm) the joist spacing must be 16in (406.6mm) unless you want to cross cut trim every 2nd or 3rd sheet you lay down. When laying out leave a gap of 70 to 100mm from the last joist to the wall (otherwise you can't get the hanger in). I always take my measure from the end wall rather than stepping off the 400mm, which can lead to a creeping error. You'd be surprised how often some of the site guys I've worked with get this wrong (particularly some of the agency lads - but maybe that's why they are on agency)

If using hangers there is no need for solid strutting at the outside walls as the joist hangers should resist any tendency the joists may have to twist under load - solid strutting (noggins are in walls IMO) is technically only necessary where the span exceeds approx. 16ft/5m. In the case of this floor I'd probably want to run two rows, though, at the ends of the tails, which would also help tie the tails in

 

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Ta

 

[Lower]

Out of interest, why can joist hangers only be nailed, not screwed?

 

[padstar]

Shear strength apparently. Nails r something like 10 times stronger than screws in shearing failure.

 

[Deleted member 174758]

Is your board actually 2400, though? (in other words, what material are you actually using? Don't make the assumption that all sheets are metric - square edge sheets of plywood and chipboars are invariably Imperial sizes) First board should be spaced maybe 50 to 70mm off the end wall (it isn't critical - but you should ensure that the last joist cannot rub against the wall, it makes sense to allow enough space to get plumbing, CH pipes, etc up between floors/from the underfloor void, but you also want the ends of your sub-floor boards supported - but with no more that about 150mm/6in overhang. ideally less). All the other joists should be spaced from the wall with provision for a 5 to 10mm gap between the sub-floor and the wall. The gap is to prevent wicking of moisture from the wall into the floor boards and also to prevent creaking which can be caused by floor boiards rubbing against masonry as someone walks across the floor - it is normally covered by the skirtings. The joists are spaced out either at 16in or 400mm centres depending on the board type, measured off the wall. That means that your joists (on METRIC flooring) would be on centres approximately at 115 (not exact), 410, 810, 1210, 1610, 2010, etc):

Note where I have put the solid strutting, as suggested above, but only one row as there is no need for any more, and why do more work than you need to? At the other wall end you need to ensure that your last joist is 50 to 70mm off the masonry. It doesn't matter if it is only 200mm spaced off the previous joist, so long as the joist spacing doesn't exceed 400mm/16in (depending on what you sub flooring you are using).

The sub floor goes on on brick pattern or hit and miss (there is a 10mm gap all round on this floor):

If you start with a full sheet, and the next one is a part sheet, try to trim and use the offcut to start the next row. Also, make sure that you aren't going to end up with a stupidly narrow strip at the end - measure the room before starting and if you end up with a 50mm rip, try to start by taking, say, 100mm off the leading edge of the first row so you get at least a 150mm final strip

With traditional planked floors this was all so much easier as you just sawed the floorboards to length as needed!

 

[Deleted member 174758]

Out of interest, why can joist hangers only be nailed, not screwed?

Sherardised twist nails have good pull out resistance and are far less likely to shear under shock loading than screws (e.g if somebody drops a heavy item of furniture on its' corner on a floor). So by using screws you are reducing the shear strength of the floor structure. BTW BCOs will generally pull screw fixings of joist hangers if they see them

 

[padstar]

JAK the boards are 2400 x 600 UFH boards. So I am setting out at 400 centers.

 

[Deleted member 174758]

Sorry for being so anal, but I've seen this done wrong more than a few times!

To speed things up you may want to consider making up a cross cutting guide for your saw (unless you already have a rail saw, like the Festool or one of the numerous copies)

 

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Tha ks for all the help today. Day 1 is over and I THINK i have it off to a good start other than 1 mistake. Spent a lot of time measuring setting out and cutting material but it then made actually installing the joists very simple.

Is there anything from the below that looks wrong before I carry on?

Also what sort of tollerence should be expected. The engineers laser is showing approximately 2mm over the section laid so far.
The mistake I made was I didn't back the ledger board with DPM. I have so much dpm in the house I can't believe I have missed this! It is treated timber fixed back to a cavity wall with approximately 500mm or more of free ventilation below and arround it. I know this should have been installed but given where I am now how much of a risk is it to leave as it is?

 

[Deleted member 174758]

Is there anything from the below that looks wrong before I carry on?

I'm a little mystified about the joist you have fixed to the RH side wall and how you intend to infill the area between that and the first installed joist (dwg to follow - it's late, it's been a long day, and I'll be fresher in the morning)

Also what sort of tollerence should be expected. The engineers laser is showing approximately 2mm over the section laid so far

2mm is neither here nor there - you won't notice it. Lasers are NOT perfect and you won't notice 2mm as you wal across the floor (10mm and you may well). What is more important is to sight down each joist before installing it so that you know if it is crowned or not (i.e. bowed allong its' length in the vertical orientation). Joists should always be installed crown uppermost as the crown will flatten out when the sub-floor goes on and the floor is loaded, however, there is a limit to how much crown is acceptable (to my mind about 4mm). Joists which are too crowned should be cut and used for shorter joists, solid strutting, etc where cutting to a shorter length makes optimum use of the timber

The mistake I made was I didn't back the ledger board with DPM. I have so much dpm in the house I can't believe I have missed this! It is treated timber fixed back to a cavity wall with approximately 500mm or more of free ventilation below and arround it. I know this should have been installed but given where I am now how much of a risk is it to leave as it is?

The DPM together with treated timber is really just belt and braces. You have treated timber, so I wouldn't worry about it too much

Glad to see that you've put a DPM beneath the bearer atop the pygymy wall _ I missed that point. The joists can be toe nailed (skew nailed) to the bearer for stability

As another aside, I tend to use another physical check as I go - I set a 6ft level across the tops of several joists to gauge any crown (or dip) there might be at various random points. In the days before we had 6ft levels we used to take a piece of straight planed timber (or even straight CLS - checked by eyeballing it) and tape a shorter level , often a 3ft or 1m level, to the top of it. The straight edge helps you verify that adjacent joists are more or less level

 

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Thanks for the help JAK. It's much appreciated.

On your points. Good to hear you dont see the lack of dpm to the ledger as being a major issue. Dpm has been installed bellow all "wall plates" to the dwarf support walls.

The area to the right will have 2 more joists installed with a 1200 ledger plate to the external wall. These 2 sit arround a steel so needed to be cut and I ran out of time.

I have been using the rotary laser together with a 6ft level and all seems good. The bubble sits nicely between the lines in all areas.