andy43
Original Poster
3,868 posts
146 months

I have a beam and block floor, beneath which we'll have a void, then a plasterboard ceiling supported by timber joists. What size timber do I need working on 4.0m clear span with no connection or help from the beam and block above, 400mm centres, no imposed loads  the 12.5mm plasterboard dead load only  so 0.2kN/mm2? I can't find a free sizing calculator anywhere, and any load tables assume I'll be above the joists jumping up and down on them. I know I should pass silver across the palm of my SE, but it's easier and quicker (and cheaper) asking on here. TIA!


Skodasupercar
718 posts
69 months

170x38mm C16 (regularised) will do the job at 600mm centres.
That is based on a 0.25kN/m2 dead load and NO imposed load
Edited  nuts, just noted you said 400mm centres  That pushes the depth down to 145mm at 38mm wide.
Edited for a second time  Use info above at your own risk (although it does come from legit source!).


andy43
Original Poster
3,868 posts
146 months

Beautiful, many thanks!! Forgot to say C16 Just to be a complete pain in the arse, would 150 x 47 C16 work? As I may have a couple or three long enough lengths that I can use to get started


Skodasupercar
718 posts
69 months

150x47 is bigger than the specified size for 400mm centres, so fine!!
If it were me, I would always oversize slightly as the cost is negligible and am prone to over spec things slightly.


andy43
Original Poster
3,868 posts
146 months

Great, thank you again, very much appreciated. I agree with the over speccing of stuff  the quality of modern timber is frightening, whether it's got a stamp on the side or not.

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z4emsee
121 posts
36 months

You'll probably find 150x50 (or 147x47 ish when sawn) will be cheaper than 147x38  I remember requesting a 38mm wide joist at a timber merchant to be told that they could do it, but would only take a wider piece (50mm or 75mm) and cut it down! You pay for the timber and the price of cutting it! Best just go with good old 6x2's.


fergus
5,376 posts
167 months


Skodasupercar
718 posts
69 months

^ that is what I used, but I used the new Eurocode 5 tables.
I would agree with the above comments though... Find what your local merchant has at a decent price, and then check it against the tables to find a spec that works (at the right price).


eps
4,034 posts
161 months

Just out of interest, which table did you look at, if I consider it as a Ceiling (i.e. table 8 or table 5.1 in the link above!!), I come up with 50x170s C16 for 4m span.
50x150 are at about 3.27 clear span.
All those mentioned are at 400crs.


Skodasupercar
718 posts
69 months

I used the new EC5 span tables  you might get a different result depending on assumed point imposed load. Because it is a ceiling below a concrete floor structure, there would be no issue of point imposed load (person walking on ceiling joists) therefore you can reduce the point imposed load.
With a reduced point imposed load, the timber sizes can be reduced slightly. But  there is no harm going bigger!


andy43
Original Poster
3,868 posts
146 months

This is what I came up against, hence the reason for the posting  there aren't any tables I could find that size the joists assuming zero imposed or point loadings. They all assume my christmas decorations and dead motherinlaws will be stored above. 6x2 seems cheapish and chunky enough, but not too heavy as to make it difficult holding the timbers in position waiting for the Nonails to set at both ends. Kidding


TA14
9,920 posts
150 months

andy43 said: This is what I came up against, hence the reason for the posting  there aren't any tables I could find that size the joists assuming zero imposed or point loadings. That's because zero imposed loading is unrealistic. Ceiling loading is already very low. In your case what if step ladders collapse and someone grabs onto a joist, etc. ?


andy43
Original Poster
3,868 posts
146 months

TA14 said: andy43 said: This is what I came up against, hence the reason for the posting  there aren't any tables I could find that size the joists assuming zero imposed or point loadings. That's because zero imposed loading is unrealistic. Ceiling loading is already very low. In your case what if step ladders collapse and someone grabs onto a joist, etc. ? Then I, as chief plasterboarder, with suitable ladder training certification, would plummet 2.4 metres (+/ 10mm) to certain death, my fall only cushioned by liberally scattered polar bears and small children, placed there as part of 'H&S In the Workplace' EU directive Protection against wobbly ladders 39787f/663b (ii).


Little Lofty
1,062 posts
43 months

In old money the general rule is half the length plus one.So a 10 feet span would be halved to 5+1 = 6, so would require a 6x2. It certainly not an exact science but its never that far away.In this case, to span 4.0 I'd be using minimum 7x2 (170x45) regularised joist.


eps
4,034 posts
161 months

and consider putting some noggins in as well.


Skodasupercar
718 posts
69 months

Yeah that works for floor joists, but you will well overspec if it is for the support of the ceiling plasterboard only.
Remember this spec if for a ceiling below a concrete floor  there will be no people load or any other imposed load on the floor. The only load is the weight of the ceiling lining  as spec'd by the OP at 0.2kN/m2 (I assume he meant m and not mm as in his post!).


eps
4,034 posts
161 months

Skodasupercar said: Yeah that works for floor joists, but you will well overspec if it is for the support of the ceiling plasterboard only.
Remember this spec if for a ceiling below a concrete floor  there will be no people load or any other imposed load on the floor. The only load is the weight of the ceiling lining  as spec'd by the OP at 0.2kN/m2 (I assume he meant m and not mm as in his post!). and the joists themselves...


andy43
Original Poster
3,868 posts
146 months

T'will be noggin'd at every board joint. 4.1m span so 1 and a bit 2.4 metre boards staggered should do it. And 6x2 seems decent enough. 12.5mm boards  yes, it should be 0.2kn/m2 not mm2. Adding the selfweight of the timbers just confuddles things


eps
4,034 posts
161 months

I've looked at the documents and I can't see 150x50 being up to the job, I've only found that 170x50 are up to the job, from all the TRADA documentation I have in front of me, for the span in question. If we consider that it is in effect a ceiling, which I guess it is really.
The joists also have to support their own weight, i.e. if they were too skinny they would deflect (bow) a lot, too much for my liking.


Skodasupercar
718 posts
69 months

The self weight of the joists is considered in the calcs but ISN'T included in the dead load figure. The dead load figure is for the linings alone.
The reason you can't find 150x50 (well 145x47) in the span tables is two fold. 1) I suspect you have the 2nd edition span tables to BS5268, not the new timber sizer software to EC5 2) the standard span tables assume an imposed load of 0.9kN. Being as there is only a tiny void between the ceiling and the floor structure above, there will be no access or additional loading. Therefore the imposed load can be reduced.
With timber sizing you should always expect a deflection of span/250. Unless you overspec the members that is!
Now for floor joists at this span and centers you would be looking at 245x47mm for a domestic intermediate floor. So, a floor that could take the weight of furniture, many many people, the weight of the ceiling and floor linings and the weight of some timber partitions needs a 245mm deep joist, whereas a ceiling joist that is only holding 1x sheet of plasterboard and nothing else needs 150mm.
I hope that puts the sizes and the loads into perspective!

