# Span tables for deep joists

Discussion in 'Building Regulations and Planning Permission' started by garyo, 17 Jan 2019.

1. ### garyo

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Hi Chaps

Does anyone have an online source for span tables for domestic floor joists deeper than 220mm? All of the ones I have stop at 47x220 and 63x220 etc.

I'm looking for one that will give me spans for ~250mm or other commonly available sizes between 220 and 280mm so that I can assess my options for spanning 4750 and 5600mm without using engineered wooden joists or steels.

Thanks

2. ### Nakajo

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I think a domestic floor joist deeper than 220mm is a contradiction in terms, which is why you cannot find the span table.

3. ### Notch7

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I did a job with 75 x 300 floor joists.

I rang the SE and he told me there and then. Dont know how he knew, I didnt hear any dice being rolled though......

4. ### garyo

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Sound promising. My room is only just over the limit for a 47x220 and I don't want to have to run a steel down the middle to break it in to two. I'll draw a diagram in a sec and see what you all think

5. ### rsgaz

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The tables appear to have a fairly linear relationship if you look at the right hand column I just knocked up in Excel. Therefore as a guide(I'm not a SE, you still need to check with one!), you can extrapolate the figures...

So, to span your 5600mm, you take 5.6m multiplied by the ratio, let's go with 50 to be on the safe side, equals 280mm exactly.

edit: these figures are for C24, 0.25 to 0.5kN/m² dead load, 400mm centres BTW.

Last edited: 17 Jan 2019
6. ### Notch7

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From memory it was a span of 5.6m ground floor extension. I seem to recall actual spec was 300 x 63 but my merchant didnt have the stock.

I wouldve thought your most likely size available would be 300mm but expect to try a proper timber merchant rather than a builders merchant.

7. ### garyo

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Thanks chaps. This is the actual situation in the diagram below. As you can see most of the run is 4.7M but there's a significant area at the top that's 5.5M. I've drawn it up with some doubled up trimmers to reduce the span but I'll have to let the SE give me a verdict on whether that would be acceptable.

At this stage I'm trying to work out what's feasible without reducing the head height in the garage below too much (which will also require a thick layer of insulation...)

Edit: Looking at your ratios rz, 250mm is probably more than enough for the 4700 section. Maybe I can close up the centres to 300mm for the 5.45 section or find a way of using trimmers to shorten the span.

Last edited: 17 Jan 2019
8. ### Leofric

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Floor joists are usually at 400mm centres ,reducing the spacing reduces the loading on the joists of course , but if you are already using a structural engineer why don't you let them come up with a solution ? Have you thought of 'I' section timber joists or those wood and metal joists ( forget what they are called just now )

9. ### DIYhard

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Maybe an old-timer accustomed to doing the "half span plus two" calculations in his head?

• Like x 1
10. ### tony1851

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Why do you not wish to use steels if headroom is important?
You could span a couple of inverted T-beams across the 4745 widths, and probably get your joists down to 150 deep.

11. ### garyo

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I'm planning on 400c/c. The drawing shows 600 because I was experimenting with putting two 203mm section UB's down the middle to half the span, but then realised that that couldn't work due to the position of the new en-suite above, drainage etc. At this stage I'm designing the extension so that I know if can all work before getting an SE involved. For reasons such as the one above, to get an SE to come up with a solution to the specific problem would probably mean getting him to design the entire extension, which I'd rather not do.

I did look at the I section JJ beams, but they didn't look like they'd weather well, and as I intend self build I expect they'll be exposed for a good 6 months.

12. ### garyo

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Hi Tony,

I'd talked myself out of steels when I considered that I'd probably need 200mm of rockwool insulation between the joists, so I figure I need to be looking at a minimum depth of 200mm anyway. In my mind, splitting the span with a UB means doubling the number of joist hangers, timber bolted into the webs, and another 1000 square twist nails, so it wasn't worth it. You suggestion above seems quite different though.

The current house ceiling height is 240 based with 170mm joists. There's already a 150mm step down in to the garage, so I have a modest amount of headroom to play with before the garage starts to feel too weird.

Edited to say - I'm currently negotiating upstairs layouts with the wife, and I can't help but think about which way the joists are running and whether there are steels blocking services!

Cheers

Gary

13. ### garyo

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If I understand you correctly then Tony, the inverted T's would run across the 4745 and then I could run much shorter joists North-South on the drawing, and probably select a depth that's convenient for the insulation - say 200mm?

14. ### Notch7

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All timber construction would probably be the cheapest, once you include the labour cost. If the joist depth works for you then you could use thicker joists or double up 47mm thick joists for the longer spans.

Steels can have holes drilled in them for services, although I think that needs confirming with the SE as position and size is limited. But then so it is with timber joists -you cant drill for a big waste pipe at half way!

15. ### garyo

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The position of the toilets means that the soil pipes aren't a problem, so as long as I can make the odd 40mm hole for central heating, electrics, and hot/cold water supplies then I think that can work. Running a couple of steels across the short span would actually make that easier because on the current upstairs layout that will mean that the holes would all be made in the wooden joists.

I'll revise my drawing and see what the spans look like. Thank again everyone.

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