Weight capacity of wooden joists

Discussion in 'Builders' Talk' started by breanting, Aug 24, 2020.

  1. breanting

    breanting New Member

    I’m looking into buying a recording booth to put into my home studio in a first floor flat. The booths I’m looking at weigh from 611kg up to 886kg (138x175cm) and I’d like to know if my floor can take that kind of weight.

    The wooden floor joists are 370cm long, 5cm thick, 15.5cm wide and 40cm apart, meaning the booth will be sitting on roughly 4 joists, and will be situated in the corner where the joists meet a supporting wall.

    I have a heavy-ish large table, a sofa bed and a bunch of recording equipment in the room, and will have usually no more than three people in the room at a time.

    Is there any danger at all of the floor not being able to hold all of this weight? Anyone who has experience with this kind of thing would be much appreciated.

    I’ve attached a floor plan below.

    [​IMG]
     
  2. Bobby Dazzler

    Bobby Dazzler Active Member

    First of all, I'm a DIYer, no training whatsoever in structural engineering.
    But I'd suggest your joists are a fraction under spec'd anyway for the lightest dead load weight rating.
    From the timber tables, your joists ought to be 170mm X 50mm in C16 timber even at the lowest dead load rating. (which I believe is something like 1 X 25k bag of cement per square metre, the second column, 0.5 is the same as 2 bags of cement per square metre.)
    upload_2020-8-24_12-10-13.png

    In C24 timber the situation is no better.
    upload_2020-8-24_12-11-58.png
     
    breanting likes this.
  3. breanting

    breanting New Member

    Thanks very much for the reply. I'm trying to get my head round the numbers in the tables such as kN/m2 and how it translates to kg, as I'm around 80kg and can stand fine in a space less than 1 square metre, and I'd presume that wouldn't do any harm. But it does seem like the booth would be a little too heavy unfortunately going by what you've posted.
     
  4. Bobby Dazzler

    Bobby Dazzler Active Member

    I'll repeat what I said before, I'm only a DIYer, but my understanding is:
    If you assume that the total weight in the room, with booth, furniture and people, is about 1000 kilos, (although people are not dead loads) then you're interested in the third column of figures, i.e. more than 0.5 less that 1.25, with joist spacing at 400mm.
    Your current joists are maybe OK for a span of about 3 metres, but you have 3.7 metres to span. Therefore, for your loading, span and joist spacing, in C16 timber, you'll need something like 50 X 220mm joists or 44 X 220mm.
     
    breanting likes this.
  5. stevie22

    stevie22 Screwfix Select

    OP your existing floor is fine as it is so don't be panicked.

    In this context "dead" loads are immoveable things like partitions while "live" loads don't necessarily breathe: they are simply not fixed. It is actually quite unusual to use anything other than the first column in the charts. Also the spans are determined by deflection not strength so joists can be substantially overspanned without them breaking.

    To explain the units: a kN (Kilonewton) is a unit of force (which is what we use for structural calculations). It is approximately equivalent to 100kg. This means you can clone yourself and then both of you can occupy 1 sqm.

    Now to add a large lump of extra weight is a potential problem as you say it's a flat so your floor is someone else's ceiling so you need to get a Structural Engineer to look at it. I don't think it's going to be safe to do what you want to do without some strengthening and you will probably need permission from the freeholder to that,
     
    breanting likes this.
  6. breanting

    breanting New Member

    Thanks for the explanations guys, definitely appreciated. Going by what both of you are saying, it seems as though my joists are basically fine for general normal use - just - but if I'm to put this booth on them it'll probably be a little more than they are expected to handle.

    You guys are probably right, I might have to get a structural engineer in to check my specific situation. If that happens I'll post any results here in case it's useful for anyone.
     
  7. Greentram

    Greentram Member

    To add to the previous, in your favour the booth is sitting over the ends of the joist spans, not the centre. Assuming it's just you + booth in the room (don't invite six mates to come and watch), you've got about 1000kg/10kN across a floor area of 2 x 3.7 = 1.35kN/m2, a little less than the assumed imposed load in the table above, so should work fine. But if there's a flat below, to cover yourself you should get an engineer's opinion, and if he's being prudent he'll take the weight of the booth and 1.5kN/m2 on the remaining floor area, which may well mean that some strengthening is required.
     
    breanting and stevie22 like this.
  8. breanting

    breanting New Member

    Thanks a lot Greentram. I usually have one and no more than two clients in recording at a time, so 3 people max. in the room at a time. I'm definitely getting mixed messages all over the place, so hopefully a structural engineer can draw a line under it. I've got a quote from one company I've contacted, £350 for just a yes or no answer which seems a little excessive, plus I'd have to remove some of the flooring for them to see the joists (I have access to my loft where the joists are exactly the same and are exposed), but I've got a few more companies that seem like they'll help for a little less.
     
  9. Severntrent

    Severntrent Screwfix Select

    Notwithstanding the TRADA tables a C24 50 x 155 joist just complies with deflection limits and utilises about 75% of the available bending moment strength in the joist,C16 fails on both criteria, not by much, but fail. Assuming your sound box is placed on the other side of the room right up to the back wall and is supported on 2 steel beams so the the load is spread across all 8 floor joists the extra bending moment induced in the floor joists would mean that their available bending moment strength and deflection limits are exceeded
     
    breanting likes this.
  10. breanting

    breanting New Member

    Thanks Severntrent. What I took from your post there is that even if I put 2 supporting steel beams to spread the load, the weight limit would still be exceeded. Unless I'm missing something.
     
  11. Severntrent

    Severntrent Screwfix Select

    Correct but this is assuming standard live loads are applied to your flooring. If you put a system of access/use into place where only a certain number of people were allowed in certain areas at the same time and any furniture was specifically allowed and calculated for, a SE could possibly prove that deflection/strength could be adequate but of course useage of the room would have to be adequately policed, a bit like shops and Covid?
     
  12. breanting

    breanting New Member

    Hahaha, there's plenty of that happing just now. Thanks for the clarification, thinking about it in those terms makes sense, I'd have to make sure the room is being used correctly.
     
  13. breanting

    breanting New Member

    Just thought I'd post my findings here. I've consulted a structural engineer and his analysis of the existing joists in accordance with the requirements of the timber design code (BS 5268-2:2002) concluded that they have insufficient capacity to support the resultant loads from even a small 434kg booth. He said that "unfortunately we require to satisfy the loading and timber design code requirements". So I guess I'll have to wait until I move and find a place with stronger floors (it's not worth it to reinforce my current floor if I'm moving in a few years). Thanks a lot to everyone that helped out in this thread, it is much appreciated!
     
    Bobby Dazzler likes this.
  14. stevie22

    stevie22 Screwfix Select

    Thanks for the heads up: good to know the outcome.
     

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