Question about Floor Joists

A little bit of extra dead load on the walls shouldnt be a problem from extra joists..

You have a couple of options, either add extra joists between the existing or reduce the existing joist centres maximum of 350mm centres.. 300mm works best as you can leave in every 3rd joist in and move the other two appart to drop a new one between them.. therefore you will only need to cut new holes for services every 1.2m or so.. still a pain..

Beam and mid span on a long thin room will not work so dont worry about that option any more..

Thanks for the reply Static, and everyone elso who have chipped in regarding my problem.

I need to digest what everyone has said, as whatever I do is going to cause mess and disruption, and whilst I would like it right, I need to work out which way is the best way forward for my circumstances. At present, the floor is down upstairs, and the stud partition walls for the dwarf walls have been built, and plastered throughout. The room downstairs is the lounge, and at present the ceiling, although some screws have popped their plaster when the floor was put down, is holding up ok.

The floor has been glued, screwed and nailgunned down, so taking it up will be a "nightmare", and then there are the ch pipes which have been drilled central in the existing floor joists from back to front, through each joist, so again even putting joists inbetween will cause me problems.

I must admit, I am leaning towards Theoldun's idea, of laminating extra to the bottom of the joists, with ply either side of the joists, that appears to be the least disruptive for the floor above, and dwarf walls which have been plastered already. It would appear that most of the work can be done from ground floor, which once the ceiling is down, will not affect the ch pipework or the electrics.

Perhaps theoldun could let me contact him privately to go through what would be involved, so that I understand fully what I am doing, if that is possible. Please let me know how to contact you, if you are willing.

I would like to thank everyone who has offered advice on this matter, as quite honestly I thought that I would not be able to solve this one, but it goes to show there are quite a lot of wise people out there!!.

Thanks again to everyone.

treecott said:

Perhaps theoldun could let me contact him privately to go through what would be involved, so that I understand fully what I am doing, if that is possible. Please let me know how to contact you, if you are willing.

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Be only to pleased to help you out. Send me Private message, and in your profile edit your message box to allow messages from all registered users.
oldun

1. The structure will not fall down - it's quite strong enough.

2. BUT... the deflection under the "normally accepted" design load is likely to lead to greater deflection than is normally considered "acceptable". These are all variables, and your case may differ.

3. This means that the ceiling will crack along the middle of the span. (it usually does - check out other people's houses !) Does this matter ? The crack will open and close as the loading changes - small change, small movement. You probably wouldn't notice this, unless you try to seal the crack with a rigid filler - you could use silicone, say, which may not look too bad. The floor may squeak - though if it's glued and screwed this is less likely.

4. Perhaps the MOST important point is - what does the Building Control person say ? This is VERY important if you ever want to sell the house ! Your buyer and his surveyor will want to be satisfied.

jeds said:

theoldun said:

We are not S.Es but to the best of our knowledge the dead load for a timber first floor and ceiling is approx 43kgm2 or say 42KN/m2, which means that you are in the 0.25 to 0.50KN/m2 joist sizing table.

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Except this floor was not designed to have a ceiling so it would have just crept in under 25kN. The builder altered the design to add the ceiling which tips it over 25kN.

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if any floor was designed, and it cannot take the weight of a ceiling. i can only suggest that your SE is not one...

geraint said:

if any floor was designed, and it cannot take the weight of a ceiling. i can only suggest that your SE is not one...

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All floors are designed to have a ceiling. Except the ones designed not to have a ceiling - like this one.

Thanks everyone for the replies. Sorry for the late response, but have been unable to get onto the site - problems accessing the post.

I have read with interest what everyone has said, and as we need the Building Control Certificate at the end of the work, have decided that the best course of action is to go back to the Structural Engineer, and ask for his help, and advice as to what to do, and what will be acceptable to B/C.

As RobinClay has said, it will not fall down, and yes we could live with it, with minimal furniture in the bedroom, but that is not solving the problem long term. And if we do sell the property, the we are passing on our problem to others, which is not fair.

I have today sent all the details to the SE and asked his advice, and when I get some info back, I will post the info on this forum, which may be of help to others on the site. In the meantime, thanks for all the advice, I am really touched that so many of you would want to help with this mess.

All I would say is that the builder could possibly be a good candidate for "Cowboy Builders", and if he does appear, I will let you all know about it, so that he cannot do it to anyone else. I would love to post his name, but feel the Moderator would not allow this.

Thanks again to everyone, and stay tuned for the next instalment, when the SE gets back to me with what to do.

jeds said:

geraint said:

if any floor was designed, and it cannot take the weight of a ceiling. i can only suggest that your SE is not one...

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All floors are designed to have a ceiling. Except the ones designed not to have a ceiling - like this one.

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so what extra load would it involve....and please pray tell me that it would fail...

geraint said:

jeds said:

geraint said:

if any floor was designed, and it cannot take the weight of a ceiling. i can only suggest that your SE is not one...

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All floors are designed to have a ceiling. Except the ones designed not to have a ceiling - like this one.

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so what extra load would it involve....and please pray tell me that it would fail...

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Extra load would be the plasterboard and skim. You're right though, it almost certainly wouldn't fail just by adding in the ceiling load, but that has something to do with the factors of safety taking into account the fact that *some* people don't bother to read or follow the drawings that they have been given. Which is what happened in this case.
The engineer designed the floor without any ceiling load as the existing joists were carrying it. After the builder removed these joists the new ones then had to carry load they weren't designed for...resulting in a springy floor...

Feel like we're repeating ourselves here...

Ronny, pleased to see your last post on this subject, as we would like to ask you or any other kind person if they could provide answers to the following questions.

We have always been led to believe that a first floor dead load for a 50x195 joist size at 400mm centres with 18mm chipboard deck and 12.5mm plasterboard and skim ceiling complete with ironmongery and services is between 0.45 and 0.48KN/m2. Correct or wrong?

A 12.5 plaster board and skim ceiling dead load is approx 0.10KG/m2. Correct or wrong?

Many thanks .
oldun

BS648 gives:

50x195 joists @ 400c/s = 0.117kN/m²
19.1mm MD chipboard = 0.098kN/m²

Total = 0.215kN/m²

It gives 12.5mm standard plasterboard @ 0.087kN/m².

Soundcheck is a bit heavier and the minimum to comply with building regulations in a normal floor would be 0.010kN/m² but this wasn't a normal floor and was not designed to have a ceiling so those figures aren't really relevant to this situation.

These are the loads that we use for a standard timber floor:

Timber Floor
Boards 0.15
Joists 0.15
Ceiling 0.20
Live (Residential) 1.50
Total Dead 0.50
Total Imposed 1.50

Was going to look in BS648 but Jeds has done it for me
When I first started in this job I did comment that going by the weights of materials given in BS 648 the loadings could be reduced significantly...before I realised that taking things down to the bare bones is not what we do

If it's a loft floor and the architect is struggling for head height then I might take off the ceiling load, and possibly reduce the joist size by 25mm or so.

Deflection should never exceed 0.003 x span for dead and live loads...although it's unlikely a loft conversion floor will ever see anything like that loading.

A lack of noggins would prevent load-sharing which, according to BS5268, can increase deflections by 50% (for C16). This could account for some of the springy-ness...

John, Ronny,
Thank you so very much for taking the time to answer our questions. You have no idea how much I appreciate your kindness.
Unfortunately, whilst I do not dispute anything that either of you two gentlemen have told me, I wonder whether you could possibly help me out again, as there are a couple of items that I do not fully understand.
BS648. I understand the joists. Have based them on Scandinavian spruce at 460kg/m3 = 0.115kN/m2, which is plenty near enough for me.
Understand the 12,5 plasterboard. Only trouble is that at 8kg/m2, I make this 0.078kN/m2, where as you list this as 0.087kN. Could this be a typing error?
It was the chipboard that I did not understand John, bur after a lot of thought realised where I was going wrong. To the best of my knowledge MD chipboard weighs in at 625kg/m3, therefore 1metre square of 18mm = 0.111kN/m2. Slightly different to yours.. Perhaps I have not got the cube weight right?
Ronny,
Not doubting you for one moment, and note your comments with regard (to the bare bones is not what you do), but even so, your loadings are very high compared to BS648. As stated previously we have always worked on an average of 0.42kN/m2.. In your opinion, even though we now know BS 648 should we reduce our figure or stick as we are.
Trust you now wear the statutory SE dress of the day Ronny. Funny hat with a feather, belt and braces, brown underpants, plusfours , brown socks and smoking clay pipe.
Regards. oldun

theoldun said:

Understand the 12,5 plasterboard. Only trouble is that at 8kg/m2, I make this 0.078kN/m2, where as you list this as 0.087kN. Could this be a typing error?

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oldun, don't know about BG but Lafarge list 12.5 std as 8.0 to 8.7kg/m². Maybe John and Ronny are using the upper figure?? Just a thought, god knows why there would be a range??

theoldun said:

Ronny,
Not doubting you for one moment, and note your comments with regard (to the bare bones is not what you do), but even so, your loadings are very high compared to BS648. As stated previously we have always worked on an average of 0.42kN/m2.. In your opinion, even though we now know BS 648 should we reduce our figure or stick as we are.

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I'd keep it as it is. Even with the loadings that I usually take, imposed load accounts for 3/4 of the total, so reducing the dead isn't going to have much of an impact.

theoldun said:

Trust you now wear the statutory SE dress of the day Ronny. Funny hat with a feather, belt and braces, brown underpants, plusfours , brown socks and smoking clay pipe.
Regards. oldun

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Haha, yeah, know what you mean, don't think I sit too comfortably in the traditional SE uniform though...
*Sometimes* the "belt and braces" approach does annoy me, but then I'm not the one paying the P.I. insurance...and it does have its benefits...on a job recently we supplied building control with calcs for a loft conversion specifying a 203x203x46 UC. Somehow a 203x102 UB (with less than half the capacity) was shown on the drawing, and subsequently installed by the builder.
When the building inspector picked up the error, we managed to manipulate the figures etc slightly to verify that the installed beam would be satisfactory, and wiped the sweat from our brows