Question about Floor Joists

[treecott]

Hi, Wonder if anyone can offer any advice.

Doing a loft conversion, and have all the structural calcs. We have a span of 4.3m and the structural eng. has specified 50 x 200. The floor is down, and was put in by a builder. The floor was to span a single storey extension with a flat roof, the floor was to be put above the ground floor ceiling. Unfortunately the builder took the roofing felt off to build up the exterior brickwork, and the heavens opened, bringing down the ground floor ceiling. To cut a long story sort, we decided to remove the roof off the extension, and the builder took the decision to use the loft timbers of 50x200 as the upstairs floor, and downstairs ceiling without checking with structural eng. The Building Inspector has passed this and work commennced. The Building Inspectors change on a regular basis, and we do not get continuity hence not sure if it is ok.

The questions I have are:-
Are the joists adequate to deal with this span, bearing in mind it is the floor and ground floor ceiling? They are on joist hangers on one side, and built into the outside wall on the otherside, with noggins down the centre, and on either side of centre to brace the floor. Builder says it is fine.
and
To make matters worse, when we have measured the joists exactly, they are actually 47 x195. It appears he has ordered 50x200, but this is what has been supplied, and he has used them.

Floors are down, and the ceilings have been plastered downstairs, but the floor is "slightly springy". Could anyone advise me what to do please as I am worried that the floor will not be strong enough to cope.

Any advice would be appreciated.

The structural eng. has retired, and we cannot get hold of him anymore, which is why I am asking the questions, I suppose I could employ another structural engineer, but if in the meantime, any advice could be given on this site would be gratefully received.

 

[treecott]

Sorry forgot to say, the floor joists are placed at 400centres. and the timber is C16 structural grade.

Thanks for any advice

 

[Static]

For that kinda span/load you should be using either 50x220 C16s at 400 or 50x200 C24s.. or as you say your going to get an amount of bounce in your stride.. this will result in the ceiling finishes cracking over time..
Make sure you dont put any stud partitions on the floor unless they have doubled up joists under.. also ensure the boarding is well screwed into the joists and perhaps a few extra noggins.. but all that wont fix the problem perminantly..

47x195 is the sawn cut joists.. the 50x195 would have been a plained.. not alot of difference most engineers will spec the plained one but design the weaker sawn so either can be used..

 

[JoeMar]

hi

on a recent extension project our structural guy specified 63x225 C24 @ 350 centres for a 4.5m span.

47x195 C16 on a 4.3m span is definitely going to be springy. as suggested by Static, you need to double up the joists and insert a few more noggins.

Good luck.

 

[geraint]

hi

on a recent extension project our structural guy specified 63x225 C24 @ 350 centres for a 4.5m span.

47x195 C16 on a 4.3m span is definitely going to be springy. as suggested by Static, you need to double up the joists and insert a few more noggins.

Good luck.

why double up the floor

 

[jeds]

Your in a tricky one. The engineer could argue that the joists were ok for the designed dead load. They're on the limit and personally I would have at least gone to grade C24 or upped the spec a size either way but they do technically comply. So I don't see any point pursuing the engineer.

The problem really arises because the design has changed and you now have ceiling load as well as just floor load. That means 50x195 C16 is not good enough. It won't fall down or anything but it will always be a bit springy.

Really the builder is at fault so if you can't put up with the spring then my advice is to ask him what he proposes to do about it.

 

[RonnyRaygun]

hi

on a recent extension project our structural guy specified 63x225 C24 @ 350 centres for a 4.5m span.

Sounds overspec'd unless also carrying possibly a dormer wall and some roof load?

The problem really arises because the design has changed and you now have ceiling load as well as just floor load. That means 50x195 C16 is not good enough. It won't fall down or anything but it will always be a bit springy.

Really the builder is at fault so if you can't put up with the spring then my advice is to ask him what he proposes to do about it.

Yeah, they would have been OK, just, without the added ceiling load.
You could go after the builder, but whatever the solution it's going to be disruptive. I would think the easiest solution would be doubling up the joists.

Either that or put up with the spring.

 

[JoeMar]

hi

on a recent extension project our structural guy specified 63x225 C24 @ 350 centres for a 4.5m span.

Sounds overspec'd unless also carrying possibly a dormer wall and some roof load?

definitely over spec'd, no doubt about that...better to over spec than under.

 

[treecott]

Hi, and thanks to everyone for the replies,

I guess I do have a problem, but from one you say, its not going to fall down, hopefully!!!!.

Could I ask another question. Would it be a solution to remove the ceiling and put in new ceiling joists below the floor joists, to create an independent ceiling, not attached to the floor joists. It would lower the ceiling quite a bit, but if it would be a solution, it is something I would look at.

With regard to doubling up the joists, this will pose problems, even if I take the ceiling down, as there are noggins inbetween the joists in three rows, and also speedfit pipe has been put through the joists for the ch rad. It will therefore be a mega job to get that done. If I took the ceiling down to make it an independent one, then I could add more noggins from underneath, but would that be necessary if I take the load of the ceiling off the floor joists.

I really appreciate the advice given, as I am trying to do the job right, and have been let down by a "cowboy builder", and now I am left with the problem of sorting it out. Any further advice would be much appreciated.
Thanks.

 

[Static]

Simplest solution would be to put a beam about mid span supporting the floor.. course this may not even be possible but is a solution..

Ceiling joists may well be similar size to original joists so effectively doubling up the joists so back to square one..

Only other real contender is to take up the whole lot and replace with deeper or higher strength timbers..

Or simply live with it, timbers are within stress limits so wont fail but will deflect if you have a party in the room or if the room is a bathroom etc.. if the room is a bedroom.. just remember when you and 4 young ladies all need someplace to sleep to use the other bedroom..

 

[treecott]

Thanks Static,

A beam mid span is not feasible. The room is a lounge, and it would look awful. The room above will be a bedroom, (at present just plastered out with floor down). As long as it will not fall down, just be a bit springy, I think in all probability I will have to live with it as it is. As it is a bedroom, then there will be minimum furniture in, just bed, cabinets, & perhaps a fitted robe. Could I just ask if I live with it as it is, what should I expect to happen in the future with regard to the springy floor?

Thanks for the help.

 

[theoldun]

Been reading this post with interest, and will give you our thoughts.
Nearly all timber these days is machined to 47mm width and unless you specifically stress that it must be 50mm, you will receive 47mm.
As with 200mm depth you will receive 195mm due to the timber being regularised.
We stopped using solid timber joists eight years ago due to the trouble we were having with springy floors, squeaky floors, distortion and shrinkage.
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.
This table gives for a 4.3m span at 400 centres
47x195 = 4.04 span
47x220 = 4.55 span.
Plasterboard and skim to the best of our knowledge weighs in at approx 12to14kgm2 or say 0.14KN/m2, therefore in our opinion it would appear your S.E has slightly undersized your joists. as 50x195 only give a span of 4.13m.
Would suggest as Static has said slip a centre beam underneath. You say it would look awful. Ask your self how many times a week do I look at the ceiling. If you think it looks awful spring out a half arch each end.
If you are determined to spend money, you could consider pulling down ceiling and laminating a 47x50 to the bottom of each joist. Glue and screw every 225mm. Cut your bridging out and laminate two number rips each side of joist of 125x12mm ply with staggered vertical joints each side, and the ply to the bottom of laminated 47x50 increasing the strength of the timber you have laminated on. Re-cut and fix your bridging to suit. We did this on a springy floor number of years ago, and it worked. By the way fitting extra noggins will not help.
One other thing, if you used chipboard for decking, unless you glued and screwed it well, in six months time when joists have shrunk out a bit, you may have a very squeaky floor due to the springing.
oldun

 

[jeds]

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.

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.

 

[Static]

Could I just ask if I live with it as it is, what should I expect to happen in the future with regard to the springy floor?

Can expect upto 15mm deflection in the joists at mid span with domestic loadings.. potentially some warping of the timber over time so the floor sinks a little in the middle.. and cracks where the ceiling plasterboard meets in the lounge that never go away no matter what you try..

Could try a laminant layer on the underside like theoldun says may help some but if your doing that might as well replace the joists.. extra noggins will at least ensure the floor acts as one rather than individual members (which could actually fail) but wont cure the deflection problem..

 

[treecott]

Thank you for the replies. I will try to explain a bit more. With regard to static and theoldun's suggestion of putting a beam mid span, The lounge and first floor bedroom is 4.3m x 25ft and the joists run across the shortest ie 4.3m so putting a beam midway over 25ft or so will a) look awful, and b) cost a fortune.

The SE calculated 200 x 50 to span the 4.3m, but as there was a flat roofed extension underneath, all that was to do was brick up the gable end, and raise the edges of the extension to the same height as the existing bungalow. When the roof edging was taken off to brick up, that was when the heavens opened and it brought the ceiling down. On that flat roof extension, the joists were 7 x 2 and from what I can deduct from the SE's plans was that those joists should stay in situ and be used for the lounge ceiling with the 200 x 50 for just the floor joists to the first floor bedroom, and nothing else. As said the builder decided to use the 200x50 joists to put the plasterboard on as well, without checking with the SE. As you can imagine, we were in no fit state to sort this out with the builder as we were moving furniture everywhere in the house, following the flood and the ceiling coming down, - perhaps we should have taken more notice of what he was doing - our fault.

The reason I asked if I could take the existing ceiling down, and put in new ceiling joists to lower the ceiling in the lounge was purely as this is what the SE calculated for, and I figured that if I take the weight off the 200 x 50 floor joists upstairs, it would assist with the springy floor.

I hear what you say about laminating onto the bottom of the joists theoldun, but as static says, by doing that, I may as well rip the whole lot out and start again, which if possible I would like to avoid, hence the question about what the future holds if I do nothing. In trying to sort this matter out, my thoughts were to lower the ceiling in the lounge by removing the ceiling off the floor joists & putting new ceiling joists beneath the floor joists, and making the ceiling independent, but therein leads to another question, if I put extra ceiling joists either on joist hangers, or into the walls, will it cause problems with the loadings on the walls? This is now starting to go beyond my expertise somewhat, but I am worried that if I dont do something soon, I will be storing up problems for the future.

Thanks again for the replies.