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General: Has joist removal affected structural integrity?

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  1.  
    Sorry for no pic here, but the best I could do is a hand-drawn sketch and the files always come out too big.

    Typical 2-storey Victorian offshot with kitchen to GF and bathr to FF. Approx 2.4 wide x 3m long. Monopitch roof falling across the 2.4m dimension. Built with roof void and suspended ceiling.

    I did a survey a few years ago, and the client, having done some work themselves, has asked me to price for IWI and vaulted ceiling underdrawing to the bathroom. In order to get the full vaulted effect they have cut out all the ceiling joists......

    I think the first thing they need is a structural engineer. What do you think?

    I have not visited yet, and don't know whether the roof is original weight or heavier, but I think the joists were almost certainly acting as collars preventing spread.

    My gut feeling says if they *must* have 'the vaulted look', they should consider a spreader plate on each wall at the former susp ceiling height, and one or more 'decorative' (discuss!) stainless tie-bars, to make it look as if they meant it to look like that.
    • CommentAuthorEd Davies
    • CommentTimeMay 22nd 2013
     
    With a monopitch roof, where would the spreading force come from?
  3.  
    Ed, that is indeed a good question!! But Equally on a dual pitch roof, unless the rafter tops actually slid down the ridge-board, how would that, technically, spread? Nevertheless I have seen walls visibly affected at eaves level by spread (on a dual pitch roof as described). Maybe I am worrying too much? The roofer who fitted a Velux apparently did not comment, and the window-man did, as did I (albeit I have not yet seen it - only had the description). If it turns out that the ceiling joists were pocketed into the brickwork on both sides, and not spiked into the rafters at all, then I shall stop wittering, but if they were spiked in....
  4.  
    Duo-pitch roof rafters only spread if they are unsupported at the ridge. If you have a ridge beam then the rafters tend to rotate inwards, not outwards. If the inboard end of the rafters is well supported then I'd have thought the same would apply here.

    David
  5.  
    To quote myself...

    ''But Equally on a dual pitch roof, unless the rafter tops actually slid down the ridge-board, how would that, technically, spread?''

    If the ridge-board sagged.

    Maybe I am citing a trigonometrical impossibility....
  6.  
    A ridge board will not normally do anything to stop the rafters falling as it is not usually load bearing. Its job is to tie the rafters together, not transfer their load somewhere else. In the absence of a load bearing member, the load will be transferred at the wall plate.

    A ridge beam is load bearing & so shares the load with the wall plate, removing the tendency for rotation.

    I think a mono-pitch roof is more like the latter.

    David
  7.  
    Many thanks, David.

    ''A ridge beam is load bearing & so shares the load with the wall plate, removing the tendency for rotation.

    I think a mono-pitch roof is more like the latter.

    David''

    So you agree with my dawning suspicion/realisation that what I am worrying about is impossible?:shamed:
    • CommentAuthorchuckey
    • CommentTimeMay 22nd 2013
     
    In general the roof loading is equal down its slope, so there is no force making it rotate. So is all down to how the end of the joists are constructed that will determine what effects there are. If the bottom ends of the joist are cut off to terminate horizontally on the wall, then they will not cause any side thrust on the wall. If however the top ends are made off to a wall plank spiked into the face of the wall, as the roof settles the joist will rotate and push the top end outwards. If the top end is "bird mouthed" or has a wooden wedge nailed to underside so the wall/joist interface is horizontal then there is no force pushing the wall out and the situation will be stable.
    In a roof with joist that are not tied side to side, the ridge board as it cannot actually support the joists on both side of it, it bends in the middle and the joist then spread because the angle of them has changed, but their length has not. This is the problem with traditional barn roofs in Wensleydale, the roofs spread, the wall go unstable and they fall down!! ( 2 in ten years within 1/2 mile of me!).
    Frank
    •  
      CommentAuthorjoe90
    • CommentTimeMay 22nd 2013
     
    Nick, in my opinion the ceiling joists do not stop the spread of the walls on a mono pitch roof, as Ed Davies says above there is no spreading force. Simples !
    • CommentAuthorCWatters
    • CommentTimeMay 22nd 2013
     
    Nick - I think you are right to have some concerns. What Chuckey said.. I reckon it depends on how the ends of the rafters were cut and fixed.

    Just in case anyone missed the distinction... A "ridge beam" and a "ridge board" are two very different things.
  11.  
    Thanks everyone. I'm going there next week, and will then find whether it really is a monopitched roof or if, like many terraced-house offshots, it effectively 'shares' a pitched roof with next door. This of course only applies if the street is level, and not many in Sheffield are. nevertheless, if it is, we could be back to the sagging ridge-board issue, as the party wall often was not built up in the roof space.

    Nick
    • CommentAuthorCliff Pope
    • CommentTimeMay 23rd 2013
     
    If you have two brick walls of different heights and you rested rafters on top of them to produce a sloping roof, the roof would tend to slide off.
    If you attached the rafters to the higher wall there would be no sideways force on the lower wall but the sliding force would be resisted by the higher wall.
    If you attached the rafters to the lower wall instead there would be no sideways force on the upper wall but the lower one would have a sideways force.
    If you attached the rafters at each end there would be no spreading force but each wall would experience a sideways force in the same direction.
    If the rafters were fixed but the roof sagged then each wall would experience an inwards force, not a spreading force.
  13.  
    Thanks Cliff. I think I have been moved to re-learn a lot about triangles! I have always been nervous when people remove, for example, the collars from pitched roofs in attic bedrooms, and (perhaps wrongly) applied the same concerns here.
    • CommentAuthorEd Davies
    • CommentTimeMay 23rd 2013
     
    Posted By: Cliff PopeIf you attached the rafters at each end there would be no spreading force but each wall would experience a sideways force in the same direction.
    No! The net of the forces acting on the rafter/roof is zero (or the rafter is accelerating which is normally considered to be a bad thing). Gravity acts vertically downward. Apart from wind loads, the only other force on the roof is that from the tops of the walls. These must sum to the opposite of the gravitational force so any horizontal components must cancel out.
  15.  
    Dammit I wrote a long post and got timed out and lost it.

    In short - think about a ladder leaning against a wall- gravity 'wants' to push the top down and the bottom outwards until it rotates to horizontal, its lowest energy state.

    Monopitch the same. Bottom support is pushed outwards. Equal/opposite outwards force on the top support maintains horz eqbm.

    Another analogy - a dual pitch roof is just two mono pitches leaning against each other. If you take one pitch away and replace with a wall, the other doesnt 'know' you did that so exerts same force on supports as before.

    My original post explored all this using Virtual Work (something that struccies love to do all day)

    I have a monopitch leanto conservatory that works like this, got a couple of ties to resist the spread.
    • CommentAuthorEd Davies
    • CommentTimeMay 23rd 2013
     
    Doesn't it all depend on how the rafter is supported on each wall?

    If it is sitting on top of the wall with an appropriate birdsmouth¹ or wedge under it then the forces on the rafter at each end will be vertical and there will be no spreading forces. If, on the other hand, it's leaning against one wall or the other then, yes, of course there can be spreading forces. But wouldn't that be a silly design to chose unless you're not able to put the rafter on the top of the higher wall? (E.g. 'cause it's only half way up it)

    ¹ Should there be an apostrophe in "birds" and why's it called that, anyway? Doesn't look much like a bird's mouth to me.
  17.  
    Posted By: WillInAberdeenAnother analogy - a dual pitch roof is just two mono pitches leaning against each other. If you take one pitch away and replace with a wall, the other doesnt 'know' you did that so exerts same force on supports as before.

    Not quite, if you had 2 mono pitches either side of a wall with the rafters joined you would have a stable roof. If you removed the wall the rafters would either push the lower wall out or slide off depending on the lower fixing either way the roof collapses.
    IMO
    Posted By: Ed DaviesDoesn't it all depend on how the rafter is supported on each wall?


    Back to the original question, I would look at how the joists were attached to the walls e.g. if they were just put into joist hangers slotted into the wall then not much chance of acting as ties, if they were firmly fixed somehow then they could act as ties if such were needed. Best to look at the rafter support on each wall as Ed suggested
  18.  
    P-in-H wrote:

    ''Back to the original question, I would look at how the joists were attached to the walls e.g. if they were just put into joist hangers slotted into the wall then not much chance of acting as ties, if they were firmly fixed somehow then they could act as ties if such were needed. Best to look at the rafter support on each wall as Ed suggested''

    Yes, thanks. I was just collection background views first. I'm going next week, an will come back with some firm details then.

    Nick
    •  
      CommentAuthordjh
    • CommentTimeMay 24th 2013
     
    Posted By: Ed DaviesShould there be an apostrophe in "birds" and why's it called that, anyway? Doesn't look much like a bird's mouth to me.

    I've never seen an apostrophe; I guess the term's been around long enough to have lost it. If you look at the cross-sections of birdsmouth fence uprights, or the pieces used to make a birdsmouth mast, you can see the bird's mouth shape more easily.
    • CommentAuthorEd Davies
    • CommentTimeMay 24th 2013
     
    Posted By: Peter_in_HungaryBack to the original question, I would look at how the joists were attached to the walls…
    Isn't how the rafters are attached to the walls more relevant? If they're leaning against either wall then, yes, spreading could be a problem; if they're sitting on top of both walls then, no, it won't.
    • CommentAuthormike7
    • CommentTimeMay 24th 2013 edited
     
    Posted By: Ed Davies
    Posted By: Peter_in_HungaryBack to the original question, I would look at how the joists were attached to the walls…
    Isn't how the rafters are attached to the walls more relevant? If they're leaning against either wall then, yes, spreading could be a problem; if they're sitting on top of both walls then, no, it won't.


    I don't think PiH is disagreeing with you Ed, just that if the joists were doing no restraining as fitted, removing them won't cause a problem. You get my vote btw for the best grasp and statement of the principles involved.

    The only comment I'd add is that as an alternative to the horizontal bearing, the required vertical force can also be provided by securely pinning the rafter to a wall plate, provided that wall plate cannot itself sag. The underlying condition is that whatever the fixing, it must be able to provide the required vertical force without introducing any lateral force or vertical movement. (This is perhaps a more general version of a point already made by David Freeborough). Meet this condition and there will be no spreading.
    •  
      CommentAuthorjoe90
    • CommentTimeMay 24th 2013
     
    Is there no SE on here that answer this one?
  23.  
    Update: The client got a SE in. He noted 2 things:

    1. Velux was fitted w/out doubled-up rafters. He wants these fitting. There are no purlins, so my take is that they'll have to span form party wall to wall-plate. The client believes that he was talking simply of screwing a bit of timber alongside the Velux, but this would increase the load, not reduce it. I have only ever fitted 2, but we were asked to span between the 2 purlins.

    2. He suggests underdrawing the entire ceiling in 9mm OSB3. Our plan had been to insulate 50mm between (75) rafters (breathable membrane) and 100 below.

    SE sugg'd go ahead and do that, with the OSB3 between the 2 layers.

    I expressed concern that if we did that, but with OSB3 half-way thro the sandwich, there might be a risk of interstitial condensation if the VCL ever got breached. The SE apparently told the client there was no problem, 'because the OSB3 is waterproof anyway'. Exactly my point, and why I think it *might* be a potential problem.

    What does the team think?
  24.  
    OK, so an SE has had a look, what about the question in the OP - will there be a spreading risk due to the removal of the ceiling joists ???
  25.  
    From a structural point of view the OSB needs to be tight against the rafters. It can go above or below. Generally speaking, below is better from a condensation risk point of view as the OSB will be warmer.

    What type of insulation are you using? If using any kind of plastic (EPS, XPS, PIR or PUR) then the amount of water vapour transmission will be low & the OSB will not form any kind of barrier. The biggest risk will be bulk air movement caused by air leakage through the VCL, so concentrate on sealing the VCL.

    David
  26.  
    P in H: Interesting point! I am not sure of the extent to which OSB slabbing would addresss spreading issues, but then that's why I sugg'd they get a SE!

    David, we are using Celotex, and will be extremely careful with the VCL. We mastic, foam and tape every layer. Can't get too tight as far as I am concerned.

    So you don't think there is any issue with (from top):

    slates/battens
    Breather membrane
    25mm vent gap
    50mm Celotex (75 rafters)
    9mm OSB underdrawing
    100 Celotex taped as VCL
    25 x 38 Battens/service void
    Plasterboard (I would use 9mm, but accept as per another thread that this is not to everyone's taste!)

    Thanks for your comments.

    Nick
  27.  
    I would put the ventilated gap on top of the breather membrane as this allows the breather membrane to be laid flat & taped, improving windtightness. How about:

    Slates & battens
    25mm counter battens forming ventilated cavity
    Breather membrane pulled tight & taped as wind barrier
    75mm Celotex between 75mm rafters
    9mm OSB racking board
    75mm Celotex taped as air barrier & VCL
    25mm battens forming service void
    Plasterboard

    The overall thickness is the same, but the OSB should be a little warmer, you won't need such long screws to fix the service void battens & there will be 25mm more headroom assuming the rafters stay in the same place.

    David
  28.  
    David,

    Thanks for this. However we are not re-roofing - that's already been done. AFAIK there are no counter-battens, and I want to leave a 25mm air gap. I don't have a problem in principle with long fixings - I think if I use 19 x 38 battens I will be ok with 150 screws - (31mm 'bite'), but that bite will be only 22mm into the rafters. OK I guess if the OSB is really well screwed. The main thing for me is that you feel the OSB will not act as a vapour check in the event that my rather obsessive sealing-up leaks. I'll go with that, then!

    Many thanks.

    Nick

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