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Surely modifying the roof this way isn't legit?

Discussion in 'Roofing and Guttering' started by Roof_question, 9 Nov 2019.

  1. Roof_question

    Roof_question

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    I'm looking to raise my ceiling in the garage to make room for a lift so getting quotes for the roof taking off and put back together like an attic space.

    But one of the roofers has suggested he can get the desired result by reinforcing the existing roof and then cutting a foot from the middle? Obviously that was my first idea but searching the internet literally EVERY single person says you CANNOT do it that way, not least without getting a structural engineer in.

    1) Is it even achievable, with the guidance of a certified structural engineer, to do this?
    2) Should I even consider using this roofer as he made no mention of drawing plans/getting an engineer?

    It just doesn't seem right to me that you can cut out sections of a roof which is very delicately designed, I thought the wood going across at ceiling height was pretty integral to the roof structure, and cutting it out will upset the balance.

    Thanks!
     

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  2. bernardgreen

    bernardgreen

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    The small triangles will try to rotate with no triangulation to oppose that rotation. Only the rigidity of the rafters will prevent that rotation.

    upload_2019-11-9_18-36-58.png
     
  3. noseall

    noseall

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    Is this a joke thread?
     
  4. Roof_question

    Roof_question

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    Why on earth would it be a joke thread? I'm not here saying "I'm going to cut my roof into bits, does that sound OK?"

    I'm asking the opposite. My limited research and, to a lesser extent - common sense - says that this kind of modification is an absolute no-no.

    But now I have a highly rated local roofer who is not only offering it up, but came up with the idea off his own initiative (I didn't prompt, or even mention anything, except why I was getting quotes for a new roof).

    Hence why I'm now doubting myself, and asking out if it's worth getting an engineer to take a look, or just go with my gut and exclude any modifications to my existing roof.
     
  5. Roof_question

    Roof_question

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    Unless of course by "lift" you think I mean something you step into to move you between floors. I mean car lift. 4 post ramp type thing.
     
  6. catlad

    catlad

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    I am sure it could be modified, but it would still be expensive and it would be a compromise it terms of what
    you wanted/need.
     
  7. Notch7

    Notch7

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    Yes it is......the weight of the roof tiles is trying to push out the sides, the bottom of the truss is acting in compression.

    I think the general view is the best way is to strip off the roof, pitch a different: roof scissor truss for example. I suppose a structural ridge would give you a vaulted ceiling.......
     
  8. Roof_question

    Roof_question

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    Exactly that, I'm not an engineer (obvs) but I do read things and take note. The sides being pushed out was the most prevalent issue I read with respect to cutting bits of roof trusses out. And you might not have issues for months, even years, but eventually it could happen.

    And what Bernard said was also the first thing I visualised - even from a non-engineering POV - when he described what his intentions were, adding massive weakspots into the structure.

    I can smell bull a mile away with things I'm knowledgeable about but with roofs, I'm not, and this guy got me second guessing myself. So I'm now wondering if it's worth paying an engineer to scout "modifications" or go with my gut and just get quotes for the roof taken off/put back on with a new unimpeded structure.
     
  9. datarebal

    datarebal

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    cut without an SE at your peril.
     
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  10. ^woody^

    ^woody^

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    What that builder is trying to achieve is a raised collar roof. This can be done, and not necessarily require an engineer, but would need new timber sections and not just adapting thin truss timber - as trusses are designed to work as one unit, and timber sections selected and braced on that basis, and can't be altered.
     
  11. Notch7

    Notch7

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    Thinking about Woodys post that talks about raised coller roof makes me think it may be possible.

    Temporary support the roof -acrows or timber struts, (a bit of thought required as a truss roof is made of piddly timbers).


    substantial new rafters could be fitted, say 8 x 2 and sistered on each truss.
    It would be impossible to fit full length rafters due to cuts at bottom and insufficient space between each truss, so first rafters in half length!
    Then glue, screw and bolt another rafter, this time almost full length, leaving say 300mm from ridge. That rafter will span the butt joint of the 1st rafters that are in 2pieces

    Bolt raised collars to the new rafters -with steel plates if needed
    Add struts between rafters and collars.

    Cut out existing trusses as required for additional height.

    I think there is a goid chance it could be done without stripping off existing roof if carefully planned -but it comes down to whether it would be cheaper.
    I think it could.

    Do you have a pic of the garage roof from inside -assuming there is no ceiling in the way?
     
  12. bernardgreen

    bernardgreen

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    Add the timbers shown in green and it might work. They would be under tension and require secure fixings at both ends. A little bit of space lost but better that than a roof that is likely to collapse.

    upload_2019-11-10_10-9-21.png
     
  13. Roof_question

    Roof_question

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    Interesting, thanks for the info. So maybe I'd be wrong to criticise this roofer and shouldn't immediately discount his ideas then. I'd still feel safer having an engineer look at it first mind.

    Pic attached, hard to get a good position.
     

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  14. Notch7

    Notch7

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    Looks like a standard fink truss.

    I think its possible to do without roof removal.

    My feeling is that the top chords of the trusses wont be strong enough to support the roof if the bottom chord is removed and a raised collar fitted, even with other webbing added. Trusses are more than the sum of their parts -cut anything away and much of the strength is lost.

    So I feel if the roof is to be altered as it stands, it will need new timbers that will form a traditional cut roof and fully support the roof as thouuh the trusses didnt exist.

    New substantial rafters and raised collers could be fitted with the existing roof in position -it would take a bit of thought to design suitable parts that could be fitted with the existing in the way. The good thing is that the trusses will all be the same -so once one set of truss reinforcements is cut, all the rest would be the same.
     
  15. Alec_t

    Alec_t

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    It would probably be cheaper to forget the lift and dig an inspection pit instead :) .
     
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