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Building Regulations and Standards: Joists around chimney

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    • CommentAuthorajdunlop
    • CommentTimeJan 7th 2020 edited
     
    I am soon going to be installing a demand controlled MEV system.

    We are going to make use of a small chimney breast that runs from the ground floor ceiling to the loft for extract ducts to get between floors and use the chimney stack for the exhaust. The chimney is small (about 455 x 470 mm), probably used for a kitchen stove, and is up my neighbours wall so is within the thermal envelope of the building, Any chimney breast that existed in the ground floor has been removed by the brickwork being stepped at ceiling height (think it was done at least 20 years ago).

    The ducts are rigid 125mm spiral metal ducts. Two of these will go down the chimney from the unit in the attic to the ground floor kitchen ceiling. One will then go one way along the ceiling and the other the other way.
    Diagram of this: https://www.icloud.com/iclouddrive/0ad5p_MnT8l3zc16PT58yCYsw#Bottom_of_chimney

    I had been hoping to have the ducts go between the joists in the ceiling but had forgotten that there are short support joists either side of the chimney that go between the wall and the joist in front of the chimney to take the ends of the joist running parallel to the wall.

    Plan diagram: https://www.icloud.com/iclouddrive/05WUJoYtjQrCcT3XctCmknL7w#Joists_around_chimney_2"
    Dodgy Isometric diagram: https://www.icloud.com/iclouddrive/0Th6iJQd3G9hCsCQlwQwggykw#Joists_around_chimney"

    What (if any) would be the best way to achieve this? What alternatives are there to support the joist closest to the wall that is broken by the chimney?

    Thanks.
    • CommentAuthorphiledge
    • CommentTimeJan 7th 2020
     
    How deep are the joists and trimmer(support for the end of the cut joists)?
    •  
      CommentAuthordjh
    • CommentTimeJan 7th 2020
     
    Sorry, how to see the pictures? I just see a logo, the title and some buttons about logging in.

    Posted By: ajdunlopWhat alternatives are there to support the joist closest to the wall that is broken by the chimney?

    Joist hanger?
    • CommentAuthorajdunlop
    • CommentTimeJan 7th 2020
     
    @philedge the joists and trimmers at the sides of the chimney are all around 80 x 180 mm and the joists are at around 360 - 370mm centres.

    Hi @djh there should be a 'Download a Copy' button that will download the image files without having to log into / sign up to iCloud. If not let me know and I'll try to find somewhere else to put them.
    • CommentAuthorajdunlop
    • CommentTimeJan 7th 2020
     
    @djh Are you suggesting using a joist hanger in the wall beside the joists with the gap for the chimney? Is there a special sort of hanger for this as I imagine the forces are a bit different to hanging the end of a joist?
    •  
      CommentAuthordjh
    • CommentTimeJan 7th 2020
     
    Posted By: ajdunlopHi @djh there should be a 'Download a Copy' button that will download the image files

    Ah, yes. It wasn't working for me earlier but is now; still very slow. Ta.

    @djh Are you suggesting using a joist hanger in the wall beside the joists with the gap for the chimney? Is there a special sort of hanger for this as I imagine the forces are a bit different to hanging the end of a joist?

    I was thinking of suspending a joist hanger from the chimney. If that won't work then I'd suggest some custom metalwork to substitute for the stud joist, with a hole for the duct to pass through. A local metal shop should be able to knock a pair up for a few pounds. (Unless there is indeed a special type of joist hanger)
    • CommentAuthorphiledge
    • CommentTimeJan 7th 2020
     
    Id replace the trimmers either side of the chimney with fabricated steel trimmers along the lines of 2 x 6mm steel plates the same length and depth as existing wooden trimmers . Cut 130mm holes where the duct needs to run and then join the plates with welded in 50x25 box section running all the way round the edge of the plates to give a finished thickness of 62mm. The plates will only be 25mm wide either side of the 130mm holes so the box section will run nicely passed the duct. Youll need to weld on drilled angle iron to fix the new trimmer to the supporting joist and attach the cut joist to the new trimmer. Bolt these 2 attachment points to the respective joists.

    You might need to drill and tap fixings for floor boards or if theres enough room bolt a batten to the trimmer between the trimmer and the chimney.
    • CommentAuthorajdunlop
    • CommentTimeJan 8th 2020
     
    Thanks for the replies.

    I like the idea of hanging off the chimney for simplicity except that there would be a large gap between the end of the joist to be supported and the chimney. Also as the chimney itself is not being supported I don't know if adding more weight would be a good idea.

    Given you are both talking about fabricated metal replacements for the trimmers I think we are probably going beyond what I would be happy doing as a DIYer so I might have to think about solutions that don't interfere with the joists (boxing) or get a professional in to do the job. But thanks to your replies I have an idea of what sort of approach to expect someone I pay to do it.
    •  
      CommentAuthordjh
    • CommentTimeJan 8th 2020
     
    Posted By: ajdunlopGiven you are both talking about fabricated metal replacements for the trimmers I think we are probably going beyond what I would be happy doing as a DIYer

    You'll want to get a metalworking shop to make the pieces. It may be they can supply the expertise to measure and specify them too; otherwise you'll probably need to find a structural engineer.
    • CommentAuthorgyrogear
    • CommentTimeJan 9th 2020 edited
     
    I would not hang off the chimney, I'd use a joist hanger on the back wall.
    (If no wall plate, maybe easiest to use a strongback on the floor-above).

    For the hanger, you can fabricate it yourself from either threaded bar, nuts, washers and a metal crosspiece, or from bashing flat a length or two of drywall stud track, with appropriate holes to take suitable nails.
    (I have used both of the above to effect).

    Then remove the timmers.

    Once you've got the ducts in, fabricate a liner around them (such as hollow clay pantiles..) then I'd fill in the floor void with lightweight keyed-in (reinforced) concrete, period.

    IANASE (I am not a structural engineer).

    gg

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