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Cutting of roof truss for Stira installation - normal?

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  • 13-09-2021 4:16pm
    #1
    Necronomicon
    Registered Users Posts: 11,589 ✭✭✭✭


    I've gone sale agreed on a property, and had a conveyance survey carried out.

    On the whole, looks pretty good. The main flag raised by our engineer concerns the Stira installation. One of the truss bottom booms was cut to install the stira hatch, which our engineer was concerned about. That said, he noted that a 'substantial' transfer timber was laid across several trusses in that zone for load sharing purposes.

    In a few of the upstairs bedrooms, there are hairline cracks in the ceiling plaster. Our engineer says he can't be certain whether this is just down to plaster shrinkage, or if it is a result of the modifications made to the trusses.

    I figure this forum would have a lot of folks who have had stiras installed. Can anyone share if they had similar work done on the roof trusses as part of the installation? I'm trying to get a sense of whether this is a common thing to have to do, or is something that shouldn't be done at all.



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    #2
    Penn
    Registered Users Posts: 33,620 ✭✭✭✭Penn


    Prefabricated roof trusses are specially engineered and designed. Each part of it works in combination with the others, in a balance of compression and tension to transfer and spread the load throughout evenly.

    If you cut a part of it (such as the bottom stringer/joist) you need to replace the strength that was lost elsewhere to try maintain that balance. If that wasn't done, it could very well have contributed to the cracking noted.

    Your engineer should be able to assess and design what's required to address the issue. However it would be separate to the house survey.



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    #3
    listermint
    Registered Users Posts: 33,915 ✭✭✭✭listermint


    Yes it's common. Yes it's easily handled for.



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    #4
    mickdw
    Registered Users Posts: 23,351 ✭✭✭✭mickdw


    Never like cutting a truss as it destroys it ability to sustain any kind of loading.

    That said, it does happen regularly and can be ok if strengthening works are carried out around the area to attempt to negate the effects of destroying the truss.

    Generally the strengthening works could involve heavy duty trimmers with cut truss securely fixed to trimmer. Additional runners in that area spanning numerous trusses and fixed to each including the cut truss. Then depending on location, additional bracing / triangulation added to the cut truss to put back strength.

    2 trusses cut without major structural works is a complete no no in my book.



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