Roof Lanterns strength support

Good afternoon, The most appropriate way to ensure the lantern is properly supported would be to engage the services of a Structural Engineer, in my opinion at least. Whilst doubled-up or tripled-up joists will often suffice, and have been used for several schemes I’ve worked on, I’m also aware from experience that some engineers would not be prepared to ‘put their name’ to such a scheme if it isn’t backed up with definitive calculations, taking into account the span and loading(s) etc., i.e. if they can’t quite get it to ‘work’. Fully understandable of course – as one engineer recently put it to me, they wouldn’t want to be in a position to explain [to clients] why the glazing in their lantern has cracked further down the line – and so it's definitely worth having the scheme looked at by a suitably qualified professional. I hope that helps.

We would put a flat steel plate in between 2 joists and bolt through, but you definitely will need it on paper from a structural engineer... hope this helps

Engage a Chartered Structural Engineer (like me) to carry out the necessary structural calculations to ensure an economical and safe design which will comply with the Building regulations. Cost about £125.00

I have an extension on my house which has a lantern 6.000m x 1.500m built 3 years ago. The support for this was to double the joists which Building Control were happy with. Since installation there has been no movement detected or glazing problems. Yes you have the option of checking with a Structural Engineer but a fee will be charged. Not to forget that Building Control who which you pay a fee for are there as an advisory body and will only ask for calculations if absolutely necessary. Hope this helps Dave A&D Design.