What size Joists?

[Hpd]

I have a single story GRP flat roof that is 5m x 5m. The joists are 225mm x 50mm and are spaced at 400mm centres.

I want to put a glass skylight in the centre, it is 1.7m x 1.3m and weighs approx 100kgs.

Could anyone advise on what size joists and trimmers I'll need to put in to support it properly? The 1.3m is the going across the joists so I'll need to cut out 3 joists.

Any help would be awesome

 

[Hpd]

No ideas?

 

[Ian H]

I'm no SE or builder but I'd guess double up the full joists either side and a double trimmer each end all in 225x50.

 

[pilsbury]

On my 4.5m x 6m flat roof I used 200x50 C24 joists and doubled up where the rooflights went. I too am no SE, but it seemed about right.....

 

[Hpd]

Thanks for that

 

[IWalkLikeHimaMartian]

I think the general rule is:
take 1 or 2 out, put 2 back in, i.e. double each side.
take 3 or 4 out, put 4 back in, i.e. triple each side.