Are steels required if I fit large lantern into roof

[happyhero]

Hi I have a flat roof on an extension that is 6.3m wide and 4m deep. The felt is in a bad state and so I want to get it recovered but we would now like to fit a large lantern into the roof of 4m x 2m.

Will we need steels or will timber still be ok?

Ive spoken to one guy who is a builder and he said no because there wont be a major difference in weight compared with all the timber joists and ply sheets and felt etc, but I wondered because the span on one side will be 4m for the opening, what do you think?

I understand that you double up on the timbers around the opening but is that enough?

The window cleaner will still walk on the roof to get to everything.

Any help/advice appreciated.

 

[tony1851]

Your builder is advising you incorrectly. If it is a 'large lantern' (4m x 2m is big), there will be a trimmer each side of the opening which will be carrying a considerable load. It is unlikely that the joist-section sizes you have would be suitable to form trimmers, even doubled or tripled; you could find you have excessive deflection which could lead to ponding on the roof surface.
Steel is the correct way to go.

 

[catlad]

If you have lived in the property and never had a skylight then to go to a massive 4m x 2m might sound great but in winter you might not think its so great. Just something to consider.

 

[noseall]

but in winter

It's the same for a lot of (thermally) sub-standard buildings. I'm sure many are sold to gullible customers in the warmer months.

 

[Notch7]

The glass for a lantern that size will weigh about 250kg.

And you would need to allow a snow load (from memory 75kg per square metre on a flat roof).

So existing timbers wont be ok given a 4 metre span.

You need either steel or glulam beams.

A 4m x 2m lantern would work wel, in that size room, but depends how much insulation in the floor, walls and roof. A new orangery with a lantern that size built to current regs will stay warm enough without excessive heating cost. Feedback I get from custoners is that their room stays quite warm, especially during the day as solar gain warms the floor and screed quite nicely.

If you want to do the job properly it will need building regs if open to the house, which wil, entail a SAPS test.

 

[Leofric]

The usual comments about building regulations application being required with structural engineer's input for the structural design apply I think.