Hello, hoping I am in the right forum. I am putting a flat roof on an extension. Span is 3.7m at 400mm centres. I'm using 47x170 c24 timbers which I believe are over what I need based on span tables I have found. However I want to add a roof lantern that is 160kg. I was just going to double up the timbers around the lantern.

 

I'm struggling to work out if it will support the load. I keep looking at the kN/m2 but I have no idea what weight it translates to.

 

Any advise appreciated. I think I'm just asking if the joists will be okay? Cheers.

 

 

 

Upping the dead load I would go to 47x195@400 centres and doubled at the sides of the lantern assuming it is running with the joists rather than across them. 

That depth will also allow you to use more insulation assuming this is a cold roof. 

21 minutes ago, PeterW said:

Upping the dead load I would go to 47x195@400 centres and doubled at the sides of the lantern assuming it is running with the joists rather than across them. 

That depth will also allow you to use more insulation assuming this is a cold roof. 

Plus one 

Exactly what we did 

Thanks both.

 

@nod is that a warm roof?

18 minutes ago, carlosdeanos said:

Thanks both.

 

@nod is that a warm roof?

It is 

 

For what it's worth, our architect - who also did the structural calcs for other parts of the project - specified 47x170 C24 joists at 400mm centres for our 3.2m extension span and tripled them up either side of a 220kg(?) lantern. I don't know if there were any calcs behind this - could well have just been experience / rule of thumb. Building Control were happy, but that doesn't mean it wasn't over-specified. 

Edited September 3, 20206 yr by MJNewton

You may be able to get away in asking Building Control to advise on the additional load implications. They may however ask for you to seek advice from a structural engineer.

@MJNewton Great so I should be okay then really as I think mine is about 160kg.