6M x 6M Roof question..

[AronDip]

Hi all, currently pricing a single storey 6m x 6m flat roof extension under PD (detached house). We was originally going for 3 flat roof lights but have now decided on a 3m x 3m lantern. My question is are double joists sufficient as load bearing or am I going to need x2 6m steels across the span to take the load and joist in between according.

Many thanks

 

[frutbunn]

With those spans I would be requesting calculations, though that will almost certainly mean using steel.

 

[^woody^]

Doesn't your plan drawer know what to do?

 

[frutbunn]

Doesn't your plan drawer know what to do?

No doubt he doesn't do B Regs, and advised his client to submit a Building Notice, this is very common now, even by the few plan drawers who actually know what they are doing.

 

[^woody^]

Madness, let alone bad advice. I'd bet this type of person is not cheap either

 

[Notch7]

Hi all, currently pricing a single storey 6m x 6m flat roof extension under PD (detached house). We was originally going for 3 flat roof lights but have now decided on a 3m x 3m lantern. My question is are double joists sufficient as load bearing or am I going to need x2 6m steels across the span to take the load and joist in between according.

Many thanks

When I was doing orangeries, I used Glulam beams and for the lantern support beams I used 90 x 405, cut down to 90 x 250 at flat roof perimeter...it meant cutting the insulation around those bits. I know 405 seems awfully tall but it forms the lantern upstand too, however your perimeter is 1500 so maybe too much.

We used Glulam beams because it was easy to fix being timber, but for most builders steels would be easier to get hold of.

I doubt whether there is any structural joisting that would be sufficient, bear in mind you will need to allow for a snow load of prob 0.75kn per sq metre as well as the dead load.


By the way, a 3m x 3m roof lantern will make the room feel more spacious than 3 flat roof lights. The ceiling upstands around 3 flat roof lights wouldn't look very good.


Your project will need structural calcs anyway so speak to the SE.

You may well need lateral stability calculations too if you don't have sufficient masonry to act as a buttress.

 

[tony1851]

On that span, to prevent excessive deflection and yet to keep the joists to a practical depth, you'd be looking at something like 152 x 152 x 37.

 

[AronDip]

On that span, to prevent excessive deflection and yet to keep the joists to a practical depth, you'd be looking at something like 152 x 152 x 37.

Iv been told timbers required are 63 x 220 c24 @400 centers but this dont account for the skylight just a flat roof (warm deck with GRP) ..Iv spoken to someone who said 203 x 203 x 46 steels would be fine along with the above timbers?

 

[tony1851]

The 203 x 203 x 46 is unnecessarily large for that span and loading. Bear in mind also that if you want a flush ceiling you are looking at timber joists which would need to be about an inch deeper than the steel. This is to allow for shrinkage of the joists when they dry out and to ensure that you get no humps in the top finish, which can cause ponding.

With a 203 deep beam, you would need 225 joists, which are overly-large for the spans you would be left with and - as with being a warm roof, you would end up with a deep fascia

With 152 deep steels, you could use 175 x 50 C16 joists 400c/s.

 

[AronDip]

The 203 x 203 x 46 is unnecessarily large for that span and loading. Bear in mind also that if you want a flush ceiling you are looking at timber joists which would need to be about an inch deeper than the steel. This is to allow for shrinkage of the joists when they dry out and to ensure that you get no humps in the top finish, which can cause ponding.

With a 203 deep beam, you would need 225 joists, which are overly-large for the spans you would be left with and - as with being a warm roof, you would end up with a deep fascia

With 152 deep steels, you could use 175 x 50 C16 joists 400c/s.

Thanks for the advice.. whats the difference in the steel sizes 152 x 152 x 30/37/44 uc .. its the weight per metre right? So the 44 would be a thicker grade steel making it stronger? Or am I way off lol

 

[tony1851]

Yes, weight in kg/m. The internal depth of the beam is the same but the heavier ones have thicker metal.

The heavier the better; the 44kg beam is - I think - one rolled by Tata and may not be as easy to obtain as the others, but I might be wrong?