Single storey extension flat roof height

[Brickymatt]

Hi I am planning on a single storey rear extension filling in the L shape quite straight forward with a flat roof but the problem is I have a low window for the stairwell in the corner which restricts how I build the warm flat roof I planned , can I hang the roof joists lower to keep the deck down giving me the space for the roof and pitch ? I will post a section up f what I mean soon

 

[pilsbury]

If you hang your roof joists lower, you'll lower your ceiling height - no? A possible option is to go for a cold roof instead of a warm roof where you insulate between the joists. That way it's not as tall as a warm roof and you loose no ceiling height. But I'm told they're not as good as a warm roof.....

Or reduce the size of the stairwell window?

 

[RichA]

Or have a cut out/flat section infront of the stairwell window.

 

[Brickymatt]

Thanks for the reply I am not to concerned with the lower ceiling height I wanted to avoid adjusting the window if possible,in regards to a cold roof I did consider it but i thought a warm roof would be better

 

[Brickymatt]

Basically I have 525mm from existing lintel to the window , so when I build up and have new doors I think my cavity lintel will be about 225mm high leaving me 300mm and 150mm will be taken by joist height leaving only 150 for insulation and board/pitch, that's why I thought having the Lintel with 1 course of blocks above then hanging the joists down but this also concerns me as the joist won't sit past the outer skin giving a issue forming the fascia

 

[^woody^]

You can't have a cold roof, because you can't ventilate it easily.

Without knowing any spans, is hard to determine joist depth and say how much you have to play with.

You don't have to worry about taking joists through to the outside, as that can be designed out

What distance do you have from the door heads to the ceiling?

 

[Brickymatt]

From door head there is currently about 225mm to ceiling the depth of the new extension will be no more than 3250mm but I may bring it in to 3000mm if it keeps the joist size down

 

[noseall]

Take the window out and put a less deep one back in.

 

[tomfe]

So you have 2600mm from floor to the window height.
6x2 is fine at 400c
So 150 joist, 18 deck, 150 insulation, you might get away with 50mm slope (I'd do at least 100), 12 pb
So that leaves you a ceiling height of 2.2m-2.1m
So it is possible but cosy.

 

[Brickymatt]

Rough sketch added is this feasible as I really wanted to leave the window alone
If not it will have to be cold deck

 

[^woody^]

Just going by that sketch, I think it would look crap. Change the landing window, or alternatively agree a roof construction with the inspector that involves complete insualtion within the joists and a little beneath - just like a SIPS panel roof.

 

[Brickymatt]

I spoke to a labc inspector today where I work he said go with a cold roof with possibly 50mm insulation on top of the deck he recommended felt over rubber Edpm which I was going for ? I thought the rubber would be better ?
The inspector said I would need to retain a air void with the cold roof and not to fully fill

 

[^woody^]

It's difficult to ventilate a cold roof in that situation. Not impossible just awkward.

Not sure what that inspector is thinking when he suggests insulating a cold roof deck. That's a warm roof!

 

[tomfe]

That's a warm roof!

I think you'll find it's called pointless

 

[Brickymatt]

I know it should be vented back to front ideally but would a few pipe type vents on the roof do the job ?