Flat roof gradient - firrings or offset joists

[macadonald]

Hi Guys,

Can someone tell me if it is best to use firrings on top of the joists or actually offset the joists to get the run-off on a flat roof. Mine will have a parapet around it, running off to the alley with a small hole in the parapet into a down pipe and away.

It would be easy enough to set the joists so that they drop off in this direction, but I guess I would have a slanted ceilign as well. Guessin gthat firrings are better for this reason?

Thanks

Mac

 

[^woody^]

If you want a sloping ceiling that looks neither here nor there, then slope the joists

 

[macadonald]

Sorry woody, not sure if there was some sarcasm there or I just misunderstood

Clearly I don't want a sloping ceiling, as it's a flat roof, if "looks neither here nor there" you mean that the slope will not be noticeable, then that is an option. I guess another option would be to trim the top of the joists by 2-3mm over every 500mm centre?

 

[HERTS P&D]

He means use firrings as that is what they are made for.

Andy

 

[freddiemercurystwin]

Or if you slope the roof joists you can fit additional horizontal ceiling joists to the sides of the roof joists.

 

[^woody^]

if "looks neither here nor there" you mean that the slope will not be noticeable,

No, it wont be flat, and it wont be sloped enough to look like you meant it - it will just look out of level

 

[^woody^]

Or if you slope the roof joists you can fit additional horizontal ceiling joists to the sides of the roof joists.

Do you charge extra for overcomplicated design and build cost?

(note to mac ... that was sarcasm)

 

[freddiemercurystwin]

Or if you slope the roof joists you can fit additional horizontal ceiling joists to the sides of the roof joists.

Do you charge extra for overcomplicated design and build cost?

(note to mac ... that was sarcasm)

Hardly overcomplicated is it? Or don't you understand the principle?

 

[^woody^]

Is it more complicated and costly than a joist with a furring on the top?

 

[macadonald]

Thanks Guys.

Sorry for throwing this into the same post, but for an extension of 3m, hence joists spanning 3m, what joist size would you recommend for a warm deck flat roof? There will be 2 velux sealed skylights 1m x 1m each.

I plan to use timber plates, the same size as the joists and bolt these to the existing house wall and new extension inside skin and then attach joist hangers to support the new joist. Does this all sound okay to you?

 

[freddiemercurystwin]

Is it more complicated and costly than a joist with a furring on the top?

More costly granted but not complicated.

 

[^woody^]

More costly granted but not complicated.

The joist would need to be notched both ends to give it a flat seat, and then each end cut plumb too. As oppose to the [less complicated] plonking the joist on

<waits patiently for the oldun to get home from work and set us all straight>

 

[catlad]

It will feel like twice as much work from below never mind the cost.

 

[macadonald]

Thanks all,

Not sure if my additional question above was missed, but in case:

"Sorry for throwing this into the same post, but for an extension of 3m, hence joists spanning 3m, what joist size would you recommend for a warm deck flat roof? There will be 2 velux sealed skylights 1m x 1m each.

I plan to use timber plates, the same size as the joists and bolt these to the existing house wall and new extension inside skin and then attach joist hangers to support the new joist. Does this all sound okay to you?"

I was thinking of 150mm deep C16 joists regulised at 600 centres? These would span 2.7m or a bit less with timber plates bolted to either side?

 

[Alastairreid]

http://www.dfpni.gov.uk/d.pdf