Rafter Upgrading

[Rich_DW]

Hi,

I was hoping for A little advice with regards to a single story extension roof.

We have an existing tiled pitched lean-too extension. Roughly 4m long and extending 2.1m out from the original house wall with a 15.5degree pitch.

The roof is constructed using 2x4 rafters spaced at 400mm. It has a 9x3 purlin running the entire 4m span supporting the rafters.

I want to remove the flat ceiling, remove the purlin, install 2x velux windows and lift the ceiling.

I think the 2x4 rafters are too small to cover the span without the purlin.
To change the rafters for larger 2x6 or 2x8 would be exceptionally difficult. Is there a recognised way to make the 2x4 setup work over the 2.1m span. Can I add twice as many? Can I sister each rafter with another 4x2?

Any advice would be greatly appreciated. Thanks.

 

[garyo]

How deep is the pulin?

Have you considered leaving the pulin where it is but box it all in with a faux set of under-rafters. You'll need to use some of the depth anyway to get a ventilation gap and 100mm +50mm of insulation in there.

 

[Rich_DW]

Hi, thanks for the comment, however, on a 2m span, the purlin (about 1m from each end) makes it virtually impossible to fit in the velux windows. I need it gone. I’d planned on creating a deeper viid to accommodate the insulation either way.
Thanks.

 

[catlad]

I would say doubling the rafters would be enough.

 

[Rich_DW]

Hi, when you say double.. do you mean adding more in between to go for 200mm spacing, or would you glue and screw a second 2x4 to each of the existing rafters at 400mm spacing. Effectively making each rafter a 4x4.
Thanks.

 

[garyo]

Isn't a 2.1m span within spec for a 2x4? Admittedly with a birds mouth for the purlin in the middle they'll be weak but you could sister up to mitigate that.

 

[Rich_DW]

I thought a 2x4 was borderline for 2.1m and guessed since the builder went to the trouble of putting a 3x9 purlin in that I’d possibly got that wrong.
Although The purlin looks huge in comparison to the rafters, I understand this is mainly due to its own span (4m).
I think for the sake of cost, I’d rather be on the safe side and double the rafters.
Possibly with deeper ones that will give me the required room for insulation as theres not too much in it cost wise.
Do you think thats a sensible approach?
Thanks.

 

[cdbe]

Surely you're going to have to double up a lot of the rafters anyway to support the trimmers for the roof windows. Also, consider the effect of removing the ceiling joists (if a vaulted ceiling is the plan) although maybe they're not doing much with that big purlin being there?

 

[noseall]

I want to remove the flat ceiling, remove the purlin, install 2x velux windows and lift the ceiling.

I think the 2x4 rafters are too small to cover the span without the purlin.
To change the rafters for larger 2x6 or 2x8 would be exceptionally difficult. Is there a recognised way to make the 2x4 setup work over the 2.1m span. Can I add twice as many? Can I sister each rafter with another 4x2?

Lol, you basically are re-building the roof.
This is what will need to happen here in any case. And it will require an application. You are basically re designing the roof and are moving the ceiling line upwards - thus the insulation detail changes with it as will venting etc.

I recommend you re-build the roof using 6" x 2" rafters birds-mouthed onto a 2" thick bearer plate/pole pate that has ben fixed to the wall. The high level joists can be fixed to the rafter sides and onto their own bearer pate that is fixed to the wall - probably all 4" x 2". These will need joist hangers on the ends. The slopey part of the ceiling will need 100mm Celotex between (50mm air gap above) and 50mm across and foil back plasterboard. The flat part can be filed with 300mm fibreglass quilt. Use breathable membrane on the roof with over-fascia vents. Double up the rafters each side of the Velux dependant upon Velux width etc.

 

[garyo]

I seem to remember Tony talking about a job once where they're screwed and glued rafters directly beneath the existing, effectively 'upgrading' them. It might be an approach that suits your situation. I'm sure he'll be along soon.

 

[Rich_DW]

Thanks for the info guys. I’m interested in hearing about Tonys job..