Roof design for extension

[Robert_123]

Hello all, I am new here so hoping this is in the right place. I'm looking for some advice regarding timber sizing for a roof:

I plan to build a room on top of an existing attached garage. The overall dimensions will be 5.5m wide x 8m long (brick and block with 50mm cavity). The room will be open with no internal walls.

The roof pitch will be 30 degrees to match the original house. The plan was to use 4x2 rafters and ceiling joists (which will be ~8m long) with binders and purlins running from gable wall to gable wall. However, using the TRADA span tables I can't find a purlin size that will span 5m (e.g. from inner skin to inner skin). Question is: Is there a purlin size that can span 5.2m and if not, what could I support the purlin on to break the span, given that there are no internal or load bearing walls that a strut would sit on?

Thanks for the help,
Robert.

 

[^woody^]

Do you have plans drawn for this or making it up as you go?

Steel beams instead of purlins. Purlins on struts on timber beams. Increase rafter section size - which you might have to do anyway for the insulation.

 

[Robert_123]

Hi Woody, thanks for your reply. This will be my third DIY extension with a cut and pitch roof so although I will be drawing plans and getting full plans approval, I'm hoping not to need an architect.

I like the idea of purlins on struts on timber beams (I presume that this timber beam would sit on top of the ceiling joists same as a binder and be built in to both gable walls?) but the question then would be: what size timber beams would be needed to span 5m and take the weight of the struts from the purlins?

 

[^woody^]

You would need to design/calculate any beams based on the loads. There are several options to achieve a cut roof and they will all typically involve structural calculations, or you can use attic trusses.

 

[Robert_123]

Ok, thanks. I was hoping there might be another table that I hadn't found that would cater for longer spans (similar to the TRADA tables detailing floor joists, rafters, purlins etc). The maximum permissible span for purlins seems to stop at around 3.5 meters using a 9x3 timber. Apart from that the roof will be nothing out of the ordinary. Thanks for your advice

 

[tony1851]

Can you use a 12" x 3"as a purlin?

Alternatively a steel beam would do,with a 4" x 2" bolted on top to pick up the rafters.

As Woods noted, you may need deeper rafters - typically 6 x 2 to allow for insulation + air gap.

 

[Robert_123]

Hi Tony, thanks for your reply. I could use a 12" x 3" purlin but I would still need to satisfy the BCO that it was sufficient - hence looking for a standard table or similar that details purlin size vs span.

The loft space will have fiberglass in between and across the ceiling joists. Is any insulation needed between the rafters at all? I thought just breathable felt? The loft will not be used for anything and won't be part of the room

 

[^woody^]

Are you building a normal square extension room and roof above or some sort of room in a raised collar roof?

If the former, why cant you just put trusses on it?

 

[Robert_123]

It's mostly normal (square) with the exception that the wall plate at the back will be about 1m lower than the wall plate at the front (reason being is that the back wall of the garage is set back from the house and I want to keep the same roof line at the back, so I will need to drop the wall height down and have a slightly sloping ceiling at the back end). This was another reason that I was asking about purlins as the ceiling joists will need to stop on a pole plate at the back of the room, rather than the wall plate as normal.

 

[Robert_123]

Roof design aside, I'm considering whether to have the inner skin of the second story built in blockwork, or to have a go at building it timber framed myself (purely to save cost). The outside will be a brick skin in any case and the existing cavity is 50mm. If the inner skin is blockwork then I'll line the inside with 50mm cellotex to achieve the insulation value, leaving the cavity open.

What are the pro's and cons of each method and am I going to save a significant amount from building the inside myself, timber framed rather than paying a bricklayer to lay blocks?