DIY rafters?

[elmsoft]

I am building an extension and wondering if I can make the pitched roof structure- less hassle than ordering trusses maybe?
The extension is 5200mm by 4400mm with 20 degree pitch and hip end
The 5200 dimension is the one I need to construct rafters to span.
Any thoughts on how simple it would be to do this?

 

[noseall]

Depends how good you are at math's and how skilled/experienced you are.

It also depends largely upon what tools you have at your disposal.

Common rafters are simple to make once you have a pattern. Hip end jacks however are not so easy.

 

[elmsoft]

Thanks.
I reckon my maths is up to it. Would I need to design my own pattern or is there likely to be one " out there"? ( I have a copy of the TRADA span tables)

 

[noseall]

Once you have worked out the rafter, joist and ridge section and possibly the purlin section, then it's just a case of using a bit of trig a wee bit of pythagoras and a whole load of wood cutting skills to achieve the pattern rafter.

Actually getting started is the hardest bit and understanding how to 'pitch-up'.

 

[noseall]

There is no pattern no. Each roof is built according the wall plate distances and pitch. The birdsmouth cuts, plumb and seat cuts etc, are relevant only to your roof depending on pitch, eaves overhang or soffit size and position of purlins etc.

 

[elmsoft]

Thanks
Sounds fair enough.
I already know that my span is 5200mm and pitch 20 degrees.
I would want 400mm on centre and use 147 x 47 mm C16
Intuitively, that would seem plenty strong enough to support the Marley tiles- say 0.75KN.M-2

It's the "intuitively" bit that worries me.

 

[catlad]

Are you planning to use a purlin?

 

[elmsoft]

Are you planning to use a purlin?

Hadn't thought so. Where would the purlin sit?

 

[noseall]

A 5.2m wide roof at 20 degrees gives you a rafter length of 2766mm between seating points.

If your building is 5.2m then I guess the measurement from plate to plate is much less, correct?

What is the plate to plate measurement?

 

[elmsoft]

A 5.2m wide roof at 20 degrees gives you a rafter length of 2766mm between seating points.

If your building is 5.2m then I guess the measurement from plate to plate is much less, correct?

What is the plate to plate measurement?

The plate to plate measurement is the 5.2 metre. That is the "truss length" to sit on the timber frame. Obviously there is a cavity and an external block wall to make up the rest of the building ( plus a soffit)

 

[catlad]

sorry I did not get what you were trying to explain.

 

[elmsoft]

sorry I did not get what you were trying to explain.

The extension will be timber frame with a 50 mm cavity and 100mm block outer wall. A truss bottom chord would measure 5200mm . The pitch is 20 degrees, so the top chord would be 2766mm with an overhang of 200mm for the soffit and a ridge height of 946mm.

What I want to do is achieve the same thing as such a truss but using traditional rafter construction.

 

[catlad]

Only you can decide if its going to be easier or not to cut your own roof, trusses will be quick if access is ok and you have someone to help you, have a look on you tube for video,s