How to build pergola ceiling joist and rafters?

[jasonlee]

I'm trying to build a 6x3.5m pergola, the span of the rafter will be roughly 3.5m at 600mm spacing.

The idea is trying to create a level enclosed ceiling underneath and a pitched roof between 1 and 5 degree towards the front of the gutter. The whaling plate will be fixed to the existing house structure. 

My question is can I use one rafter for both ceiling joist and roof rafter to support metal sheeting, I'm thinking to use 140mmx45mm H2 MGP10. Is it possible to rip the top of the timber at a small angle from one end to the other to create that slope but at the same time to keep the bottom straight use for ceiling joist? I attached a drawing for your reference, green part to keep. 

Thanks

Jason

[EricL]

Hello @jasonlee 

Thanks for sharing your question about your pergola. I don't see any drawbacks with your plans, except maybe if you had to run an electrical conduit for lights. You would need to drill into the timber to allow the conduit to pass through. Please remember to spray the cut section with TWA Woodcare 300g Ecoseal Tanalised Timber Treatment.

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you have any other questions we can help with, please let us know.

Eric

[jasonlee]

Thanks for your prompt reply Eric.

I want to know if 3500mm span for the ceiling/rafter joist is safe to use 140mmx45mm timber? 

Jason

[MitchellMc]

Hi @jasonlee,

For a span of 3500mm you'd need to go for at least 190 x 45mm. However, if you then want to cut it down to achieve that slope, you'd have to step up to 240 x 35/45mm. 

Please let me know if you have any questions.

Mitchell

[jasonlee]

Hi Mitchell,

In stead of using 240mm beam, can I add another beam on top of all joists in the centre to break 3500mm in half, would that give them more strength ?

[MitchellMc]

You could certainly add a central support to break up the span to 1750mm @jasonlee. However, since your other dimension is 6000mm, that would have to be a huge support beam to span that distance.

Mitchell

[jasonlee]

Can I have 2x3.6m support beam overlap each other around 600mm for the 6m span?

[MitchellMc]

Would you possibly be able to draw a sketch of what you'd like to achieve, @jasonlee? 

Mitchell

[Dave-1]

Good Morning @jasonlee 

Going throught the posts I am just getting my head around what you want.

So I get you want to shave down one end of the beam so that its level with the ceiling space you are creating.

Having a 5 degree drop is the first part I question, thats not a surface that will help water drain especially if you get dirt and leaves in your area.

Shaving off the the beam will make it weaker. Instead of doubling the height of the beam as I think you are height poor for the space, how about having two beams side by side chamford the way you want them. That way you still get the strength, plus the angle you are after?

Dave

[jasonlee]

This is what I’m trying to achieve, support beam and roof joists acting both rafter and ceiling joist. 

If I want to separate the roof rafter and ceiling joist which I can, do I need to build a dawrf wall or something at the back so the rafter can fix on to a new walling plate? Because the support beam for the pergola is free standing, there is no structure for me to fix the walling plate onto. That’s why I was trying to use just one beam for two purposes. 

Jason