# Forum Home Renovation Roofing  Rafter size on pitched roof

## rickw0708

I have the Alan Staines book, the Timber Assoc book on Carports/Pergolas and also the Timber Solutions computer program and I find that I get different answers for timber sizes from each of them. 
I am designing a pergola with a pitched roof (not sure yet whether it will be gable or hip ended) that will be 8.0m long and 5.5m wide attached to the house on the 8.0m side via Roof-Extendas (I think!). It will be covered by polycarbonate. 
I have a couple of questions-
- when using the span tables to determine the size of the rafters in the pitch, do I work on the span being the distance between the 2 side beams (ie. 5.5m) or do I use the length from the side beam to the ridge (ie. about 3.0m) ????
- how do you determine the size of the ridge beam?
- assuming I end up going with gable ends, the ridge beam would need to be 8.0m long, can you join 2 beams together or how do you handle that? 
OK, I better stop with these questions for now. 
Thanks

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## ausdesign

Side beam to ridge ( single span; 10 kg )
2 scenarios for the ridge  - either the ridge beam needs to be supported by a propping beam ( side beam to side beam ) or you will need an engineers design for a coupled roof without support.
Ridge spans are in AS1684.
Where you join ridge beams they will need to be propped.

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## rickw0708

Thanks for that Peter. 
Can I ask how many side beam to side beam rafters you need? Is one at each end enough considering the total length will be 8.0m? 
I have been looking at pictures of pergolas on the www and seem to find many that have a pitched roof where there appears to be no side beam to side beam rafters and the only thing that looks like it is helping hold it all together is collar ties. I have been wondering how they manage to do this.

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## ausdesign

We may have a confusion on terminologies here so let's start with - 
the side beams are fascia beams if the roof is pitched. Side beams if its flat as in a carport etc.
So you will have 1 fascia beam supported by the 'extenda' brackets & one supported by posts (assumed) running parallel to the 8.1
The fascia beam size needed is calculated by inputting the required span between posts or extenda brackets by the 'roof load width' & the roof load ( which will be 10kg)
For a 'supported' ridge, which you will need to work on unless you have an 'engineered design' the roof load width (RLW) is half the length of the rafter from the fascia to ridge beam.
To determine the ridge size, the roof load width will be the rafter length. The options in sizes will give you a maximum length that the ridge can span.
At each end of the ridge span you will need to provide support.
This support is carried by a 'propping beam' which runs from fascia beam to fascia beam.
The propping beam size is determined by the squarage and weight (10kg) of the roof supported and the required span of the beam. 
Makes sense to me but I don't think you'll find the info in Alans book.

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## rickw0708

Sorry, I think I may have used the wrong term.  The "side beam to side beam rafters" that I mentioned are (I think) what you are calling the propping beam running between the 2 facia beams. 
I haven't quite got my head around how you get the member sizes for the ridge, etc but I will sit down over the weekend with your explanation and my span books and I am sure it will all become clear. 
I have attached a few pictures of the effect I am trying to get. In these pictures they don't seem to have any propping beams apart from the ones at each end and they seem to be covering a fair distance with the ridge beam without any intermediate propping beams. Also, the member sizes of the ridge beam and rafters seem quite small. I see they also use collar ties(?) but I was told they didn't do much as far as preventing the rafters from "pushing out" the facia beams. This is where my confusion lies.

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## ausdesign

The pic's are of a 'coupled' roof without a ridge support (propped ridge).
Building surveyors will require an engineers design for this type of construction.

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## thebuildingsurv

90% of the carports / pergolas I approve have collar ties and not a loadbearing ridge we would generally accept something up to about 6m wide like this. We never ask for an engineers design. I dont know many that would. If you are happy to keep the collar ties fairly low ( bottom third is best)  i dont think you will have a problem. Why dont you have a chat to the local council. Peter, I think who do you use for your permits ?

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## ausdesign

I'm with you 'thebuildsurv' but I fear we are a dying race.
We've come across this increasingly over the last 12 to 18 months - not that we do a huge amount of this type of design.
The last was from East Gippsland Building Services . . . "further we do not permit coupled roofs without engineering certification or provide an appropriate supported ridge beam."
We had similar with, from memory, Frankston City Council,  one in Kalgoorlie which admittedly was pushing my comfort zone at 7.2 meters.
We have also had a case with an unsupported ridges approved with 'collar ties' every second rafter (1800) at pitching beam height with the post & footing designed for bracing. ( attached to a class 1 )
I know it's going over old ground, but I am still optimistic that one day we will see uniformity across the board.

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