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Hi,
I'm designing the roof for a new house I hope to build next year. The length of the rafters as I've designed it are 6.1m. As it's not always easy to get timber in this length, my question is. Is it normal or acceptable to join rafters over an underpurlin?
Cheers
Bruce
What timber are the rafters? You can get Hyspans in that legth ... or you can get muscle beams, which have nail plate joins rated at the strength of the timber (according to their documentation).
I am intending to use 140x45 pinus radiata at 900 centres, as this is the most widely available framing timber in NZ.
As long as the underpurlin is designed to carry the intended load.Quote:
Originally Posted by bmw
Speaking to an engineer is your best bet.Quote:
Originally Posted by bmw
Here's one where I built the framework on contract. The pitch would have been around eight metres long. You can see on the left there that it's not complete with the 90x45 rafters hanging in the air. I installed the underpurlin near the end next, and extended the rafters down to the slab;
http://i7.photobucket.com/albums/y26...h_Beecroft.jpg
The engineer designed the footings and slabs, but had nothing to do with the framework. It was just done according to the framing code and span tables. :wink:<br><br>You can also see the flag that I installed on the ridge there. I usually don't throw that up until all the eaves are complete and everything's tidied up. It's my standard signal to the builder that he'd better have a slab under his arm the next time he rocks up on site :U<br>
Thanks for all the replies :).
I'm building an earth walled house following the NZ earth building standards (except that I'm using poured earth, under an engineer), and the roof is following the NZ timber building standard (NZS3604). From memory, NZ3604 says something like "Rafters shall span between any of the following - top plate, ridge beam/board, underpurlin and valley rafter" This implies to me that joints over underpurlins would be ok. but does not say so explicitly. I'm interested to know what is common practice. I expect most builders would just prefer pre-made trusses! But I don't want trusses under my sloped ceiling.
Cheers
Bruce
You can join over underpurlins.
you can join under over-purlins to :U
I've never seen an over-purlin, but I suppose that they would be used somewhere :UQuote:
Originally Posted by thebuildingsurv
Interesting. The NSW Framing Manual says:Quote:
It was just done according to the framing code and span tables.
Could be an oversight, I don't have the BCA or relevant standards to check against.Quote:
Rafters shall be single length members which are nominally fixed to top plates and adjoining ceiling joists at one end and at the ridge board, hip or valley rafter at the other end. They may also be supported by one or more underpurlins between the ridge and top plate.
you will have to check your AS but you should be able to join over a point of support but will either have to overlap your two lengths (double check this but I think its 3? times width of timber) or use a nail plate which would be 6? times width of timber. There is also a min fixing rating i.e. 6? Hand driven 75mm nails or 8? machine driven.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
hope this helps, sorry I can't be more specific<o:p></o:p>
BTW an overpurlin is just a purlin. My shed has them. They hold the roof on. :)
I don't know where you'd get a rafter eight metres long. Six metres is about the max unless you know someone at a sawmill.Quote:
Originally Posted by silentC
Of course there's gang nails nowadays, but we didn't always have them.
edit:So your purlins are above your rafters, or are they on top of your roofing:?Quote:
Originally Posted by silentC
Actually, no, it's quite specific. It goes on to say:Quote:
Could be an oversight ...
The fact they went to the trouble of detailing how underpurlins should be joined but there is no such information for rafters indicates to me that rafters may not be joined.Quote:
Where possible, underpurlins shall be single length ... Where they must be joined in their length ...
Me neither. I'm just telling you what it says in the book...Quote:
I don't know where you'd get a rafter eight metres long.
In my case the rafters were lapped a fair distance, but they wouldn't have to be if you've calculated the spans as a 'single span' as opposed to 'continuaous over two or more spans'.Quote:
Originally Posted by jrock_au
Hyspan does rafters up to 11.9 metres.Quote:
I don't know where you'd get a rafter eight metres long
The purlins are on top of the portal frames and the roofing is on top of them. Of course they don't call them overpurlins, but an underpurlin is so called because it is a purlin that goes under the rafters.Quote:
So your purlins are above your rafters, or are they on top of your roofing
Talk to an Engineer. You can definately splice a rafter if it is at the right location or lap the joint if you provide enough fixing. Best place to lap is about 0.2-0.3x Length.
But do not attempt to do this on your own without professional advice.
AS1684 has been deliberately set up to guide people on what is safe and only experienced professional are allowed to step outside the limits if they can prove that the design is adequate.
That's what I was going to say. If it's not in the framing manual, it's probably still possible but would need an engineer's design to pass.
Ive never heard of overpurlins i was just trying to be funny
Don't give up your day job :wink: