roof design

[bmw]

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

[OBBob]

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).

[bmw]

I am intending to use 140x45 pinus radiata at 900 centres, as this is the most widely available framing timber in NZ.

[toecutter]

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

As long as the underpurlin is designed to carry the intended load.

[Dindas]

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

Speaking to an engineer is your best bet.

[pawnhead]

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;



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. <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 <br>

[bmw]

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

[ausdesign]

You can join over underpurlins.

[thebuildingsurv]

you can join under over-purlins to

[pawnhead]

you can join under over-purlins to

I've never seen an over-purlin, but I suppose that they would be used somewhere

[silentC]

It was just done according to the framing code and span tables.

Interesting. The NSW Framing Manual says:

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.

Could be an oversight, I don't have the BCA or relevant standards to check against.

[jrock_au]

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-comfficeffice" /><o></o>
hope this helps, sorry I can't be more specific<o></o>

[silentC]

BTW an overpurlin is just a purlin. My shed has them. They hold the roof on.

[pawnhead]

Interesting. The NSW Framing Manual says:

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.
Of course there's gang nails nowadays, but we didn't always have them.

edit:

BTW an overpurlin is just a purlin. My shed has them. They hold the roof on.

So your purlins are above your rafters, or are they on top of your roofing

[silentC]

Could be an oversight ...

Actually, no, it's quite specific. It goes on to say:

Where possible, underpurlins shall be single length ... Where they must be joined in their length ...

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.