common rafter with purlin joint

[Alexis]

Hello,

I would like to know which joint are used when you have a frame with common rafter with intermediate purlins? I suppose the bottom is a step lapped rafter feet but what is the top? Also do you make it one long timber or two different timber?

thanks

Alexis

[Rooster]

Hey there,

I'd like to help, but I was wondering if you could give some specifics related to your question.

Is this pertaining to a specific project of yours?...or are you asking just out of curiosity?

If it is a true project, what is it? House? Shed? Barn? Mother-in-law guest house? 

How big is it? Dimensions?  Roof pitch?  Length of Common rafter?  Type and size of rafter material?   

Will the underside of the rafter be exposed?  Are you using a ridge beam?


Let us know...so we can help. ;)

Rooster

[Jim_Rogers]

Without knowing the answers to the questions mentioned above, it's really hard to say.

I've seen the single long rafter done with a middle step lap rafter seat cut into the rafter for it to rest on the purlin.

And I've seen two piece rafters that go into a pocket on either side of the purlin.

Also, I have a question. Is the roof slope continuous or does it change at the purlin to a different angle?

Part 5 of the Historic American Timber Joinery shows roof joinery and there are some examples of different ways to do what you're asking.

Here is a link to the page where you can download a pdf copy of this part and then view it on your own computer:
http://tfguild.org/joinery/joinery.html

Hope that helps....

Jim Rogers

[Alexis]

Sorry for the vague question:

The building is a house 29X42 with a 12:12 roof pitch. It will not have a ridge beam and the underside of the rafter will be exposed. The size of the rafter is probably 4X6 but can be upgraded.

Ideally, I would not have had purlins at all, but with the answer from a previous post, I think I will have to...

I live in quebec, canada so i have an important snow load.

thanks for your time!

Alexis

[routestep]

I'm slowly working on a barn that has principle rafters supporting a purlin. Common rafters as two pieces will fit into the purlin. The top rafter, going up to the peak is about 12 feet long. The joint at the peak is an open mortise with peg, at the bottom is a snub tenon one inch long. I'll timber loc it to the purlin.  The lower rafter runs from the purlin to the plate. It will have a snub tenon with timberloc at its top and a step lap at the plate.

The pitch is a constant 10:12, common rafter size is 6.5 by 4.5

Because the purlin is 14 feet long and only an 8 by 8 I have two posts coming up from the deck underneath it to help support the roof load. I have to have a 14 foot 8 by 10 sleeper resting on the deck, running between the bents to support those two posts. The engineer gave me grief about mortising the posts into the sleeper, he wanted just screws, but he signed off on a shallow 1/4 inch mortise to help locate the posts. I'll screw the tops to the purlin.  And my sleeper also rests on a floor joist ( 6 by 8 14feet) and has to be screwed to it at multiple locations but not in the middle third of the sleeper.

I'm in central Virginia so my snow load isn't bad at all, but with this design I think the roof will take a hurrican driven blizzard.