Purlin scarf joint - which to use...

[MarkH]

Hello,

Brilliant forum! I'm not sure I've seen one with this much knowledge and information. It's possible this has already been addressed somewhere (I have looked) but anyway:

I'm building a simple rectangular house in the UK, masonry walls but with a green oak cut roof. The roof consists of one masonry gable and then three trusses with the other gable being one of those. My question has to do with the purlins, there are three runs of purlins on each side with each run in three lengths and therefore I'm going to joint the purlins at the trusses. I wondered if anyone could suggest the best joint to use? Is a simple half-lap sufficient if the joint is over/near a truss?

The purlins are 8" by 4" and the largest span between trusses is 13'. The purlins (and all the truss timbers are oversized relative to the engineers spec by 25%).

Thanks for any help!

Mark

[Heartwood]

Rather than scarf them in-line, why not stagger the center purlins above or below the outer ones, running the ends past the trusses a bit. I think this is the way it would have been done traditionally.

[Roger Nair]

Mark, I suggest that you should refer to Cecil Hewett's book Historic English Carpentry.  You will find documentation and commentary on the methods used and relative strengths of the various methods of scarfing in Appendix 1 and joining purlins to principle rafters and analogous joining of joist to beams in Appendix 3.  The range of buildings analyzed spans a thousand years.  It's a very useful book with clear writing and the best line drawings documenting timber framing.

[MarkH]

I'm considering the staggered option - hadn't thought of that. There's some appeal in the neatness of lined-up purlins though.

I'm going to make a half-lap with some offcuts I have left over from the trusses, get a feel for it...

Thanks for the replies!

[redbox13]

Edited, due to being in the wrong thread.