Seeking advice how to attach roundwood rafters to roof

Hi, I’m getting ready to fix the roundwood rafters onto my roundwood frame and I can't find what type of joint to use to carve out from the poles and fasten the rafter. I can’t find any examples online and my Ben Law book on the topic only shows how to use milled timber for the roof, which is obviously easier, but I'm not going that route.

An acquaintance mentioned you can also tie the rafters with rope for added strength. Anything else that might help?

Here's a photo of where I'm at as of now. https://photos.app.goo.gl/ZfCcxDchd1A7KL5J6

Nice build! Your frame looks great.

I attended an excellent roundwood timber framing course last year that was run by a guy called Adrian Leaman and his company, Wholewoods. He'd worked with Ben Law on previous builds and really knew his stuff. I've not put it into practice (yet) so much of this is from memory. Others with more practical experience might be able to offer more insight.

I've attached some photos showing a few joints for both a roundhouse and a cruck frame (like yours).

The roundhouse has a ring beam, a series of half-lapped logs that are fixed to the uprights with mortise-and-tenons. The rafter then sit on top of the ring beam with a scribed notch/saddle cut out of both rounds to ensure a firm fit. A peg or bolt through the rafter will hold it firmly in place. An alternative joint to use is the butter pat which would be stronger but is harder to lay out - it may probably be easiest to strap the rafters in place, scribe the locations of the joints and then remove the rafters for cutting.


Butter pat joint from Permaculture Eden

I'm finding it a little difficult to picture your finished roof but I think that a saddle or butter pat would probably work for your build too. If the angle is too steep then a simple saddle won't hold the rafters in place without a peg/bolt and a butter pat might be a better option.

Thanks for the reply. I consulted with Adrian twice last year, good to hear you learned with him. I considered a butter pat, however, my concern is will it greatly reduce the carrying capacity of the top most beam if I cut that much wood out?

I was considering lightly taking off from both pieces just enough so that the rafter has a flat surface to sit on (ie doesn't slide under its own weight) then craft a joint to the rafter on the opposite side, then pretty it into place, then lastly tie all connections with rope.

It's a small world, that of roundwood building!

You're no doubt correct that a butter pat would reduce the strength of the beam. You could try and estimate the total weight of your roof and determine whether you think it would be safe to do so. I have no idea where to source the figures needed for that calculation though.

Flattening the beam and the resting surface of the rafter in some way is essential. Two round logs have a tiny point of contact and the rafter would want to roll or twist. A flattened contact point and a peg, or preferably a large diameter bolt (as it has clamping force) would be a good solution. Are you against using metalwork in your frame?

Yes, I'm against metal for this construction and I don't know either where to source the load capacity info so I'll go with my gut most likely. Thanks for your thoughts Luke

Maruf Miliunas wrote:my concern is will it greatly reduce the carrying capacity of the top most beam if I cut that much wood out?

Good looking build so far!  Keep us posted!!

I am less experienced than many on this site, but have been working with roundwood for a few years and have a handful of successfull builds under my belt.  Your concern is correct - but this joint is not a perfect example



This image shows a lot more removal of material than butterpats generally call for.  You can achieve a butterpat with very little removal of the diamater - an small fraction of the log is more than enough to ensure good contact. You really are just skimming off the outer bit of the circumference. When I am improvising joints on small projects with unknown characteristics, I overengineer by 2x, and I generally cut each flattened joint just into the heartwood, so most of the strenght still remains.

The relevant portion of this video (2:25-2:55) shows the importance of minimizing depth of cut to preserve strength of the member.  This relates to any joint where the sides of the logs are in contact, including butterpat.



If you cut butterpats minimally, you will not lose much of the strength of the member.

Here is Maruf's photo.
Looking good so far!

After some fooling around, I found a rafter joint that requires the least work and offers the most hold.

Looks like a roundwood variation of a bird's mouth notch, which is widely and reliably used in conventional stick-frame construction.  Maybe sufficient to keep the rafter from sliding downward, though it lacks the ability to resist rolling.  Perhaps once perlins are integrated and roof is a unit, rolling will not be a concern, but I would see how others weigh in.

I'm not sure but I think Maruf is drilling and wood pegging those rafters.

thomas rubino wrote:I'm not sure but I think Maruf is drilling and wood pegging those rafters.

Right, I suppose so Thomas.  However, isn't joint member contact more reliable to prevent rolling that pegs or fasteners? Again, rolling may not be of consequence when the roof is an integrated whole.

That's what the joint's called! Thank you.

Yes, they're getting pegs, though to prevent rolling, the rafters connect at the top and there's an additional peg to join the two. In the end, each pair of connecting rafters get 5 pegs.

Hey Beau;
All roofs are wobbly shaky until tied together.
I would guess that if they are pegged and tied together the whole roof would be as strong and stable as a frame roof.
I defiantly am not a wood framer of any kind, be it round wood or square so I may be mistaken (first time for everything right)
Hopefully, a true round wood builder will chime in and tell us.

EDIT) I just saw Maruf's post.   He has this covered!

Maruf, I'd trust that!  

Thomas, I think you're right on all counts.

Maruf, what are you using for purlins/decking/insulation/roof cladding?

Beau, the purlins will also be roundwood timber, which I intend to butter pat onto the rafters and peg everywhere as well (I'll need tons of pegs for the roof). For the roof, I found a thatched-roof master who insisted I indeed won't need a membrane and it will serve as insulation, as I'm going for an eco house. The roof is roughly 5,5 meters across with 5 rafters to span the distance roughly every 110cm and the purlins will be slightly smaller diameter roughly every 150cm (the master told me 10cm diameters rafters every 150cm is the traditional distance apart). As I intend to mount solar panels I'm adding the extra pair of rafters.

I'm curious what your thoughts are.

My thoughts:
Thatch is amazing, and I can't wait to see your project unfold. I have no experience with thatch, but from what I understand, all that you have said has held true for thousands of years.
What's the climate like where you are?
Please keep us up-to-date!

Will do!

I just updated my profile to reflect my location. It's a temperate climate here in Lithuania. Cold winters, hot summers and generally quite a bit of rain.

How is it coming along, Maruf?  

Here's how far I got before we closed shop for winter and left for Spain.