you'll need iron (or steel) at the arch to rafter and arch to arch/collar connection or buttresses at the location of the xframes. My read is that in the 2nd configuration, the lower arch pieces are decoration and the upper arch braces are in tension. you will also need deep rafters because you will be loading the rafters in midspan with the tension connection from the arch.

we did a hammer beam roof that sat on masonry walls with buttresses once. the buttresses were poured concrete and were basically full height walls 12" thick that stuck out at 90 degrees to the main walls. They stuck out about 8' if I remember correctly. Big old buttress. But the span was about 56'.

Hope this makes sense, I'm a bit rushed -- dinner plans.

Well - I got the span, what spacing are they on?

Point load the king post with ridge? Purlins? Common rafters?



You'll definitely need tension steel in there. Roof surface and load will tell you how much. I have a shallower pitch version that we did (sitting on posts as bents) @ 42' span. I can post a pic when I get back to the office next week.

The closer you get the lower curve to the end of the rafter the less shear you'll have - so piece sizes can be reasonable.

Note too that not all masonry can take outward thrust. You can certainly design for it - but in 1 case I designed for I couldn't get the engineer to give me any value for the stone...

Hey Guys,

Thanks for the insight. Gabel, you say that the lower arches are decoration. I think you mean that the load of the roof is carried by the rafters at the plates, and the lower arches in a sense are just "hanging" there. Is this correct? I wonder if there is a way for those to actually carry the load of the roof more than the principal rafters? This might somehow be convoluted, and to indepth for a forum, but if you load the lower arches in compression and have it's previously mentioned tenon butt against the upper arch tenon can this negate some of the tension in the upper arch? Anybody have thoughts on this? Im thinking like Thane. I see a dome or "arch" under there and all of an arch is in compression.


Mike, by the way, we just got 8" of snow this past weekend! HA!
Id also like to see the pic if you have time.

Only if the lower arch is well supported and the wall resists outward thrust. Otherwise some steel gussets at all joints will be required to hold tension. This way the wall and the truss can share the resulting forces.

More to your PM inbox, but this is a New Energy Works project from years ago. Steel framed walls with masonry, tension tie steel across the trusses, oak.