Hello out there,

I got a question. I'm trying to design a 4 hipped roof with a very steep common rafter pitch. Im thinking around a 24' run along each side of the square in plan.

For sake of discussion lets disregard what is below. Just say its concrete wall.

If the hips are 12/12 that puts the commons around 17/12. Well how do you roof such? I have the hips, pin, chords, internal bracing figured, but then you have the framing between each hip in the roof plane that is the problem.

If you use jack rafters the height dimension becomes so small due to the plumb cut driving the x-section of the hip.

I have thought about purlin down the roof from the apex until the purlin can't span between hips anymore without too much deflection.

BUT THEN, you are stuck with maybe running rafters at that point but they have to land on a purlin.....

Anybody know what this jumbled mess of typing on my part means, and anyone found a solution for this when they have come upon it?

I have to put in a picture for clarity I think.


Even small purlins rely on little relish below the bearing surface.



Thanks.

Hi Mo,

I confess that I am struggling a bit to understand your question.

I would make the comment that you should draw in the centre common rafter in each roof plane and then set your purlins at 90 degrees to this rafter letting the purlin intersect with the hips. This way the centre cut rafters will meet the purlins square on in a similar manner to which a roof gablet collar is employed to provide landing space for centre hip rafters. All other rafters can be cut as jack rafters.

Will you nail the upper small purlins into position ?

You might also want to check out English Historic Carpentry for ideas. You might be able to draw some inspiration and guidance from Hewett's illustration of York Minster chapter house roof.

Does this suggestion form part of a possible design solution to your question ?

Regards

Ken Hume

what is holding up the roofing material?
and what is the roofing material?
are you dealing with heavy snow loads (maybe metal roof?) or shingled?

i think what is on top is just as important as what is holding it up.

if you are using 3x (or 2x) decking, why not split that 24' wall into 3 or 5, then go evenly spaced jack rafters? you'll most likely need to reduce the end of the jack that hits the hip with a curve or angled reduction.

and note: this is completely theoretical, assuming that your plates and or wall combination are stiff enough to take the outward thrust of the roof... which will be pretty tricky in those corners, and with your cross tie 1/3 of the way up the rafter you'll need a fairly deep hip to take the bending moment... as you are essentially building crossing king post trusses, no? (unless there is another crossing tie / beam @ plate height - which may mean that your jack will fit nicely in the hip)

and yes, in some cases you end up chasing the size of the hip round and round. at some point you can make it larger that needed to house joinery, or make it just large enough and reduce the adjoining timbers.

and, there's nothing saying you can't lower the hips away from the roof plane - either by a bit to leave a flat on top and sneak out some extra relish, or by alot so that your purlins or jacks overlap the top. when done well this only further expresses the structure.


and personally, i hate the way purlins 'flatten' a space like this. much prefer rafters here - and they are easier to install.

Hi Ken and Mike,

Ken, yes what you suggest makes sense and I have already messed around with my model to that end.
-purlins between hips with a common rafter
-jack rafter the rest

The purlins would be housed and yes spiked, or more likely screwed from above on a slight angle.

Mike, what the specs say for now is 5/4decking, furring above that for insulation, sheathing, wood shingles.

When you speak of lowering the hips, would you back the hips or not? If you used jacks that sat proud of the hip and thus created the true roof surface, how would you fasten the decking over the hip? Its an interesting idea, and I agree it sounds good looking. More please..

As far as the section problem of sticks, I'm thinking that there is more of an axial load on rafters and such (in the roof surface) rather than bending. I would think smaller sections would work than if the slope was less. I guess this reduces thrust as well.

Thanks guys,

Moyer