Hi Guys,

So just take this as a rough plan of my thoughts on how to approach this. It's obviously incomplete (I only have one one brace drawn in, for example), but it captures the essence of what I've been thinking.

As you can see, the design does exactly what DLB says I shouldn't do; rafters hung on top of a ridge beam and also fixed to the plate. I am going to need a substantial overhang all the way around (adobe walls with earthen plaster), so I'd love to be able to extend the rafters, but A) I don't have timbers long enough to pull this off, and B) because of the hip angle, extended rafters will be in different planes on adjacent sides of the house.

Everyone now think the dragon-and-cross is unnecessary?

Capture by rannymo, on Flickr

How wide is the building?

A few observations:

First, the purpose of a true ridge beam is to hang the rafters.
The purpose of a rafter seat is to support the rafters.

Thus, doing both is redundant.

If you WANT to use a dragon beam, use the seated rafter roof. If the building isn't too wide, you can just tie the rafter pairs with a collar beam located about the top third point. If it is too wide for this to work, you can add a raised kingpost. That is, keep the collar beam up there but add a kingpost between the rafters that pulls up on it. Or bring the kingpost all the way to the tie beam.

An effective way to get an overhang is to rearrange the whole assembly. Put the tie beams on top of the plate and extend them out past it maybe a foot. The primary rafters are seated into the tie. A secondary rafter is affixed to the primary ones at a slightly shallower angle and fastened to the end of the tie beam, extending below it a ways. This is a very good way of getting an overhang. Maybe I'll have to draw some pictures.

In this assembly, rather than the end wall beam joining level with the plate, they would extend past and work just like the other tie beams. This would not allow you to use a dragon.
If you want the dragon, you can lap the tie beams through the plate rather than passing them over with a cog. Then the end wall beams would sit level with the plate and you could place a dragon on them.

Also, where you have bolsters on top of your posts and a scarf joint above, there is no need for the lightning bolt scarf. You can use a much simpler scarf and peg it securely to the bolster block. The bolster itself serves much of the function of the complex form of the lightning bolt scarf, so you can save yourself some work there.

I am not sure I understand the problem B). If you extend the rafters, they will extend naturally in the planes they are already a part of. If the hip rafters lie in both planes, extended hip rafters will remain in both planes.
Swiss houses very often have hips set at a steeper angle than the main roof, framed by extended rafters shot out over the purlins. There is no problem here.

You can use a dragon, there are other ways to do it, but it all comes down to what you want to do. It's unnecessary sure, if you do it a different way. A tie beam is unnecessary if you build the roof a different way, too. It all boils down to whether or not your overall frame design makes it necessary.

How wide is the building?

It's about 29' square.


An effective way to get an overhang is to rearrange the whole assembly. Put the tie beams on top of the plate and extend them out past it maybe a foot. The primary rafters are seated into the tie. A secondary rafter is affixed to the primary ones at a slightly shallower angle and fastened to the end of the tie beam, extending below it a ways. This is a very good way of getting an overhang. Maybe I'll have to draw some pictures.

Yes, please! It sounds intriguing, but I don't have quite enough experience to picture it.

Also, where you have bolsters on top of your posts and a scarf joint above, there is no need for the lightning bolt scarf. You can use a much simpler scarf and peg it securely to the bolster block. The bolster itself serves much of the function of the complex form of the lightning bolt scarf, so you can save yourself some work there.

That's great to know. Since it's probably going to take me a decade to build this thing, saving work where I can is going to be important for this project.

I am not sure I understand the problem B). If you extend the rafters, they will extend naturally in the planes they are already a part of. If the hip rafters lie in both planes, extended hip rafters will remain in both planes.
Swiss houses very often have hips set at a steeper angle than the main roof, framed by extended rafters shot out over the purlins. There is no problem here.

Might you be able to direct me to a picture of this method? I can't quite wrap my head around that visually, either.


And finally, thanks for all your input. If I'm going to pull this off, I'm going to need a lot of help from folks like you, and I really appreciate the effort it takes to respond. I will certainly provide you the beverage of your choice should our paths ever cross in real life!

cheers,
Randy

Here is a picture showing the kicked out rafter assembly. In German this would be called 'aufschiebling'
You could instead reverse the arrangement of the tie beam and the kick rafter, so that the tie tenons into the rafter, which extends down past it a ways.


and here is a picture I found showing a hip roof frame with passed rafters. It's not quite what I was hoping for, because the hip is at the same angle. But the concept is the same when the hip angle changes. Essentially all you do is move the top point of the hip out.


I also found this. More of a curiosity for Jay than anything else. This is a dragon in the Swiss Alps. Dragons are rare in Switzerland but here we have one!

I like that arrangement of dragons.

Don't want to dart in and muddy the waters, but...

I don't necessarily agree that redundancy is bad. We often use a structural ridge and a "seated rafter" as DL terms it. In that case both the plate and the ridge are sharing the load and that is usually a good thing so you don't need a huge ridge. As it is to my eye, your ridge looks a bit undersized. Also in this condition, you eliminate lateral loading by the rafter feet which helps keep connections simpler between ties and posts, etc. and keeps the plates from having to resist overturning or bending at their midspan.

It's a shame you can't get your overhang out of the length of your pieces. In my experience rafter tails take a lot more work and a lot stronger connection than you would imagine to meet design loads.

As you mentioned, though, overhangs with bastard hips like this get really funky trying to keep the fascia line level. When I design them I usually work from the fascia line and then try to work out plate heights, etc. you will often need to offset the hip to keep the backing depths equal on either side and sometimes you need to raise the plate on the steep side of the roof so the jacks have something to bear on since they are gaining altitude so much faster than the shallow jacks.

Interesting project.

For my purposes, redundancy is going to be desirable. The combination of my lack of experience and my shallow pockets (to pay for professional design input) means that overbuilding is my logical strategy. However, if seating rafters at the tie/plate and on a ridge beam is a bad idea for structural purposes (as DLB seems to suggest in one of his early posts), then that's a different story.

Anyhow, before I get too bogged down in asking you guys detailed questions about your individual posts, can I ask a general one? Are we in general agreement that dragons are not necessary where a ridge beam is in use? I am not tied to the notion of a dragon if it's not necessary, cool as they are.

OK, I think I get this now. I'm assuming that every principal rafter needs to seat into its own tie beam in this configuration?

And how would you deal with overhang on the adjacent ends of the house since the hip rafters won't have cantilevered tie beams to seat against? (or would they, somehow?)

Either seat into its own tie beam, or the ties carry a purlin on the end to support the intermediate rafters

At the hip ends, you would have to do the same thing on the hip rafter itself. Here you can frame a short stub tie into the hip rafter to carry the kick rafter (I don't really know the name for this in English, sorry) so that you have a triangle framed into it there. Alternately if you have a dragon arrangement like I showed, the dragon beam serves as the tie.

It all gets so complicated, there are so many different ways to do it! It's so much simpler if you have overhanging rafters.
This I think is why in Switzerland when they switched to framing roofs with seated rafters instead of hung ones, they stopped framing full hips and started making half or quarter hips instead.

with a reduced hip, the hip rafters get framed into the last jack rafter on the main roof slope instead of the tie beam, so it makes it all quite easy to do.

Sometimes you see where they made the hip ever so slightly less than full, just to simplify this. They could have used a dragon beam, but they usually didn't

Now I wanted to explain about seated rafters and a ridge beam.

I am aware of the fact that people do it, but I was taught from the standpoint that there are two systems, you do one or the other but not both.

The reason is partially highlighted above: when you have seated rafters, it complicates the whole assembly at the bottom of the roof. When rafter seating is the primary means of support for the roof, then this is all OK because the effort is justified.

BUT, when you have a structural ridge, you are inventing a whole lot of unnecessary work by seating the rafters in the plates or ties beams and, as shown here, complicating hip construction a great deal. You have to cut joints into the ties or plates that weaken them (or require you to use bigger timbers) when you could just as easily lodge the rafter on these timbers and spike them or peg them fast. The rafter seat really adds nothing in this case because of how the rafter is loaded with a structural ridge. You do not need a joint that can resist tension at all, you only need to resist compression which is easily handled by lodging things. In fact, a birdsmouth or other rafter seat is actually less effective under compression than if the rafter were simply lodges. The upward lifting force of the ridge can potentially throw the rafters out of their seats. I always heard, a framed roof will work and for this reason, the old timers never fastened the rafters to anything else if they were hung from a ridge. They seem to have had the right idea, since the pictures below are of a house that has stood strong since about 1710.

To see how simple hip construction can be with a structural ridge, see this image:


Even if you can't use rafters projected past to form the overhang, you can use rafters seated on the outside of a plate or purlin that come just past it, then tack an extension beside them to obtain an overhang, secured up the rafter a good ways with spikes or pegs.

This roof structure, by the way, has steeper hips than the primary roof slopes although maybe it is hard to tel by this picture.
This structure is about 40 feet wide.