how to frame a "dutch gable" ??

[wastelandStan]

hello, i joined nearly a year ago and after reading many threads and some of the recommended books, i decided to design a small cabin from Beemer's example in "learn to timberframe."  i increased the size to 13' x 20' (which he says can keep the original 7x7 posts).  i added a loft (his recc'd 5x6 joists) and now im onto the roof.

this is meant to be a medieval house, and so im after the iconic "dutch gable" (i saw those threads about it really being called a gambrel).  heres where im at after modifying from Beemer's example roof.  I changed the pitch to 55 degrees, and i would like some advice on whether or not i should use a ridge beam, or i can go without on like the simpler example that Beemer uses.  Or anyway advice on how to frame this, anyways... here it is!













thanks for looking

[Don P]

That's pretty cool looking, can you zoom on the joint area at the top of the hips, click on x-ray... I'm curious how you're dealing with that busy area.

@Heartwood

[Andries]

Starting to sound like a medical situation . . . 

[wastelandStan]

I'm curious how you're dealing with that busy area

ive not sorted out that joinery yet, lol.  im not sure if such a thing would work, with those short rafters set onto a stout collar to form that triangle window, so i havent bothered to model it quite yet.
the other option i was thinking was to have 2 hefty trusses to form those triangle windows, whose collars would seem certainly stout enough to bear the short rafters.  but this starts to depart into unknown territory (for me). not sure if it would necessary...or would significantly change the design, i i would rather not change the major wall post design

[wastelandStan]

Starting to sound like a medical situation . . .

lol  well here is the situation



all the rafters (jack rafters?) are 5x5", except the corner ones are more like 5x7" with the top corners shaved to match the roof deck

the rafters are all seated in the plate, even the corner rafter....is that ok?



the corner rafter then has a bit of a scallop shaved  to match up



those 3 short side rafters are sitting on a 5x8" collar.  i made it so fat to create some space between it and the corner rafter

this yellow line is 1.4", what do you think is it too close?






so i have 2 collars, the lower for bearing the rafters, and the upper will be the sill for a triangle window

[Chilterns]

This design is neither Dutch nor Gambrel instead being a "sans purlin" half hip roof that does not have wind braces. The last full height set of rafters are known as "singles" and usually these are the only sidewall rafters that are pegged to the wall plate. The roof as currently illustrated is not stable and hence the addition of a ridge would effectively join together the 2 half hips allowing any thrust received from either half hip to be resisted by the other half hip thereby effectively reducing the possibility of roof racking.

The collar that has been added should be moved up closer to the singles apex to support the ridge. The common rafters (not singles) do not need to be jointed at the apex but fixed side by side and pegged to the ridge thereby causing the rafters to push inwards rather than outwards on the wall plate.

The dropped tie shown is not typical of medieval design but more likely a "square rule" carpentry invention.

The challenge with half hip roofs is whether the corner rafters should be framed in the same plane as the sidewall rafters or the half hip rafters ? Historically both practices can be seen. 

[wastelandStan]

Thanks for the post, this is the sober reality check I need
I'm not sure how to support a ridge beam, as I really want to keep the 2 bay wall layout.  The ridge is 13.5" long, I could send a king post up the middle, but for the ends....Do trusses need to be directly over wall posts?
How about skipping the ridge beam and adding 2 purlins like this?

[Don P]

This roof was rattling around in my head, I found an article, it took forever to download on my connection so beware but there is a neat little building that is kind of similar. On pg 103 , the kitchen at Little Braxted. They refer to it as possibly a smoke gablet.
Layout 1 (esah1852.org.uk)

[wastelandStan]

Smoke gablet! Yes that's exactly what it is, thanks for the link

[Don P]

I was looking for instability and I'm not seeing it. Looking at the triangle formed by the hip to the "king common", that is a wind brace and there are 4. on this sized building the roof sheathing is fine for transferring the gable wind load cross the sheathing to the far end hip rafters to resist wind or thrust;

Thrust. The commons at 40 psf vertical load (adjust for your snow load) are going to push outward against the ties with around 1300 lbs of force. The center tie connections to the posts need to be good for better than about 700 lbs, not hard.

The hips appear to be supporting maybe 10 sf? That area is busy but I doubt you are in trouble there.

This isn't giving me heartburn and its cute as a button.

[Chilterns]

[wastelandStan]

Great pic, thanks.  So the ridge beam is resting on, or is notched into, the collar of the singles, is that right? And the singles  look like the same size as the commons?

[Andries]

Here is a photo of the roof over my workshop:



Locally known as a Dutch hip roof.
The nomenclature. What is used in Oxford, Canada or the US ?
The photos really help, thank you!