Gooday,
I was wondering in a principal-rafter and common-purlin roof, say with 4 bents, how would one create both a gable overhang and eave overhang , say 2'6", if one were to use 2x6 planks with rigid foam insulation above. Does anyone have a suggestion, or with this design would I have to use SIP panels in order to create the gable end overhang? Thank you for any input
Tucker

Are you planning on a soffit box on the side eve? For the gable ends pass the 2x decking the needed distance beyond the gable ends. Or better yet, don't do that just extend the strapping on top of the foam as the extension, allowing you to tie the side wall insulation into the roof top insulation.

I am starting to double strap the roof, two layers of strapping, so you get extensions on gable end eve.

Are you foaming the walls as well?

How important is the 2.5' of overhang?

Here's a detail I came up with and used on our house. A continuous layer of xps/polyiso around the entire frame, no thermal bridges, solid and super air tight.

[IMG]http://img.photobucket.com/albums/v190/frwinks/roofconstruction-1.jpg[/IMG]

This post is meant to understand different framing options, Im looking at ideas on how to frame overhangs on both the gable and eave. Both of you suggest the same thing, having two layers of strapping which extend beyond the frame which provide the overhang. I guess the distance one can extend is based on the spacing of the strapping and the roof loads. I like the detailed drawing frwinks sent, Im wondering now if one needed a soffit box to stiffen the eave overhang, if 2'6" were to be desired. I guess now it comes down to engineering. I do prefer the purlin roof for this design im working on, as the timbers can all be cut on my mill and I wouldn't need to scarf any joints. If, of course, the overhangs can be solid enough.


to answer your question:
-the 2'6" overhang is necessary
-and the walls would be a combination of fiberglass and foam, both on the outside of the frame

Thank you for your help
Ian

Hey Tucker,

Those above options both seem to give you a gable overhang, but there are other options as well.

You can extend your plates past the end wall posts by however much you need it (2'6"), then add another pair of rafters. It depends on you roof loads, but then you likely would need braces coming from the plate ends back towards the frame into the posts to support the roof load above. This option can work well, and does a good job solving the structural problem, but then you're left to deal with air sealing around any member that's passing from inside the frame to outside. I think you might also have to check with your local building code, as some states frown to interior structural members being exposed to the outside. However, even so, there seem to be options to getting over that hurdle.

2'6" is a lot to ask out of a SIP. As far as I know, SIPS cant overhang that much unsupported.

You can also look into roof brackets, which can support a big roof overhang, but then you're looking at the same air sealing problem as mentioned before. This time though, the brackets are purely exterior members.

A cross-hatched roof works well, which gives you an overhang by putting wide boards on edge and cantilevering them beyond the last gable. This, however, wouldnt work for your current roof system.

Hope that helps,
Dan

I'd raise the purlins up, and run them long. It's traditional and it takes less meat out of the principals. Air sealing would still be an issue, but using fiberglass is a more significant air-sealing issue.

I like to enclose the whole frame within the layer of foam to avoid the problems mentioned above, air leakage and such. The method Brad, timberwrestler mentions is one I favor as well, in this case I would dedicate the roof to attic space not living space. Insulate the ceiling instead. All these approaches should be considered before things are set on stone. It is kind of late to make decisions after a certain point, at which time compromises are made.

One other way to look at it is that the purlins are not raised up but the principal rafters are set into the building. This gives more relish on the ends of the tie beams where the rafter sits upon the tie.

Hi Tucker;

I have only seen a few buildings with a common purlin roof system with gable overhangs here in northern New England. In the examples I have seen the purlins are trenched into the principals with the tops flush and the gable ends the purlins simply cantilever a foot or so to form the overhang. The projecting part of the purlin is called a lookout: See figure 29, page 8 of this link.

http://www.tfguild.org/joinery/part5.pdf

On the sidewalls the tie beam can cantilever over the top of a post or an English tying joint with what I call an eaves purlin like this:

http://i121.photobucket.com/albums/o223/jimderby/th_IMG_0667resized.jpg

Or frame the overhang separately such as using "sprocket", see figure 8, page 3 in the same link as above:

http://www.tfguild.org/joinery/part5.pdf

In French work this sprocket is called a coyau and can be set at a lower pitch producing a bellcast (bell-cast, belcast) roof.

I have not seen a historical example but, at least in Eastern work, their are rafters called fan rafters which angle out to frame the corners of the roof overhangs. I do not know of a Western example to show you but you can see some fan rafters at this link:

http://thecarpentryway.blogspot.com/2010/10/fan-of-fan-viii.html

Good luck;
Jim

I think that if using common purlins, framed roof overhang is kind of unusual.

Which is why I don't use common purlins. ever.

But that's what you have, so I realize that anything else is not an option.

But it is my understanding that a principal purlin roof can accomplish overhangs much easier.

get ok.