Hi all,

Looking for some advice on rafter spacing and joinery options:

I need a long roof overhang (~4') to protect the straw bale walls on the cabin I'm building right now, and what I have in the plans right now wont adequately support the loads. Right now, my design calls for 4x8 Hemlock rafters joined and pegged at the peak, meeting the plate with a birds mouth, and then reduced to 4x4 tails that can extend beyond that. The rafters are spaced 4' o.c., and these 4x4 tails at such a spacing wouldnt be able to support a 50 psf snow load. I already have the rafter stock, so I'm not looking to change the rafter dimensions. So, I'm trying to figure out something that would work.

One option would be to add more rafters and put them at closer spacing. What would be the furthest spacing (fewest rafters) to accommodate a 50 psf snow load on the 4x4 rafter tails?

Another option previously recommended was to "taper them from 4x8 at the plate to 4x4 at the ends as long as the birdsmouth was minimal (perhaps 1½" x 1¼") and the plates were notched for the rafters so that there was as little reduction of strength in the rafter as possible at the plate 'hinge' where all the horizontal shear force would be concentrated." (thanks Robert R.) It seems like that would still allow for 4'o.c. spacing.

Any thoughts? Any other ideas?

Thanks a lot,
Dan

CORRECTION: The above information is not accurate. My plans right now call for the lower ends of the rafters to be step-lapped, NOT use a birdsmouth. The dimensions of the step lap are 2"x2".

Sorry.

As you may know already, my study and area of expertise is in German framing techniques, particularly in Switzerland.

In this particular style it is very common to have extreme overhangs, sometimes as much as 8'. Their reason is the same as what you desire, to protect their walls from the weather. This allows them to get away with having board siding on their houses that is 200 years old and still in fine shape, or exterior walls made of mud.

There are 2 common methods they use to accomplish this wide overhang. The first and most common is to have the tie beams which set atop the plates extend a couple of feet past the walls and then support a purlin, and have the rafters then extend another couple of feet out past this purlin, joined to it with a birdsmouth type joint. This type of roof relies on purlins to support this rafters, which reduces the horizontal sheer.

The other option they use is to mortise the main rafters into a sill or into the joists, and then have another set of rafters joint to this which extend out past at a shallower angle.

The first style genearlly has a roof pitch of about 9 or 10, while the latter has a main roof pitch of say 10 or 12.

I can supply some drawings if you dont understand what im talking about

DLB

Hey DLB,

I understand the first style, and am already cutting the frame, so that wouldnt work, but I'm happy to learn about this detail.

I think I understand the second, but am not entirely sure. It sounds to me like one rafter will end at the plate, then you attach another to give you your tail? Is that correct? A drawing may help.

Thanks
Dan

Dan.
According to my 2005 handbook for spans, 4x4's 4'o.c. are adequate for a 50lb. snow load for a duration of two months, with the critical value being bending. With proper rafter length, cantilever rules apply (1/3 out 2/3 in). Due to the fact that most snow loads are not evenly distributed throughout the duration of load, great care should be exercised in the design and attachment of kingpins and/or collar ties to distribute the concentrated loads.
Depending on your roof covering and your design two options of reinforcement come to mind: the addition of a fascia member to connect the rafter tails and spread the loads to adjacent rafters, members could be added horizontally from fascia to beam to assist in supporting the large overhang; another option would be the application of tensioned, steel strapping to the top of the rafter, which would supplement the tension chord of the beam reducing bending.
Caution should be taken to ensure the absence of defects near the plate attachment, knots,splits,raising grains, can all reduce the strength of the member, leading to failure.
A good reference for custom applications, just adjust the values for the specie selected. http://www.spanwood.com

dan,
don't feel like make a drawing right now, just got home from work...

So here I found just a quick picture on google. The technique is in German called Aufschiebling


These examples both involve extending the tie beam out past the wall, as described earlier, but it does not have to be done this way. Instead, you can have a purlin (I guess you would call this a purlin, in the German tradition it would actually be called a sill) out past the wall, supported by angled support braces, which can be made to look quite attractive.

The two shown are the simplest form of the aufschiebling, the other that I described is a more complex variety.

Eric,

Thank you. That is helpful. I was able to use that link you posted and saw the same thing (bending being the critical value). Upon some more thought, I realized I had another option. Rather that reducing the 4x8 rafter to a 4x6 before the joinery at the plate, I could just reduce it to a 4x7.5, or a 4x7 (depending on the depth of my rafter at hand). This clearly would affect the beauty of the 2" reduction before the plate, but would allow me a 5.5" or a 5" rafter tail depth with all the joinery remaining the same. That in itself should push me a little further into the safe zone.

In your suggestion for evenly distributing the snow load over the rafters, what do you mean by "members could be added horizontally from fascia to beam to assist in supporting the large overhang." It sounds like you are suggesting horizontal members be added connecting the beams (rafter tails) to each other in order to distribute the load, but I don't understand how these additional member would be attached to both the fascia and the beams. Also, are you suggesting these members be attached in between the beams, and under the roofing?

Also, any suggestions as to where to place that collar tie as to most effectively distribute that load?

DL,
Thanks for the diagram and pictures. Very helpful indeed.

Dan

Dan,

When you get to timber spans, fractions of an inch in depth make big differences, with the change you mention the numbers get well within the safe zone. Again this is providing no deficiencies are in the area of the rafter/plate connection zone.

Typically a fascia would attach to the rafter tails ends, as a stringer so to speak tying all the tails together. If you are wanting the exposed tail look, they can be inserted between the rafters, however, care should be taken in attaching to compensate for tension stresses applied to both top and bottom chords of the members.

The additional horizontal members would run from rafter tail/fascia to the structure; supposing the overhang would project at an elevation somewhat below the plate, the bracing would need to run to a solid member such as a beam or post, depending upon your design. This bracing would not need to be done at each rafter to provide a lot of support, ends and equal spacing,or every bent would ensure your piece of mind. all of this would take place below the roofing.

AS far as locating the collar ties, I would place them as close to the bottom third of the rafter span in order to help the overhang, if a second floor is planned, keep the collar ties within the lower 2/3's of the rafter span; if you have no second floor and the truss is decorative, the collar tie can run just above the plates with a king pin to the ridge and queens to 1/2 span.

How is the weather there, we finally got some rain in this section, accompanied by some welcome cooler temps.

Eric

Structural Soffit.