joist vs. rafter cantilever (overhang) on a hip roof

[charles]

This is something of a theoretical question:

The general rule for joist cantilevers is that the joists must extend at least twice as far inside of their support than beyond it. Does the same rule apply to rafter overhangs? If so, the overhang on a stick-framed hip roof would be severely limited by the fact that the hip jacks are so short near the hips. Just how wide of an overhang is safe for a stick-framed hip roof?

[TSJHD1]

Re: joist vs. rafter cantilever (overhang) on a hip roof

This is something of a theoretical question:

The general rule for joist cantilevers is that the joists must extend at least twice as far inside of their support than beyond it. Does the same rule apply to rafter overhangs? If so, the overhang on a stick-framed hip roof would be severely limited by the fact that the hip jacks are so short near the hips. Just how wide of an overhang is safe for a stick-framed hip roof?

Well, it's more than a theoretical question, but one that's hard to answer with some 'rule-of-thumb' type answer.

The joist rule is a poor way to determine a floor cantilever. I learned it too, but it's just too general. The joists may be sized to carry the floor spans, but when they're stuck out in the air with wall and roof loads on top of them, they might be undersized even with the 2-to-1 ratio rule. And forget about using any of that when framing with I-joists.

Rafters don't have the wall and roof loads placed on them, so the cantilever amount can certainly be more liberal. How much more? No more than a Frank Lloyd Wright roof. :) Ever seen these? They look nice-they're quite cantilevered, but any more than that might start to look weird, imo. And they're hip roofs. So it can be done.

I know that load-concerns aren't the only thing to get right. But the roof doesn't carry like a floor does, so they're not a huge issue. Look at wind uplift issues, and some creative framing to support the small jacks. I have made the sub-fascia stronger at corners, with mechanical connectors at the hip, and that has worked well. And the hip size definately needs to be looked at in this situation.

I would get something like this designed by an architect, or sized by an engineer, then make sure the inspection department approves it before it gets built. And of course there's always trusses.

Tom

[George Roberts]

Re: joist vs. rafter cantilever (overhang) on a hip roof

charles ---

I believe the joist rule of thumb is based on equal deflections ratios (d/L) for the main portion of the joist and the cnatilever.

For roofs, the deflection ratio of all the tails will be the same regardless of the rafter length.

The rafter tail is limited by the nails at the hip supporting the moment due to loading the cantilever. At 50 pounds/sqft 3(?) nails at the hip/rafter connection near a corner should support 2' of overhang.

[TSJHD1]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Sounds to me he's looking for an overhang bigger than 2'.

Tom

[George Roberts]

Re: joist vs. rafter cantilever (overhang) on a hip roof

TSJHD1 ---

I just ran some rafter numbers. I can cantilever 1' for every inch of rafter depth.

5.5' for #2 SYP 2x6s @16", 8/12 pitch, 40pound/sqft live load, 5 16d nails at rafter/hip connection.

[Sweep8]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Use 3/4" plywood sheathing and double the jack rafters 48" away fron the corner and use metal connector plates at the hip rafter. The hip rafter overhang should support the corner. You could even double all of the jack rafters. The length of the overhang would depend on the code required loading.
5.5 ft. overhang with 2x6's ? I don't think so.

[George Roberts]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Sweep8 ---

I guess that is why you are an architect and I am an engineer.

I gave all the numbers I used in my computations. By plugging them into the correct engineering equation, you can verify my result. (I guarantee you will use the wrong equation.)

With some 1/2" plywood I can kick the overhang up to 7', but I don't engineer that close to the edge unless I supervise the construction.

[Sweep8]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Sweep8 ---
I guess that is why you are an architect and I am an engineer.

Bad guess. After 2 years of studying structural engineering I decided I wanted to be responsible for more of the design of a building than is usually given to an engineer.

I would limit the span of an overhang not only by the maximum deflection at the eave but by the long term permanent deflection that I thought the eave detail and the client would tolerate. This is a judgement call typical of many required of an architect in the design of a house. I would not be comfortable with 2x6's @ 24" o.c. cantilevering 66 inches especially on a hip roof.

[Joe Carola]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Sweep,

Are you guys talking about a 66" overhang with a level soffit or open cathedral soffit? Level soffit you can go 66" with no problems building up the level soffit framing with angled 2x's going from the level soffit to the bottom of the jacks to support the overhang or using plywood gussets.

[George Roberts]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Joe Carola --

I did my math assuming an open cathedral soffit.

But you are correct a level soffit will add strength to the overhang.

[charles]

Re: joist vs. rafter cantilever (overhang) on a hip roof

George, I too have a hard time accepting that you can overhang 1' for every inch of rafter depth. Seems totally implausible. Can you get 9' of overhang if your hip jacks are 2 x 10? The 2' you mentioned earlier sounds much more plausible. What impact does roof pitch have on the calculation? How does 1/2" plywood help?

Tom, I have seen a few FLW houses and would never trust his engineering skills. Seems like most of his roofs were flat, not hip. How do you make the sub-fascia stronger at corners? For purposes of this discussion I'd like to conservatively assume single-ply (nonstructural) hips. Let's also assume a live load of 20 psf and dead load of 20 psf, and an open soffit.

Sweep, how does 3/4" plywood help?

[Sweep8]

Re: joist vs. rafter cantilever (overhang) on a hip roof

Sweep, how does 3/4" plywood help?

I believe the original concern was the short backspan of the jack rafters near the corners of a hip roof. Since the jack rafters farther from the corner are longer and work as cantilevers, thicker sheathing can span the distance from those long rafters to the hip rafter since 3/4" sheathing can span twice as far as 7/16" sheathing. The thicker plywood also distributes more of the cantilever backspan forces diagonally upward to the larger roof area where full sized common rafters span to the ridge.

This is only speculation on my part; I've never seen it done. Very large roof overhangs, open below with no boxed or enclosed cornice would be considered strange in this area since they would be unlikely to survive the first winter due to ice storms or freeze-thaw weather cycles not to mention the awful appearance of downspouts returning to the house wall.

[George Roberts]

Re: joist vs. rafter cantilever (overhang) on a hip roof

charles ---

I used the size of the rafters not the size of the hip. Yes, 2x10 rafters will allow for a 9' overhang.

I don't know how pitch affects the results. I only do 8/12 pitch roofs.

I never include the plywood in strength calculations on new construction. But 1/2" plywood glued to both the top and bottom of 2x6s decreases the deflection by 50%.

[charles]

Re: joist vs. rafter cantilever (overhang) on a hip roof

9' is just incredible. Are you talking about common rafters? I'm talking about the shortest hip jacks right up within 16" of the hips. The amount of torque that a 9' long lever would exert on the hip is just phenomenal. Let's see: tributary load on the shortest hip jack would be 9'*1.33'*40psf = 479 pounds. The torque arm is 9' long, so the torque is 479*9 = 4311 ft-lb. Now divide by about 1.33 to get the shear force on the shortest hip jack/hip connection, comes to 3241 lbs. 5, 16d commons have a shear capacity of 1105 lbs. Something's not right!

[Timbersmith]

Re: joist vs. rafter cantilever (overhang) on a hip roof

We always use a structural fascia or a "beam" (usually the rim) in the insulated cavity above the decking to carry the short jacks.