Pyramid Framing with Chimney 1

[jlleiber]

I am working on a relatively small (14'x14')sunroom style addition where the owner wants a 4-sided, pyramid hip roof with a vaulted ceiling on the interior and no ceiling joists. I was originally going to do the standard pyramid style wood roof framing and detail it to use the wall top plates and fascia boards to resolve the thrust in the corners. However, there is a dual-sided brick fireplace on one wall and the owner has requested that it continue above the roof line, which will interrupt my top plates on this wall.

Is there a way to frame this using conventional lumber? The fire place will be a gas insert so potentially I can have them run a strap or 2 all the way through the brick to connect the top plates. Or do I need to design a rigid kinked wide flange beam to put diagonally across the roof that does not introduce thrust into the walls? If I go the wide flange route, can I get by with just one kinked beams?

I have attached the exterior elevation and a very rough framing plan.

The other thing that might help is the contractor suggested "boxing" out a section on the interior where the roof meets the walls so we can hide all the connections. Think like a vaulted tray ceiling. The contractor also wants to put a "flat" section of ceiling at the peak too so I can hide connections in these spots.

Just looking to brainstorm the cheapest/easiest option to build. Thanks

 

[TheDW]

I've not had to do it, but I've been curious if it would be valid to rely on the roof sheathing for something like this to resist the thrust...if your diaphragm is a beam spanning from wall to wall and is installed properly, I don't see why it couldn't, provided you've got plenty of extra shear capacity...you've just got dead and live load applied to your diaphragm now.

 

[KootK]

- I like the kinked beam idea and believe that it would be possible to get by with one.

- If you could get a sufficiently tall heel height, you use a parallel chord wood truss with a flat bottom near the top. That's probably a prohibitively tall heel height though.

 

[jlleiber]

Daywalker - that’s kind of what I’m thinking. It’s a small roof so I’m thinking the decking could resist the thrust or even use straps across the corners where the hips meet and get them into a couple jack rafters on each side. Then for good measure, beef up the corner connections and go ahead and anchor both walls into the brick chimney. That combined with beefing up the fascia board corner connections and even anchoring the nearest jack rafter into the chimney should give it adequate stiffness to hold the pyramid together. If I get enough roof pitch, it’ll be even easier.

KootK - thanks for the input. And I agree that I don’t think I have enough heel height to make that style of truss work

 

[dhengr]

Jlleiber:
Wasn’t (isn’t) there a bldg. code prohibition against bearing of (mixing of) a wooden structure on a masonry chimney? There are two things at issue, fire rating/clearance issues and issues with different bldg. materials and differential deflections or movements. I would expect the roof sheathing diaphragm to take or distribute a good share of the lateral thrust at the roof bearing elevation, but you still need a tension member in the plane of the ext. wall and up at the elevation of its top plates. You do the exact detailing, but I would move the entire fireplace masonry structure outward (to the left) a foot or so, so that the vert. plane of that exterior wood framed wall fell just to the left of the interior masonry veneer plane. Then, you could get a vert. masonry layer/wythe to the left of the tension/bending member at the top plates, for fire rating/proofing needs. To the left of that vert. masonry layer/wythe, you would have vert. space for the two flues, and then the outer (exterior) vertical masonry wythe. And, without really looking at the code, can the flue sizes be somewhat smaller for a gas fired flue, than they would be for a wood fired flue?

 

[jlleiber]

dhengr: That is a great point about not wanting/being allowed to bear on a masonry chimney. The contractor already wanted to ledger into the existing exterior brick wall for rafter bearing and told him he wasn't allowed to do that. Excellent idea of sliding the entire chimney out so that I can run the wood wall in front of it and just create a "window" into the fireplace. Is it still a good idea to use brick ties from the chimney to the wood wall? Also, I believe they want some kind of stone hearth on the inside and outside but they can just construct the interior one in front of the wood wall and I can easily frame the floor joists around it. Or if they don't build it up from the foundation, they can apply a stone veneer to the wood wall.

After the homeowner chooses a see-through fireplace, I will figure out how large the chimney needs to be for the flues.

 

[BridgeSmith]

Some people would consider some small steel rods crossing the space to be aesthetically appealing. Maybe you could talk them into adding some tension ties of that type.

 

[jlleiber]

Hotrod - we discussed that option as well and the homeowner was OK with it but I don’t think it will work very well since they also want a ceiling fan in the middle of the room.
Unless running the cable around the perimeter can resolve the thrust and act as my tension ties?

 

[BridgeSmith]

I would think that running the cables from the corners of the chimney either straight across or to the corners across the room would what you would need.

For a ceiling fan, I think you'll want something to stabilize it anyway, since you'll have to drop it farther than a typical ceiling fan mount is made to accommodate. Without bracing close above the fan (or multiple hangers) it'll dance all over the place when it gets off balance (and it will). I envision cables or rods running straight across the room from the framing at the corners of the chimney, with a couple of cross ties between them in the middle, passing on each side of the drop tube for the fan, with a small stabilizer plate (or mounting plate) tying the cross ties to the drop tube, with the fan mounted directly under.

 

[jlleiber]

Ah, I see what you're saying. Sounds like a pretty industrial/modern kind of look too, which I think the homeowners will apprecaite. And although the thrust goes out to the corners through the hip ridge beams, I should be able to tie everything together well enough to get the tension over to the cables/rods. Does it make sense to run those cables across the other direction of the room at the same spacing for a symmetrical look?

Is there enough rigidity in the ties and cross-ties system to hold the fan in place? If I understand you correctly, the stabilizer plate would be there for the fan drop rod to pass through. I guess you aren't trying to hold the fan rod completely still, rather keeping it from large swinging motions when it gets off balance?

 

[BridgeSmith]

You could run cables or rods in both directions if you want (or the client wants), but as you said, the thrust can be carried to the corners, so it doesn't seem as if they're really needed, other than on the side where the chimney creates a discontinuity.

It won't take much to keep the fan from wandering. I would probably fit the plate with a rubber grommet around the drop tube. To give it a little more visual interest (and ensure that it wouldn't move), you could pull the cables in at the cross ties (make the cross ties shorter). That would work with cables, but probably not steel rods.

Steel rods, are usually in a dark brown or black, and would have a less industrial, more rustic look. It depends on what look the client is going for. I've seen rods with turnbuckles in the middle threaded eyes at the ends, too, either fully threaded or just at the ends and turnbuckles. It's the kind of stuff you normally see in log or timber construction, but you'd probably want to scale it down for this application.

 

[boo1]

consider a vent less fireplace?

 

[jlleiber]

A vent-less fireplace did get brought up at the last meeting but was ultimately dismissed (can't remember why). But the homeowners also prefer the chimney look

 

[HouseBoy]

I agree that the decking MIGHT resist the thrust but (when I come across this issue) I think the connection details might not be so clear cut and I prefer to use a structural system that I can easily verify when possible. (Don't we all?) Nothing necessarily wrong with other approaches, just relating what I have done in a similar circumstance.

Anyway, I did almost the exact same thing a while back.
I used a steel flitch up and down both hip ridges with flat steel plates on top of the walls. Maybe one would have worked but this seemed more reliable to me and the contractor was on board with the idea. I had actually done the same thing with him on another addition several years earlier on the same house (same architect too).

A few pics are attached.

Steve

 

[jlleiber]

HouseBoy - thanks for the photos and input. Couple questions - since you used flitch beams up and down the hips, were they not rigid enough to eliminate the steel flat plates on top of the walls? Or are you not relying on it being that rigid? Also, I don't see bolts through the 2x boards on either side of the steel plate. How did you connect the 2x boards to the flate plates?

 

[HouseBoy]

The steel plates on the top of the walls were actually the anchorage for the thrust of the flitch frames. I think they were 2 feet long and bolted or screwed to the top plates of the walls. The flitch pieces were welded to the top of the "big L" shaped pieces.
The architect didn't want to see the connections for the flitches so the contractor countersunk and plugged all of the fasteners.

 

[CANPRO]

HouseBoy, cool pictures, thanks for sharing. Did you connect the steel plates at the peak of the roof?

 

[jlleiber]

The steel plates on the top of the walls were actually the anchorage for the thrust of the flitch frames. I think they were 2 feet long and bolted or screwed to the top plates of the walls. The flitch pieces were welded to the top of the "big L" shaped pieces.
The architect didn't want to see the connections for the flitches so the contractor countersunk and plugged all of the fasteners.

Makes sense and very slick application. I have designed a flitch beam for this contractor before so I'll go this route too. Thanks for the input!

 

[HouseBoy]

CP - One set of plates was but welded at the peak and the two perpendicular plates were fillet welded to that.