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Roof Design Question


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This question mainly concerns rafter ties on a new construction design for a 9 in 12 roof, 60 psf snow load requirement.  The footprint of the house is roughly 100 X 33, slightly less wide in the middle half.  The goal is a habitable attic space with minimum obstructions and maximum usable area.  There are interior bearing walls along the short dimension of the house every 16 to 24 feet the length of the house (a total of four plus the end walls).  The ceiling joists for the floor below will be 2 X 12 parallel to the long ridge line.  Plan has three cross gables on the front side of the long ridge line, and some dormers on the back side of the long ridge line.  The structural problems seem to be a bear.  If I use self supporting ridge beams, the spacing of the interior bearing walls means the spans between the posts (located at the bearing walls) are so large that a tremendous ridge girder is required.  Something like 7 X 16 or 18 LVL according to the span tables.  I shudder to think what this would cost for materials and installation, even in 16 to 24 ft sections.  So that brings me to using ridge boards and rafter ties.

The layout I described means that most of the rafters on the long gable are not in opposing pairs, since the rafters that join the back wall of the house would not have a rafter to tie to on the front of the house, due to the cross gables that are part of the habitable space space.  Similarly, the rafters on the cross gables would only be in opposing pairs at floor level at the front of the house, for a few pairs.  There is no way to meet the requirements for the number and spacing of rafter ties in this situation.  And even if the joist were run perpendicular to the long ridge line, they would then end up parallel to the cross gable rafters.  So how does one use rafter ties in this kind of design?

I have one possible alternative, but am not sure if it would meet code.  That is to frame the attic floor as though it was the beginning of a third story, with double top plate from the floor below,  2 X 12 joists and then 3/4 plywood sheathing, with a 2 X 6 sill plate around the perimeter of the house.  The rafters would then be toe nailed to the sill plate.  I would think a continuous plywood sheathed floor would tie the opposing walls together better than rafter ties.  At that point the only structural issue might be uplifting which would be dealt with by strapping of the rafters to the exterior wall framing.

Does anyone have any thoughts or advice about how to handle all of this?

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Right,  somebody somewhere has designed the best layout.

Also it varies with the expected max snow load, better to go up to the next higher level.

 I took my first plan to the local municipal inspector. He shot it down, told me why, approved it after the changes. Get off on the right foot with that guy, and get advice from an engineer if possible.

Usually you use floor joists to tie the long walls to the center beam. Running them parallel, you loose that rigidity where the rafters push out on the walls. I'll have to read the long text again later.

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The thing about trusses, even those without the bottom cord, is that they do not allow the required headroom over a useful area without having a very steep roof.  With scissor trusses, I think I would end up with a 15 in 12 roof, and then with only a five foot wide floor area with 7 ft headroom.  That would also put me above the building height restrictions in my area by four feet.  To stay within height restrictions I would be limited to 11 in 12, and end up with no floor area with 7 ft headroom.

Just as an aside - the requirements for snow load are way beyond anything I have ever seen living in my area for 30 years.  75  psf on the ground, which is about 1.25 feet of solid ice, or about 15 feet of newly fallen snow.  I've never seen more than 2 feet of snow on the ground in over 30 years.

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Right, I skipped over that part about the snow load in your first post.

A mansard (barn) roof design might suit your plan better.

A properly built dormer has short rafters from double headers up to the ridge to oppose the rafters on the back side.

As far as the height restriction goes, there is always a way. You just haven't found it yet. I have seen several times where what looks like a ridge turns out to be a flat strip with a torched on membrane, a gable roof with the top cut off.

You need the help of a professional, IMO. It's too big of a project for screwups.

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Thanks for the feedback.  My take away so far is that a ridge board with rafter ties just won't work for my design goal.

Most truss designs just don't give the floor area with 7 ft headroom needed.  The only truss type I can find with no bottom chord that might work is a parallel chord truss.  I recognize that eventually I have to get with a structural engineer, but before that I would like to know I have a design concept that will work.  Then the engineer can work out the details to keep the building dept. happy.

A huge self supported ridge girder remains an option, and provides the most floor area with 7 ft headroom (in a gable design).  Since the girder is built up from smaller LVL members, it is manageable at the job site without a crane.  Anyone have an idea how the cost compares to the cost of a truss design?  I've priced the LVL components alone at just under $4,000 (7 X 18 X 100 ft, in five sections).  Each member of the built up girder is 150-200 lbs each (lengths of each section vary), and would be assembled on the roof.

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