Expert: Bruce E. Johnson - 11/28/2008

Question
QUESTION: Hi Bruce....Bear with me and I'll try to explain what I am pondering. Ok,I have an existing bungalow (1 1/2 story) that I will be renovating, it is conventionally framed. The 2nd floor has low ceiling clearence, about 7' at the "collar ty's ", with a 8/12 roof pitch. I was thinking about extending the plane of the roof on one side of the gable which would reset the ridge higher and to the side of the existing ridge to allow more headroom. I do not plan on necessarily going all the way to the existing gable ends but I am planning on maintaning the same pitch off the new ridge and creating a "bump out addition" down to grade to accept the shifting of the ridge while keeping the same soffit elevation. I have reviewed and accomplished dormers but they don't go higher than the existing ridge. Make any sense ?

Bill

ANSWER: Hi Bill, what you are describing is basically a modified "saltbox" roof design.  A saltbox is basically a cape cod roof on one side of the ridge and a shallower pitched roof on the other side of the ridge.  I call your idea a "modified saltbox" because you are actually going to move the ridge from the center of the roof and then shallow up that side like a salt box.  This can be accomplished providing you use a structural ridge beam or ridge board that is supported at the ends.  Your shallow pitched side then uses a structural wall at the exterior to support this side of the roof.  Over the last few years we have been using LVL ridge beams for structural ridges.  These are laminated beams capable of longer spans than conventional lumber.  These LVLs can be purchased through local truss manufacturers or lumber companies.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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QUESTION: Bruce, To follow up the roof will have the same pitch just a higher ridge.  Normally a dormer addition doubles the rafters at the sides of the dormer to carry an inboard header for the new framing. Can I beaf up the rafters on the sides and put a vertical support up to the new ridge beam

ANSWER: Hi again Bill, I'm not sure what you are asking here.  Let me explain a little bit about your roof framing.  Right now your rafters meet at a peak or a ridge board in basically the center of the house.  To make that ridge higher you need to offset your ridge toward the high side.  This would mean that your existing rafters on both sides will no longer work.  On the short side you will need to make those rafters longer somehow or replace them with longer rafters.  I suppose you can extend them with like size extensions nailed into the sides of the existing rafters.  Okay, now on other side of the roof your pitch is going to become steeper because your bearing point at the outside wall isn't going to change but your ridge location and thus your "span" for that side decreases.  So your pitch will be different on this side.
As far as supporting your ridge beam.  I don't see how you can support the beam with the rafters it is supposed to carry.  Even if you come in a ways on each end and keep the existing roof on each end to shorten the length of the ridge beam required you would be tranferring your entire roof load on two sets of rafters?  Theoretically all your rafters, collar-tied together, will support a normal ridge board. The problem I can see with your design is that the rafters on the longer span size no longer will reach the new ridge without being extended. Once extended the roof system will be weakened by the extensions and the balance of the support will be shifted off center.  That is why I suggested a structural ridge beam as opposed to a simple ridge board.  The structural ridge beam becomes the support of the rafters instead of the rafters supporting the ridge.  I'm afraid you will need to consult a structural engineer to have him work out the details but it seems to me that you just need to re frame the entire roof with a new pitch and new rafters.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com



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QUESTION: Bruce, to clarify, the bearing point on the outside wall is going to change, I plan on an addition on that side of the house that will match the new roof pitch apx. 6 to 7' off the existing bearing wall if I shift my ridge 4' with an 8/12 pitch. I also plan on a structural ridge to be able to extend the existing rafters (sistered). The new structural ridge bearing will be short of the existing gable ends therefore supporting the new beam would have to be off an existing (modified?) rafter/joist "KING" truss W/  modified shear bolts at the rafter joist connection. I have drawings and engineering data for adding a structural ridge at the existing ridge intersection but my main question is can I modify the king truss to load it down one leg of the rafter in order to acheive the offset. I see diagrams of standard dormers w/ an inboard header that loads one leg of the rafter/truss down from the ridge so why can't I load at the same point and go up to a higher ridge? Sorry, wish I could send a drawing

Answer
Hi again Bill, if you are going to provide a girder truss or some other structurally capable method of supporting the posts to the new structural ridge beam then yes it is possible to do this.  My only point was that it would need to be designed properly to support the weight. The girder truss would be designed with a point loading at the spot where the column or post is to be attached.  You could probably even get this girder truss built with a post already integrated into the design so all you need to do is set the ridge beam on top.  Or the girder can be built to mimic the shape of the new gable ends and the ridge beam hung on it with a bucket. I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com