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Architecture/roof truss queston

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Expert: - 1/11/2010


Question

scissor1
is it possible to make this vaulted by adding scissor supports like the picture i added
this pic is what i want to do
[IMG]http://i50.tinypic.com/3358iut.gif[/IMG]
this 2 pics are what it looks like
[IMG]http://i47.tinypic.com/35jft44.jpg[/IMG]
[IMG]http://i48.tinypic.com/14vtjlf.jpg[/IMG]
is there any measurements i am supposed to make i cant get any help from anyone please help me

Answer
Mike - thanks for your question!

Yes, it's certainly possible - you will need to do a some structural engineering to make it work safely and to code, however.

Gabled roofs - like the one you're working with - get their strength from triangulation.  That means the two angled rafters that form the "gable" shape of the roof are held together at the bottom by the ceiling joists - the third "side" of the triangle.  When the ceiling joists are removed to create the vaulted ceiling, there's no triangle and nothing to hold the roof up.  All the horizontal stress is transferred to the walls - they'll bow outwards and that's the end of the house!

Scissors rafters are an entirely different animal altogether.  Instead of completing the triangle with horizontal ceiling joists, the structure is held together by the connection between the two angled scissors rafters.  This connection is critical, as it is taking a great deal of stress.  An improper connection here will cause a complete failure of the roof!

Hence the need for structural engineering!

Frankly, this isn't a DIY project by any means.  You will need properly-sized and spaced scissors rafters and properly-designed connections where they cross AND where they tie to the middle AND the bottom of the rafters over the plate.  You will likely find that making the connection over the plate difficult - access is limited there.

You will have to install the scissors rafters before you cut out the ceiling joists, so that the roof remains intact until the scissors rafters are completed.

Finally you'll need to consider building codes.  Most codes will require a minimum amount of insulation in the vaulted ceiling; you'll need to make sure you leave room for this.  You'll also need to allow for proper venting of the rafter spaces - this is a very common place for moisture to collect and mold to grow.  You will need to allow a minimum 2" airspace above the insulation, unobstructed from the soffits (which must be vented) to whatever venting you use at the top.  In most roofs like this we use a ridge vent (CoraVent is my favorite).

I recommend you contact a local structural engineer who regularly works with houses to help you on this.  You probably won't find this guy in the phone book, I'd ask at the framing desk of your local building supply house or lumberyard.  They see architectural drawings of houses and additions all the time and know who the engineers are who do the structural work for the architects and designers.  Might cost a few hundred bucks, but it's money well-spent.

Hope this helps!

Richard Taylor, AIA
www.rtastudio.com
www.rtastudio.blogspot.com
www.rtaplans.com

Richard Taylor, AIA

Expertise

Ask me about all aspects of house, remodeling. , and room addition design and construction. Ask me about historic homes, renovation, and restoration. Ask me about materials and techniques, and about how you can get the best value for your design and construction dollar. Ask me how you can make your home a very special place. I can't, however, answer specific structural engineering questions in this forum - that's something you'll need on-site observation for.

Experience

I own a full-service residential architectural firm, and have been designing custom homes, remodelings, and room additions across the country since 1983. Check us out at Richard Taylor Architects and RTA Plans. I have written and been published extensively on the subject of residential architecture.

Organizations
American Institute of Architects, City of Dublin Architectural Review Board, Vice Chair of City of Dublin Planning and Zoning Commission, American Planning Association, Board Member Historic Dublin Business Association, Past Editor of SPLASH (a software forum), Past Editor of Open Directory Project, Assistant Scoutmaster, Boy Scouts of America

Publications
Business First of Columbus, Suburban New Publications, About.com, Cleveland Plain Dealer, Columbus Dispatch Residential Architect Custom Home Architect Remodeling Architect

Education/Credentials
Bachelor of Environmental Design (Architecture) Miami University 1983
For more about home design, check out my blog at Sense Of Place and join the conversation on our Facebook Page.

Awards and Honors
2010 Dublin Chamber of Commerce Business Person of the Year

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