Expert: Richard Taylor, AIA - 1/18/2009

Question
Hello!  I live in Tennessee and I have a very serious problem.  Hired a contractor to build a 24x40 addition.  Wife wanted an "open cathedral ceiling" - builder said no problem.  He put in a 40 foot LVL 9 inch beam across top and 2x8 rafters.  The ceiling is tongue and groove bead board.  Three months after moving in we noticed the corners coming apart and the roof is sagging.  We had structual engineer come inspect and was told to move out for fear of collaspe.  My question is - the engineer thinks we need support posts and collar ties put in but my wife waited 20 years to be able to add on to our house and really wants the "open cathedral ceiling", is there another solution? Builder is gone and won't help.  Thank You.

Answer
Good morning Gary - thanks for your question!

I'm sorry you're experiencing this very serious problem, but glad you did the right thing by getting an engineer to review it.  He was right, it may have been in danger of collapse.  The cathedral ceiling is one of those things that looks simple, but is deceivingly complex to design properly.

His solution is one option, but there are others.  To get the "open cathedral ceiling" you want without posts or collar ties, you'll need a much, much larger ridge beam.  I can't size the beam for you but to span 40' it probably need to be about twice the depth of the one you have now and several plies wide.

But that's only part of the solution.  The ends of this beam will be carrying a tremendous vertical load and need to be sized accordingly...in similar situations I've used multiple 2 x 6s or 2 x 8s and occasionally steel posts.  Very important!

Likewise, the connections between the rafters and ridge, and ridge and posts are critical.

Still not done!  The side walls - the ones parallel to the ridge beam - carry half the roof load.  They can easily bow, even with a properly-sized ridge beam.  The top plate of these walls need reinforcing, usually by a triple top plate or in some instances a "ring tie" which is a header that wraps around the top of the walls.

All of this is assuming of course that the foundation is properly sized and that the new addition is properly tied into the existing home.

The structural issues are just one aspect of this design, however; the best structual design can still fail if the insulation, air flow, and moisture-control issues inherent in catherdral-ceiling design aren't dealt with.

I'm not surprised at the solution the engineer offered - we work with structural engineers regularly and their first take is usually technically correct but architecturally inappropriate.  You need someone with a broader view to tackle this for you, I would suggest you find a local Architect who specializes in residential design.

Start by searching Architects at www.aia.org - find a few in your area and interview the.  Do your homework, check their references, and make sure you have a proper contract for the design work.

Good luck - I hope this is helpful for you.

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