Expert: Dan Griffin - 11/18/2009

Question
What is the best way to attach two roof together one has a 6/12 pitch the other has a 4/12 pitch and they are 90 degree from each other

Answer
Dennis, you will have several problems not keeping the same pitch on the two roof lines.  The valley lines will not be 45 degrees in plan view, not a deal breaker.  Jack rafters will have unusual angles and lengths.  Eave overhangs will be different.  Shingle joints will not line up.

You don't tell which has the higher ridge or which was the original.  It sounds as if you already have the two roofs up, just not tied together.

Let's assume that your original roof has the higher ridge.  Install a partial ridge using several common rafters on the lower roof line or install trusses on the new work.  Use a string line, straight edge or other means to extend the ridge line of the new work to the existing roof.  This single point becomes the apex of the angle that is formed at the top of the valley lines.  The eave line of the original roof will intersect the new eave line at one point on each side of the new structure.  Make sure you are allowing for any sub fascia, finish facia, etc to determining the point where the two do meet.  You will also want the horizontal eave height to extend to the new section.  This point now becomes the bottom of the valley.  This will determine the size of the soffit (it will be different than the original)  It will also determine the soffit elevation, and I assume you made the correct choices about the tops of any windows and doors on the new portion to work with that soffit height.

Now you can snap a line on the original from the apex of the valley to the bottom of the valley on each side.  You can extend the ridge to the old roof.  The jack rafters need to run from the ridge down to the line and continue the rafter spacing that you started on the new work (16/24/whatever) to accommodate the roof sheathing.


It is possible to have a situation where the eave heigths do not match, but this will cause more problems, and it will be best to make them match if possible.  This tie-in is plenty hard enough when the pitches match, it just gets a bit trickier when they don't.

I hope this gets you off to a good start.