Expert: Bruce E. Johnson - 9/16/2009

Question
The roof I am installing over my deck will have a 3" (plus or minus 1/4") slope in 15' and I am using 2X8" deck joist on 12" centers and would like to know if I can put the joist in level and then rip a 2X4" on an angle and nail on top of the joist to achieve the slope.  That way the ceiling will be level.  I thought it would be easier to build up the joist on top rather than do it on the bottom but want to make sure it is okay.

Answer
Hi again Jerry, I suppose you could do it that way.  Generally the ceiling is what is level and the rafters slope.  If you are doing a built up roof you can sheath the flat rafters with plywood as normal and then install tapered insulation on top of that to get your pitch.  A local roofing supply company can custom order tapered insulation to fit your deck. Another option would be to install ceiling joists inside and level the ceiling that way. By building the taper on top of the joist you set yourself up for some structural issues.  The lateral stability of the roof deck is dependent on the attachment of the sheathing to the rafters.  This creates a diaphragm of the entire structure and braces it against lateral or side forces.  When you put a tapered board on top of the rafter you now have a rafter that has a "hinge" on it where the two pieces are attached.  Now, when you attach your sheathing to the tapered piece the diaphragm is on top of the hinge so when you get a lateral force like wind against the wall of the addition theoretically the rafter can move sideways while the tapered diaphragm stays put.Similarly if the uplift on the structure is sufficient enough the tapered top could detach from the rafter if not properly secured.  Whether or not this will actually happen is conjecture and depending on wind shear and uplift requirements in your area.  Just as you worry about the attachment of the roofing material in your previous question so should you be concerned about the lateral reinforcement of the roof structure itself.  If you use some solid method to attach the taper to the rafter like a long lag bolt in a predrilled hole (to keep it from splitting)  or use a series of Simpson clips or straps to hold the piece on you might be okay.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com