Expert: Daniel Humphrey - 7/23/2010

Question
QUESTION: This is a ridge of a new higher pitched (6/12) roof that is built over old 2/12 one. Old ridge is 3.5 by 12 solid wood beam 19 ft long supported in 2 places in the middle (so span is about 6.5ft). New rafters (2x10, 12ft. long) will be placed over old ones, and we want to use two 2x12 as ridge beam (load bearing, midwest, snow). We would place 4x4 posts exactly over existing support points (so, also with 6.5 ft. span).

Any ideas ?

Thanks in advance.

ANSWER: Dear Alex,

Yes, your plan is sound. The 2x12 doubled should be plenty, supported as described.

The two improvements I can think of are to distribute the load along the 4x12 by trussing between the two ridges, and to add similar trussed midspan supports between the old and new rafter systems so that all the wood is working together. With both roofs bearing some weight, the roof would be stout, to say the least.

Daniel

---------- FOLLOW-UP ----------

QUESTION: Thank you, Daniel. We are now thinking of using 1 3/4 x 11 3/4 LVL instead of 2 2x12. Trussing would not be desirable in our case as we are planning to use glued rigid polyiso boards for insulation and trussing would introduce additional thermal bridging (in addition to posts). You think using single LVL is a good idea ? This is mostly to be able to use ridge venting (they are not designed for wider beams).

Answer
Dear Alex,

I think your plan to use the LVL is pretty good, but it will not equal a doubled 2x12 in performance. Just make sure the LVL is held upright by your supports. I would build up the supports from a sandwich of three 2x6 or 2x4s, with one supporting and the ones on the sides coming all the way up the depth of the LVL, holding it rigid.

Ridge venting is very important. Thermal bridging from the trusses as I described is insignificant.

Daniel