Expert: Dan Griffin - 5/1/2009

Question
I have a 12' x 24' deck and want to build a gable roof over it. What size lumber will I need for rafters? What size should the headers be on each end of the deck to nail my rafters to since there are no walls? I need to keep the pitch at 4/12.

Answer
Mark, I'm having a bit of difficulty.  I guess the deck is about 12' from the house to the outside edge and it runs along the house for about 24 feet.  I have no idea how high off the ground it is.  I assume the ridge line of the existing house runs the same way as the 24' deck dimension.  I am guessing that the eave soffit is about 8' above the deck.  Your existing roof has about a 4 in 12 pitch.  You don't tell where you are so I have no idea about snow and wind loading that code will demand for your location.

Now we need to decide what you want to do.  You want a covered deck.  It would be very unusual to be able to carry any type of roof structure on top of a wooden deck.  The columns to carry the load of the roof  and resist uplift should have been done before the deck or will require reworking the deck to incorporate them.  You said you want a gable roof over the deck which would imply that you intend to turn the new ridge perpendicular to the existing ridge so that the roof will span 24 feet with the carrying beams/headers at each end of the deck and tying back into the existing roof a bit like a dormer.  I'm reading between the lines that you really meant you want to continue your existing roof line out another 12 feet to cover the deck continuing the 4/12 pitch.  Continuing the 4/12 would require 48" of fall in 12', I doubt you have enough head clearance for this one.  Turning the gable ridge is quite doable, will look nice, but will require a bit more carpentry skill to tie back into the existing roof.  It would be possible to have a fairly flat roof over the deck that runs out the 12' dimension, but will need to be hot tar, steel, or some other appropriate flat roof finish.

YOu will need to design new columns to carry the loads.  YOu will either need to take some of the deck apart enough to get the columns in the proper location with the proper footings, or you will need to set the columns outside the existing deck lines which will look a bit unusual.  If you are turning the gable, I would use trusses, the columns could be steel or probably 6x6 with double 2x12 headers.  The dormer rafters could be cut in 2x6.  Not knowing your snow and wind load requirements, I cannot size a wood frame extending out the 12 foot direction, but will probably be 2x8's.  The size of the header beam will be determined by the number of columns, but again will probably be 6x6 with doubled 2x8 or bigger beam.

Your local bulding department will demand a drawing and may require an engineer's stamp.  I am not trying to scare you off the project, but I can't get you much closer without a lot more information.