A Guide to Roof Framing – Prefabricated or Conventional
Trusses are prefabricated meaning they are built to the specifications according to the blueprints specs, live load, dead load, etc… The problem with trusses is they are rated per specs at 24inches on center, that leaves 22.5 inches in between. The problem arises somewhere between 3 to 7 yrs later, you will start to see a sag between the trusses. This is not the trusses sagging but the plywood used for your roof, unless a thicker grade plywood was used 3/4 inch or more,which I highly doubt. The standard thickness for plywood is 1/2 inch, with the seams nailed every 3 inches on center alternating and 4 inches on center on your 24 inch centers. The results are you end up with a 4 foot x 8 foot x 1/2 inch piece of plywood spanned across 5 trusses with 4-22.5 inch spaces in between each truss. You will have approximately 18 to 20-8d nails on each end and 12 to16 nails in the three trusses in the center.Total you will have 72 to 86 nails per sheet of plywood.
Conventional framing of a roof will and does involve more work therefore more money, this is a fact. The outcome of conventional framing as you will see in my opinion is the better product. Conventional framing is framed at 16inches on center 98% of the time,there are times that a general contractor will have a designer or architect figure on 24 inch centers but not that often. Framing with 16 inch centers will leave you with 14.5 inches in between each rafter. I think you see where this is going,with rafters at 16 inches on center you have 7 rafters for every 4 foot x 8 foot x 1/2 inch piece of plywood. The nailing pattern will remain the same, however the total amount of nails used will increase, 100 to 114 nails should be used. You will still have 12 to 16 nails per rafter and 18 to 20 nails at each end. The result is you have 2 extra rafters than you do trusses.
The material difference is very significant in the fact that trusses are primarily 2 x 4`s, except the bottom cord, which runs across your walls for your Sheetrock ceiling. There are exceptions to this if your house or building is wider the trusses will get a size bigger 2 x 6 and so on, however typically trusses are 2 x 4`s. With conventional framing you will not see 2 x 4`s used for rafters except on a smaller roof like your bay window or fire place. There are several other problems with trusses that are seemingly worse than conventional framing. Trusses set on your front wall and go straight through to your back wall,crossing every wall in its path, after a couple of years you will notice Sheetrock cracks where your ceiling and wall meets.
General contractors love trusses for the most part, due to cost. Most general contractors due not pass the savings on to the buyer. So they get the money that they saved from the framers and the money that they saved on materials by using trusses instead of conventional lumber. The result is in approximately 5 years you will be paying to have the dips taken out of your roof so it will not look like a roller coaster. The next time you are out driving around take a look at some of the houses that are about 5 years old, that have trusses. This will give you insight into the effects that trusses have on a house.
