I have a porch roof attached to the rear of my home - size is approximately 6'X24'. One side is attached to the exterior wall of the home and the other side is supported by five 6 x 6 treated posts. Can I create my own beam by nailing a treated 2x10 on either side of the posts to carry the load and then remove the three middle posts? What is the maximum span with the method I am suggesting?
There are a lot of variables to consider when sizing a supporting beam for a porch roof or any area in your home where a load must be carried. While I know your idea seems logical and it might work, consulting with an architect or engineer is the only way to know for sure. The sizes of your porch roof framing members, their spacing, the type of roofing material, and even the average snow load for your locale are items that must be figured into the equation.
Even though a 2 x 10 is very substantial, your method will actually have the fasteners used carrying the weight of the porch. Over time those nails or lag bolts could loosen and suddenly you have a sagging porch. The way the porch roof is currently framed, each of those 6 x 6 posts is carrying part of the roof load.
I have no doubt that there is a way to adjust the framing so that a few of the supporting posts can be removed. Microllam beams are available in numerous sizes and can span large distances. The beams can then be boxed in much as you had planned to do with the 2 x 10s.
Flitch plates are another option. A flitch plate is a metal plate sandwiched between two framing members such as 2 x 10s. The metal thickness is based on the width of the space to be spanned. Metal beams could also be used and would eliminate the need for most of the supporting posts across the front of your porch. They also can be boxed in with wood after installation.
Of course, all of these solutions are a little more expensive than simply nailing 2 x 10s to the sides of your existing posts. Microllams would probably be the least expensive option depending on the size needed and they can be purchased at many lumberyards.
However, before you start shopping for any of these beams, the first thing you need to do is meet with an engineer or architect to get the proper sizing. You don't want the roof to collapse - especially when someone happens to be sitting on the back porch.