I have a 20'X20' garage that I would like to build a bonus room over. Plenty of room upstairs for it. The problem is that the builder did not plan for a second floor so he only put 2'X8' beams to span that length.

I've had a few bids that all end up being around $30K but I'm trying to stay under $20K. They have all said that the cost of materials to build the stairs and the floor is what is driving up the cost. The suggestions were to use 2 steel beams across the roof of the garage to support the 2'X8' floor beams or we could use triple LVLs. I was told the LVLs could possibly still sag over time and are very expensive; but the steel beams would be the way to go. The steel beams were outrageously priced and labor would be up there as well to get them in place.

With all that said, I've checked the loads for all the standard lumber and nothing will handle it. Then I came up with another possible solution... floor trusses or I joists. I am worried about code obviously, but I am more concerned about the cost and being structurally sound; but cost being #1.

With either of those solutions, the ceiling of the garage / floor of the attic would most likely have to be torn out and the ceiling in the garage would have to be lowered to accomodate for the 14" height of the new beams that will be needed, which of course means more labor cost along with sheetrock and paint.

I'm going to get a personal loan to get the work done and then refinance my house to bundle it together. So I want to make sure that it will also appraise for the amount that I need. That's my main concern.

What does everyone think???

I think to expect to create 400 square feet of new space for $50 per square foot is a pipe dream, especially when doing major stuructural work within a garage. Adding new beams is a fine idea provided that there are footings that can carry the concentrated loads created at the ends of the beams.

Depending on the lumber species and grade that was used for the existing ceiling joists, 2x8's can usually carry up to 12 feet or so of floor load, so only one beam will be necessary, but the loads at the ends of the beam will be high.

I don't know what your garage or attic ceiling heights are, and I don't think 14-inch composite joists will span 20 feet and perform acceptably. These joists can be very bouncy even when structurally adequate, so deeper joists with wider flanges may be called for.

I think that clear-spanning the garage is the way to go for economy, but the height problem has to be solved, and the method of bearing the new joists on existing walls has to be dealt with. A 16 or 18 inch joist will not go where a 2x8 is or was.

What you really need is a competent architect or structural engineer to investigate the alternatives and advise you, and work out the details before any demolition or construction is done.

Take Richards advice and contact a professional. They may come up with a way to modify your existing rafters to be able to make the span. I know I have used bonus room trusses that have spanned at least 20ft with a 2x8 bottom chord. But it was part of a truss system that was designed for that specific purpose. Maybe yours can be modifed for the same purpose.