I have moved to SC where it is warm and have a 7 year old house with a 12Ft. x 14Ft. cement slab, which is approx. 3-1/2 to 4" thk, no cracks, off the kitchen that is screened in with shingled roof, soffits, guters & downspouts. It is framed with 4 x 4 treated beams on 2' centers. I want to close it into a 4 season with windows, outside door, vinyl siding, raised floor, treated, insulated,and tiled. Inside drywalled & plastered. I am sure the concrete slab does not have any rebar. There is an expansion joint from the house foundation. Should I need to be cocerned if the additional weight will be a problem being that the existing structure is solid?

Richie

No you should not have any real concerns about weight. The roof is supported by existing columns which is where the real weight is going to be anyway. If you were going to have any issues I would think they would have shown up before this.

You should check with the local builing department to see if they will have any issues, but I do not assume so as this is a very common upgrade done a lot.

Just be sure that you flash the sill so no water can seep under it and into the room when it rains. A very common complaint when these four season rooms are built.

Thanks for getting back. When you say flash the seal, what do you mean? I was going to seal the 2x4 sole plate all around the in and outside before I put the 6 ml film down.
The additional weight bothers me for I am an engineer and I have calculated the existing structure which is about 1500 lbs. which is supported by the 3 walls around the outside of the slab. The additional weight added will be approx. 3000 lbs. with some of that being transmitted to the entire slab.

It's not the actual weight of the structure alone that you must design for. That weight is just the "Dead Load". You also have to design for a "Live Load", which varies from region to region. You can find that number in the building code, or by asking your local building official.

You may also be changing from concentrated loads to uniform loads, depending on the two structures.

You won't want to hear this, but the right way to do it is to build a foundation which is designed to carry all the loads required by code. Building it on the existing slab is a risk. If it were my house, I wouldn't take the risk. You only build it once. You live with the mistakes for a long time.

The building official also may ask for some shallow foundation protection. Meaning that because the new conditioned attached addition doesn't have frost protection, he may ask that rigid foam be layed flat against the existing slab and then buried under ground. I've had to do this on more than one of these cases. In the IRC 2006, it's section R403.3.1 that describes the procedure.