Q: We are considering adding a second floor to our home. Before advancing very far into the process I want to determine if it is even possible. Can you give me the "standards" used in constructing a home that will give it the strength to support a second floor? Is there a required stud size and spacing (or some other parameter) used in determining this?
A: There are certain requirements that must be met by the single story framing in order to pass the building department. Roofs must be designed for their own weight plus that of any snow that might collect...or in the absence of snow, a "live" load to allow people to work on your house safely. The short answer: Since you will be removing the roof framing, it will not restrict your addition plans.
The perimeter wall framing is often selected for stud depth (to accommodate insulation) and of a grade which is available in quantity from the local supplier. Although 2 x 4 walls are most common, 2 x 6 walls are sometimes used (for the insulation reason mentioned) and they would be better able to support a future second floor. Existing 2 x 4 walls do not preclude a second floor, but they must be looked at carefully (visually and by calculation) to assure that they will accept additional load.
The headers above your existing first floor windows may need to be replaced or strengthened to support your added floor load plus new roof (often 3 times as large as the typical roof load).
The foundation system of your house may or may not accept the added floor. If you are sitting on continuous concrete footings (poured against the dirt) or have a beam and pier system (again, relying on footings poured against soil), then chances are good you can add a floor without overstressing the soil or foundation system (because most municipalities require a minimum footing *width* which is often greater than that actually needed...the greater the width, the more a foundation can carry). If, on the other hand, your house is supported by concrete beams and drilled piers OR has a full basement, then the added load may overstress your foundations.
Clarification: An overstressed footing rarely fails abruptly and/or drastically. No crack/boom in the night. Rather, it "sinks" into the overcompressed soil and that portion of the house experiences "settlement". Very expensive to fix.
Chances are some of your interior walls will need to carry some of that new second floor. Expect significant underfloor work installing new footings. Existing basements create problems because you have to add columns and beams to support the wall line above. This may be inconvenient and leave you with a less than optimal basement plan. Beware sewer/water/gas lines when digging new footings. Rerouting them may be possible...and can cost a fair chunk.
Lastly, it will be obvious that by adding a second floor you are adding both mass and volume to the existing structure. Added mass means an increase in seismic forces if you live in an earthquake zone. Added volume means more "sail area" in a high wind zone. Either will almost always overtax your first floor wall's ability to resist this increase in lateral load...expect to add plywood to the exterior of your home to take care of this. Also anticipate the need to increase the ties from these walls to your foundation...a difficult and expensive task...but this is dependent on your proposed layout.
Scott McVicker, S.E.