Expert: Daniel Humphrey - 1/13/2009

Question
You advised someone that it is okay to build a second story if the house is sturdy and it is advisable to build the foundation independently.  What exactly does that mean?

Answer
Dear Esther,

I think the previous question was whether a 2-story addition could be built alongside an older pier and beam house. I recommend that any addition be made so that it has its own foundation and that the new structure can act somewhat independently from the older one.

It is possible to design a pier and beam foundation that will support very tall buildings, but most older houses do not have much more support than a single story requires. Perhaps some pier and beam foundations on single story houses are actually strong enough for an additional story, and an engineer could tell. The engineer would look for the bearing capacity of the piers and the strength and rigidity of the floor structure and how the piers are locked together at the ground level or below.

A pier and beam foundation can usually support a great deal of weight straight down, but when you ask more than that, it cannot resist sideways force. The real problem is when people add more weight onto a pier and beam foundation than the soil can bear or the beams can support, or when they try to cantelever a porch. In an earthquake prone area, adding a second story to a pier and beam is not advisable without doing some engineering first. Earthquakes exert sideways forces that any added height multiplies.

Daniel