We've recently embarked on a development branch of our company, where we will occassionally develop a piece of land in the interest of promoting better architecture in a typology that appears to be lacking good design. Our first attempt at this is to build a spec house in East Austin. With a speculative project, the strategy is to find the proper balance between affordablity and quality. This series of blogs will address the decisions we architects and developers make along the way and why we make them.
Part I: Choosing the Right Foundation
It all starts with a good foundation, and believe it or not there are choices. The cheapest and most common foundation for homes is the post-tension slab. It's cheap because there's very little rebar. The strength of the foundation relies on cables pulled in tension when the concrete is being poured. The problem with this foundation is if you ever want to make an adjustment - remodel a bathroom, add a bedroom, move plumbing - you risk compromising your foundation if you cut a post-tention cable.
In a conventional slab foundation the structural engineer designs a pattern of rebar that reinforces the concrete. The rebar can be cut without compromising the slab but rebar is expensive. Slab construction is great for compact houses in Austin where the soil can be expansive because we can design a monolithic slab that keeps it's rigidity even when the soil expands and contracts.
A third alternative, and one we often use although it's less common, is the pier and beam foundation. Advantages are that it's quickly built and plumbing and electrical are installed after the piers have been poured, so there's an advantage to scheduling trades. The major disadvantage to a pier and beam slab is that it doesn't work for a garage foundation. So if you have an attached garage (which is common with inner city spec houses), this foundation is not ideal. Also, from a design standpoint, a pier and beam foundation needs to sit at least 30" above the ground which means steps up to the house wherever there is an exterior door.
In the end we went with a conventional slab foundation. It costs more than a post-tension slab but would allow for future modifications.
