Post-tension slab foundations are found in newer homes. Here in the Bay Area, a structure’s foundation needs to withstand not only the load of the building, but expansive soils, and the ubiquitous earthquake. Certain foundations are better at handling these conditions, and are seen more frequently here. One of these which is gaining popularity in new construction is the post-tension slab foundation.
What is a Post Tension Foundation?
Post-tensioning is a technique that was developed and first put to regular use in the 1970s, and approved methods have been published by the Post-Tensioning Institute (PTI), a nonprofit organization, since 1976. Sometimes called post tensioning, or simply PT, this is a type of slab foundation with added reinforcement.
In essence, a slab foundation, aka a slab on grade foundation, is a concrete base only a few inches deep, sitting directly on earth. You might see this for a small shed or playhouse, but larger structures are almost always reinforced, usually with rebar, and a fabric water barrier is lain out before the concrete is poured.
A post-tension slab is reinforced with grids of steel cables cased in plastic sheathes instead of rebar. After the concrete has hardened around them, the cables are pulled taut with hydraulic stressing jacks. This pre-stressing of the concrete creates added compressive strength to the foundation.
Why would concrete need that reinforcing?
Concrete by itself has a tendency crack under certain stresses – think of bent and fractured sidewalks from tree roots – you certainly wouldn’t want that to happen to your house! Adding the horizontal compressive strength means that if there is movement under (or above) your home’s foundation, it is less likely to crack, and if a crack occurs, the pieces will remain firmly in-place instead of bending and warping. It also spreads out the load, balancing pressure exerted on the foundation across a larger surface area.
Both of these are crucial reinforcements to protect a house against expansive soil and earthquake force.
In fact, post-tensioning is such an effective means of protection against earthquakes that it is regularly used for seismic retrofit projects. These are usually done using external post-tensioning aka EPT, which causes minimal disturbance to the original concrete, steel, or wood structures while strengthening or restoring the structural integrity to a weakened or damaged buildings. You’re not likely to find this in a home, however it’s very common in commercial and public structures.
Additional Considerations
But if you’re thinking of buying or building a home with a post-tension slab foundation there are some advantages and disadvantages to consider before you do.
If you’re building, the initial cost and time to install a post-tensioned foundation are usually much lower than a post-and-pier foundation, but usually more than the standard rebar-reinforced slab foundation. So it’s an affordable option where the soil or fault lines warrant their use. However, compared to post-and-pier foundations, repairs can be much longer and more costly.
Because the concrete foundation lays on the ground, there is no crawl space. In older homes, that also means no ready access to piping, cables, etc. So if you’re planning on adding a sink, fixing a leak, or moving a toilet, you’ll need to save some of your budget for an expert to x-ray, jackhammer, and repair the concrete foundation, in addition to your other costs.
Most newer homes have solved this by moving the pipes into the attic space, and some localities may not allow new foundations to be built with pipping beneath a slab floor. While this area has a temperate climate, it can freeze and pipes in the attic are subject to more extreme temperatures than those in a crawlspace.If a pipe begins to leak in your attic that flood will go directly into your house.
In slab-foundation homes ventilation is usually contained in the walls, so repairs will involve more steps (drywall repair and painting) than projects with access via a crawlspace.
Advocates say the lack of crawlspace saves money in other areas, however. No crawlspace means no risk of pests or standing water under the house, and no air-circulation below the house means lower utility bills for heating and cooling.
Pre-stressed concrete foundations also typically use less concrete than other slabs, and may be considered more eco-friendly for using less material.
How to Check for a Post-Tension Slab Foundation
First off, if there’s a crawlspace then it’s definitely not a slab.
When you look at the foundation, if you find circular patches in the concrete 1-3 inches large and a few feet apart, the foundation is probably post tensioned.
The garage is a great place to check. Usually a PT foundation has information stamped into one corner of the concrete garage floor, readily visible. Or it might be on a sign fastened to the wall of the garage, usually near a door. If you find either of these, they usually say something like, “Post Tension Slab – DO NOT CUT.” (Improper cutting and drilling can damage the cables and harm the integrity of the foundation.)
Lastly, you can always check on paper – contact the builder or the local building department for records.
