Safes & Post Tension Slab

Why You Don't Want to Cut or Drill into Post Tension Slab

When drilling or cutting into a post-tension slab, it's possible to hit a tension cable.

If that cable is damaged, it can not only weaken or cause damage to the foundation, the cable can even snap and rip right up through the slab, causing serious injury or even death to someone standing nearby. You do NOT want this to happen.

If it's a new home, putting any holes in the concrete will also void the home's foundation warranty. 

The concrete can be drilled or cut, BUT it must be done with awareness of the location of these cables. To do otherwise could end up a much larger problem than the one requiring the concrete be cut. If you plan to cut or drill into a post tension slab, do so after hiring a structural engineer that can tell you where it is safe to do so. 

How to Tell if You Have Post Tension Slab

After the mid 1990s homes with post tension slabs will have a stamp in the concrete, usually located beneath where the garage door (if installed) would come down.

This stamp location is not universal, though, and it's important that you walk along the entire length of the opening in search of the stamp. To complicate things a bit, in early homes that had post tension slabs installed, they may have only used a piece of plastic or paper sign near the garage door (which can be lost over time).

Because of this, if you don't see a stamp you will still want to check around the edge of your home's foundation. Look for round concrete patches 1-1/2 to 3 inches in diameter, spaced every 2-4 feet (these are the patches covering the ends of the post tension cable). If you see them, you have post tension.

The best and most assured method, if you can't determine it otherwise, is to have a licensed, bonded, and insured, experienced contractor or structural engineer come out and verify. If you do have post tension, this would also be the person who could determine where and how to drill or cut safely.

What is Post Tension Slab

A post-tension slab refers to a method of construction used in buildings to increase the strength of a concrete foundation through the use of cables within the concrete that are placed in a state of tension. 

Steel cables are laid before the concrete pour. Once the concrete is hardened a bit (about 75%), these cables are then pulled into tension by hydraulics, then secured in place at either end. And we mean a lot of tension; upwards of 30,000 lbs worth. The cables are spaced throughout the concrete in a grid 2-4 feet apart.

This causes the concrete to be in a constant state of being 'squeezed' and, by the nature of compressive strength inherent in concrete, its strength increases and it prevents the slab from splitting or separating.

While originally mainly used in commercial applications, it became the prevalent way of laying home foundations in the mid-90s as it allows for reduction in construction costs by lessening the need for thicker slabs of concrete to support a given structure. 

Why are Post Tension Slabs Used?

Concrete has a high compressive strength – it can bear a lot of weight. But it has low tensile strength; it doesn't bend or stretch well without breaking.

For many years this has been countered by adding re-bar (steel rods), which does provide extra tensile strength to a concrete slab and makes it less likely to crack. But for most applications, a post-tension slab became the least expensive option to resolve a structural building issue (usually poor soil to build upon).

A post-tension slab can be thinner than one that is not, and provides supportive structural element to the slab that is absent in slab pours that don't have them.

And as  communities spread nationwide, building in poorer conditions with low quality soil, post tension became the best option to address potential issues. Here in Arizona, many neighborhoods have been built on poor soil subject to shrinking, swelling and shifting.