Digging under the foundation to put in a storm cellar (house, construction)

[Chaboznic]

Can I dig under my foundation of my ranch-style slab concrete to put in a storm cellar? I only need a small space maybe 8 x 8 x 8 and I wanted to put it under an existing house. Is it possible. I live in new Braunfels Texas. I know it's really rocky here but I think the dirt and soil around here would support structure underneath the slab.

[Hedgehog_Mom]

You can get prefab safe rooms that can be bolted to your foundation. They're not cheap but they're probably cheaper than digging one, especially under an existing house. Tornado Shelters, Storm Shelters, Safe Rooms by Family SAFE Shelters of Texas and Louisiana

I would be afraid to have someone dig under my foundation and I would be worried that anything I constructed in that space wouldn't hold up...the foundation is thicker than the part you can see when you're looking at your outside walls.

[Rustybolt]

You can also retrofit an existing room in your house to be a tornado room. Like take a closet and line it with 3/4 inch plywood and extra reinforcement. This protects from windborne debris where a 2x4 can become a missile. I took a course in it from the Corps of Engineers in the Ft Worth District. Problem with the bolt down safe rooms are that they really aren't for retrofitting. If you really want something in the ground, don't dig under the house, just put something in the yard. If you put it under your house and the house gets hit, you may not be able to get back out because of debris piled on top.

[Hedgehog_Mom]

The websites that sell the prefab safe rooms say they can be used for retrofitting...as long as you don't mind just having the thing parked in your garage, taking up half of it, it can be attached to the slab out there. You can also put them outdoors but then being able to access it in an emergency might be more difficult.

[TrapperL]

The quick answer- no. The slab is designed with the stability factor of the soil at your location for support. Removing any of it compromises the slab.

The long answer. Yeah you can but get ready to be on first name basis with yer banker. The slab would have to be modified for support beams to carry the suspended weight and all of that is subject to the surrounding soil conditions. You'd also have to install a french drain to keep water out of it and have enough grade to make the drain work or have a sump pump. My first guess would be that an 8x8 room would roughly cost about half that of the house.

[Primo]

Originally Posted by Chaboznic

Can I dig under my foundation of my ranch-style slab concrete to put in a storm cellar? I only need a small space maybe 8 x 8 x 8 and I wanted to put it under an existing house. Is it possible. I live in new Braunfels Texas. I know it's really rocky here but I think the dirt and soil around here would support structure underneath the slab.

Physically yes you can dig under your foundation. My parents had a pipe rot out in their house and the plumber dug a 4'x'4 square tunnel almost the length of the house underneath the foundation. And they did it in about a day, using shovels & buckets and then an air hammer to chisel the limestone.

They guy said it is actually quicker & easier when it is solid rock as opposed to dirt.

But as far as getting a building permit or a variance on your home for such a construction you would have to research.