Expert: Dan Griffin - 9/22/2007

Question
QUESTION: I am looking at properties that have modular or prefab homes on blocks
because they are the least expensive- it's what i can afford.  modulars can be
more sturdy that stick built homes these days.  i want to have a basement  or
partial basement to retreat to in the event of a hurricane or tornado.  These
houses are already on blocks.  how costly would it be to install a basement or
partial basement after the fact and what would the plan be in order to do so?  
would it be better to build a storm shelter right outside?

thanks!

ANSWER: It would probably be cheaper to move the house, pour the basement, and reset the house.  It is certainly possible to dig a basement, but it is very expensive and the home would probably not be habitable during the process or major accommodations would need to me made for utilities.  A storm shelter in a garage or backyard would be much more doable.  Make sure that you register your storm shelter with emergency agencies, people have been trapped in their shelters under debris.

---------- FOLLOW-UP ----------

QUESTION: What do you mean by pour the basement?  What about putting the home on a
foundation versus being on blocks?  Also roughly how much would something
like that cost?  
When you say dig the basement, do you mean when the home is still above
where you are digging?

If I just go the storm shelter route, how much would that run?  i would need
some storage space for my important papers and for 2-4 people to get
horizontal to sleep....as well.

Answer
Most basements are made using poured concrete. In order to pour the concrete an oversized hole must be dug deep enough to provide the basement.  There needs to be enough space for men to set forms on both sides of the wall and the house would need to be raised high enough to be able to pour concrete into the forms.

Basements can be made using CMU (cement block).  The digging requirements are the same.  Pouring the bond beam is quite similar.

Either choice will require a French drain system, probably directed to a sump pump, and at least damp proofing of the walls.

Most regular basement contractors will not even look at the job with the house sitting in location because of the additional work and liability.  You may be able to find a general contractor to consider the project, but you will pay substantially more to add the basement under an existing home.

Your best resource in my opiniion would be to contact your local FEMA office.  They have some excellent information, pictures, and scale drawings of the requirements for a storm shelter.  I live in Tornado alley.  there are certainly a bunch of storm shelter contractors out there.  They make them out of concrete, steel, fiberglass, and other. I know several folks who have gotten away from the snake infested, mold, leaking, smelly problems of a below ground room that is only looked at once a year  that have designed an above ground safe house.  Looks a bit like a storage shed in the back yard, but much heavier construction details.