Expert: Ted Barnhill (Principal, Design45 LLC) - 7/3/2008

Question
QUESTION: I am wanting to add a sunken patio and entrance to a garden level basement.  The entrance will be a 5' sliding door.  Structurally, the area above should be fine for the header (just 16" OC studs in the wall).  Looking at my house plans, it looks like the footings are immediately below my concrete floor slab.  With the dig on the outside going down about 3' to get to that level, will I need to do something to adjust for the frost line after digging the sunken patio?

ANSWER: Steve,

Yes, you will need to be careful that you don't modify grade such that your frost depth goes below the footings of your home.  A small area such as a window well is normally not a problem, but if you are excavating such that it begins to look like a "walk-out" basement, you should address the frost problem.

One option to look at is placing rigid insulation below your patio material, to keep the frost from penetrating below your footings.  This is known as a "shallow frost-protected foundation" and is becoming more and more common as a technique to save resources.  Water management is an important part of this system, as it would be in any sunken patio; be sure that you provide adequate drainage away from the house.

Good luck with your project, and feel free to post follow-up questions.  -Ted

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

QUESTION: Thanks, Ted.  I am planning to keep my block retaining walls very close to the door (just enough to allow trim) so the exposed area is minimized.  I was thinking of draining the patio into a trap and then tee'ing that into my french drain that is under the floor (it goes to a sump pump), so I'll have go work that under the footing (need to check elevations to verify it will drain).  Do you see any issues with this?

Is there a rigid insulation that can be put under a concrete slab?

Thanks again.

Answer
Steve,

Here is a link to a basic primer on shallow frost-protected foundations:  http://www.oikos.com/esb/43/foundations.html.

Among other things, the bearing capacity of foam insulation is discussed.  The upshot is that there are foams readily available that you can put beneath bearing walls - the load under your patio will not be a problem.

Your drain solution sounds fine if you can make the elevations work out.    You might consider also hooking it to a drywell away from the house as a backup.

Good luck.  -Ted