Expert: Richard Taylor, AIA - 11/5/2004

Question
Thanks for the information. Unfortunately, it created more questions.  You mention a drain tile at bottom of frost wall.  What is a drain tile? Would installing the black flexible piping with holes in it (not sure what the name for that is) be considered a drain tile?  What is the frost wall and how is that different than the foundation wall? How do we waterproof the foundation wall?  Thanks for your help.

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Followup To
Question -
Making a 16x16 room addition off the back of my house.  We will be removing a 10x15 deck that connects to back of house in the space where the addition will be built.  My question is around the contractor is proposing a raised slab construction due to his concern for moisture. His solution is to block in the foundation and fill with dirt then pour a slab. There is a 3 ft incline going into the back yard that will be approximately 2 ft from the foundation of the addition.  Water does flow down the incline, but typically does not puddle.  My house has a crawl space.  Is there a distance limit that the house needs to be from moisture concerns or are there alternatives to the raised slab construction?    
Answer -
Phil:

Thanks for your question!

If I understand correctly, you and your contractor are concerned that water flowing down the incline toward your new addition's foundation will seep into the foundation and cause problems.

If that's the case, you need to take several precautions - and you don't necessarily need to build the raised slab.

Making the foundation and floor system a sort of solid block of concrete will only treat the symptoms of any water problems - you need to treat the actual problems.  And the first is the slope.  Anything you can do to adjust the slope to carry water away from the foundation will help greatly.  Or you could install a French drain at the bottom of the slope to cut off the water flow and carry it away.

Second, whether you do a crawl space or slab-on-fill (by the way, use gravel, not dirt for fill) you need to waterproof the foundation wall.  If water gets in under a slab you'll have problems just as much as you will with a crawl space.  The concrete will wick water from the base and cause problems in the house.

Third, install a drain tile at the bottom of the frost wall to carry away any water that makes it to the wall.  Be sure to place it in a gravel bed, with a filter fabric over it, and more gravel above.

Finally, make sure you always keep your gutters and downspouts open and flowing - blocked or disconnected downspouts are the #1 cause of foundation moisture problems.

Do all these things to prevent the water from getting to the foundation wall, and you'll have solved 95% of your potential water problems.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com

Answer
Phil:

Yup, the black plastic stuff is the drain tile.  It's placed at the bottom of the wall where it collects water and carries it out to a storm drain, street, or other area.

Frost wall simply means a foundation wall that is just deep enough to be below the frost line (that depth will vary depending upon the part of the country you're in).

Waterproofing is done by several methods.  The cheapest is bitumen, a coal-tar based black goo that you spread on the wall.  Better systems are proprietary such as Rub-R-Wall or Tuf-N-Dry.  Your local building supply should be able to direct you to different products.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com