Expert: Dan Griffin - 12/27/2008

Question
QUESTION: Hi Dan,

Merry Christmas to you and your family.
Here is my question

Someone has brushed tar on the inside of a block wall in our basement. I found it while replacing some drywall that had broken. I have checked two complete walls on the INSIDE of the basement and they have tar from the floor to about ground level roughly. The last or top course of block do not have tar on them. I have read on the Internet that this practice does not create any problems but I am concerned so would like to know your opinion. What will happen with the interior of a block wall that has had tar applied. I assume the tar will act as a moisture barrier but does it cause any problems. I live in Canada where the winters are cold and the summers are hot. I have also bought a very old backhoe and have begun to dig up the exterior to expose the basement wall and foundation and am going to use DELTA MS membrane to seal the outside. So far I have done about 20% of the exterior wall before winter set in and stopped me. As an added note the exterior wall also has tar on it with a thin layer of cement underneath and then the exterior of the blocks that make up the basement walls.Then I went inside to fix some things and found the tar on the inside of the block wall as well. It does not seem to be on all the walls only a few of them.

What do you think???

Thanks for your reply in advance


ANSWER: Garry, the interior tar treatment seem unusual.  In some sense it is a vapor barrtier and gives you a vapor barrier on both sides of the wall which is normally not good if the eall is something that suffers from moisture.  YOur block walls are not harmed by moisture unless it freezes.  It sounds as if you are making major water proofing efforts.  I assume you are adding a French drain system either to sump or daylight.

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

QUESTION: Hello Again,
Thanks for the quick reply.

After I am done with the Delta MS dimple system on the outside of the house including the 3/4 crush with sock covered weeping tile just beside the foundation wall of the house, back filled to 18 inches from the top with porous crushed rock of various sizes and finally topsoil. This will take care of any exterior moisture against the blocks. When I am done with the back fill I am going to make sure the grade is away from the house so any water will travel away away from the house as well. I do not think it will be required to install a french drain system but it is a good idea if there is any under ground hydrostatic pressure in a certain area. I will know after I have finished the exterior waterproofing since it will leak after when the pressure builds up under the delta system and gets through to the block wall.
Will the tar on the inside of the wall cause any problems after I have done all the other alterations. Would you think it is a good idea to remove the tar from the inside or is it okay to leave it there.

Thanks again

Answer
Gary,  I'm a bit confused. You are going to dig all around the house and install drainage plane, gravel, and drain tile and you're not going to send the tile to daylight, sump, or drywell?  That is the whole point of all the other work.  I don't think your interior tar work will hurt anything, I also see no way to remove it short of sand blasting which will do more harm to the block work than leaving it.