Monday, November 23, 2015

Construction - Damp-proofing the Earth Contact Wall

Is Damp-proofing or Water-proofing Really Necessary?
Yes and No.  "Yes" because the building code requires it.  But perhaps "No" from the standpoint of needing it long term.

The probability of the earth contact wall getting wet from surface runoff is low. The insulation/watershed umbrella will extend northward from the wall for 20' and the backfill behind the wall will be sloped away from the wall in three directions and end in swales to direct the water around the house and northward to a wet-weather creek at the north edge of the property.

The probability of the wall getting wet from a rising water table is non-existent due to the elaborate French drain system we installed 10' below floor level to lower the water table 
(first post on the French drainssecond post on the French drains and third post on the French drains).

Is a Footing French Drain Really Necessary?
Yes and No.  "Yes" because the code requires it and because it may have some advantages until the backfilling is complete and the umbrella is in place.  "No" is probably the correct answer for the long-term because the chances of water reaching the bottom of the wall will be slim and none except perhaps near the ends of the wall. And the bottom of the wall is 7" below the floor level and the footing drain is below the top of the footing, making it a foot or more below floor level.

Ideally, the backfilling should be done in shallow lifts (layers) and each lift compacted.  My intention is to let nature do the compacting while I finish the house. Now that the wall is damp-proofed, I plan to backfill to a depth equal to about half of the final height of the backfill, or 5 or 6 feet, then let it compact itself over time. Eventually, the backfilling will be carried to completion and the umbrella installed.  In the short-term, the footing drain will be useful for draining any water soaking down through the initial lift so as to eliminate hydraulic pressure against the back of the wall.

Rather than encompass the fabric-covered drain with rock, I used sand that not only enveloped the drain but also was several inches deep on top of the footing so as to provide a passageway for water from the wall to the drain under the backfill.

Choosing a Damp-proofer
The fact that water at the wall will not a problem made it tempting to go with an unsophisticated material such as an asphalt based non-fibered roof and foundation coating at 11 cents per square foot.  My concrete supplier however recommended the Platon Waterproofing System at 22 cents per square foot.  The 11 cent differential amounted to $124 to cover 1,128 sq ft which seemed by far to be the best value.
Overview of the damp-proofing membrane; notice the sand
bridging from the membrane to the footing drain (click
on photo to enlarge for detail)

In order for the asphaltic coating to cover well, some areas of the wall would have required parging to cover up surface porosity in the concrete.  Some of the wall also would have had to have been power-washed to provide a reliable bonding surface. And I was not too keen on working with an asphaltic material.  With the Platon system, there is no need to "water-proof" the wall; it provides a drainage plane between the membrane and the wall so that water drains immediately to the French drain instead of going through the wall. And it could be hung without regard to the condition of the wall in less time than it would take to coat the wall, much less parge and wash it.  So, coughing up an extra $124 with the promise of less hassle, made the choice of the membrane a no-brainer.

Installing the Drainage Plane Sheets
The deadmen divided the membrane into four linear sections. We cut
The washers (left), removed from housewrap nails (top),
were mated with the recycled Tapcon screws (right) to
fasten the top of the membrane

each section to fit then stood it on the footing while fastening. We wrapped the material around the corners onto the deadmen for 9" or so. We used Tapcon screws to fasten the top of the membrane to the wall after mating the screws with the orange washers from housewrap nails. The manufacturer sells an attachment system but the Tapcon screws and washers worked fine at nominal cost, especially since we salvaged the screws from the wood braces that we had used for the insulated concrete forms. The membrane would have been unmanageable working alone so I was happy to have help from stepson, Keith.

Where the membrane got shorter as it
Close-up of the membrane; connecting the dimples on the
wall side are vertical channels to shunt water downward
approached the ends of the wall, the sections were laid flat for cutting.  We also pre-cut the membrane for the top 4' of the 12' high section of the wall and stored it until, 
several months down the road, the backfilling reaches that level.  It can then be installed while standing on the initial backfill instead of on ladders. At that time, it will be shingled under the lower section (not over as with roof shingles) which means the screws holding the lower section will have to be removed, the upper section tucked in behind it and the screws replaced.  Shingling under means that the edge of the lower section is has to be attached firmly so that it is not pried loose by soil when the backfill is done.


  1. membrane is facing backwards... dimples go on the concrete...

    1. Thanks, Eric, for your input. I guess my vendor's instructions were wrong or I misunderstood. I will definitely turn the dimples inward when the remainder goes up.


As a do-it-selfer-in-training, I welcome your comments.