HomeBUILDER Magazine discusses footings and drainage

FOOTINGS & DRAINAGE

by Jon Eakes

At times we try to read strict requirements into the building code that simply are not there and, in doing so, cause our customers expensive problems. Such is often the case with foundation perimeter drains.

Where is the "outside" of the wall?

The code says that the foundation wall has to be protected with perimeter drains on the outside of the foundation. So it has become common in some areas to lay the drainage tile or pipe right against the footing all the way around the house, including a jog under the garage slab. From a technical point of view, this creates three 90-degree elbows where there could be only one, helping to create problems at the inside elbow deep under the garage. Homeowners have discovered that it costs as much as $18,000 to dig through the slab to repair that elbow.

Fastfoot® provides a capillary break between the ground and the concrete footing

Michael Swinton, Senior Research Officer at the Institute for Research in Construction of the National Research Council suggests that following the perimeter of the roof satisfies the code in protecting the entire foundation wall against water ingress, while eliminating two elbows and making the corner of the drain easily accessible from outside of the garage. An $18,000 repair now becomes a $1,000 repair or none at all while still keeping water away from the basement.

Does slope matter?

As soon as we get a pipe in our hands, we all become plumbers and do impossible things to try and get that perimeter drain to slope someplace. First of all, unless you have roof runoff pumping into this perimeter drain as in the old days (and filling up with leaves), the fact is that there is never enough water flow in this drain to create a head nor are you trying to transport solid matter as in toilet drains. The slope is simply not important. Surprise: there is no mention of a slope in the building code.

The water rises slowly all over this drain and the pipe itself simply serves as a path of least resistance toward an outlet before the pipe fills up. Just get the outlet lower than the lay of the pipe with no dips in the pipe run - which would pool water - and it works. The only code requirement that applies here is that the top of the drain should be below the bottom of the basement floor slab, which simply means no higher than the top of the footing. There are details in the code about getting water to this drain with granular backfill and the like, but we don't usually mess that up.

Blocking "rising damp"

More and more I am hearing a debate as to just where we should place that perimeter drain, with a growing western trend of placing the drain below the level of the bottom of the footing while, traditionally, we have always placed it approximately alongside the footing. Remember that the code only says that the top of the drain must be below the bottom of the basement slab. Even though all the illustrations in the code show it alongside the footing, there is nothing to stop us from putting it deeper.

Why would anyone bother to dig deeper just for the drain pipe? It all comes back to an attempt to stop what is called "rising damp" - or the capillary action of moisture entering the footing, moving up into the concrete wall and evaporating into the basement. As you might guess, different people have different ways of solving this.

So many right answers

Years ago, the Ontario New Home Warranty program recommended dealing with this problem by simply applying damp-proofing over the cured footing before pouring the wall. This created a capillary break at the critical junction between a wet footing and a dry wall. But that was considered a "Better Building Practice" - not code - so not many people took up on the idea.

Swinton at the IRC suggests that it can be as easy as extending the wall damp-proofing all the way to the bottom of the footing - keeping the water on the drain side and out of the concrete.
In the west, partially because of a growing tendency to do a single pour for both the footing and the wall, they have begun to place the drain pipe with the top of the pipe just below the bottom of the footing, keeping the footing sitting on dry soil. Richard Kadulski strongly promoted this and drainage layers under the footing in Solplan Review in November 2003. The only caution that is necessary with digging below the footing is to remember that the structural load on the soil under the footing extends out at 45 degrees on either side. Hence, you should not dig right alongside the footing but a bit out from it to maintain the bearing capacity of the soil.

Water in wet soil can wick up by way of the footings into the walls and then the basement in surprisingly large quantities, and yet this is something that we tend to ignore. We moisture-protect the wall. We moisture-protect the slab. Yet, in this age of mould litigation, we leave the footing as an unprotected moisture gap between the two. One way or another, it is wise to provide some kind of control for "rising damp".

Faster feet

While we are talking about footings, I just want to call your attention to another western innovation: the very quick, very flexible fabric footing forms called Fastfoot®. A few stakes - reusable 2x4s - and your footings are done, including a capillary break between the footing and the soil!

As you can see, their ideal detail includes a granular drainage layer under the footing with the flexible form doubling as a moisture barrier footing membrane. See www.fab-form.com for more information.

The row housing swamp

There is another common but serious drainage error often made in row housing: the small passageway between houses too often becomes a water sink specifically designed to soak basements. When landscaping is neglected in this area and either surface runoff or rain gutters load this small space, of course the water finds the cracks in the foundation walls. Water control is critical in this small but often saturated space.

Either plan the elevations to allow raising the soil to keep water out from between the houses, or organize it to pass through unhindered to the street, or put in a surface drain and get that stuff away from the foundations.

One after-the-fact repair that I have seen done effectively is to remove all the top soil between the two houses, install a graded waterproof geotextile that goes up each foundation wall a little while creating a culvert to the street down the middle, then cover that with granular backfill followed by a ground filter followed by the top soil and grass or paving tiles. Now the drain path is below the surface, but water flow is not allowed to load the foundation walls nor the perimeter drains. HB