Footings
and 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.
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 "damp
rising"
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 "damp rising" - 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 "damp rising".
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