On its face, the location of a foundation perimeter drain seems like the simplest of details. The perforated drain line is run around the foundation next to the bottom of the footing.
At least that’s what many construction drawings show. But in some parts of the country, the drain is placed on top of the footing rather than next to it, and this discrepancy is at the root of Steven Knapp’s dilemma.
In a question posted at Green Building Advisor’s Q&A forum, Knapp writes this approach is not typical in his area, and that his waterproofing contractor is refusing to go along with it.
“Several years ago he switched to placing the drain (a rectangular pipe) on top of the footer and thinks this is the better method,” Knapp writes. “I’m annoyed and confused since I was advised by another credible expert that placing the pipe on top of the footer would greatly increase my chances of springing a leak.”
Knapp’s builder is leaning in favor of the “on footer” method, but he’s willing to do whatever Knapp thinks best. They also consulted with a Residential Foundation Repair Services company for a third opinion.
“So what is industry best practice?” he asks. “Putting the pipe next to the footer makes intuitive sense to me, but I know that what’s intuitive isn’t always correct. I just don’t want a basement that leaks.”
That’s the topic for this Q&A Spotlight.
The drawings are just plain wrong
Yes, writes James Morgan, we’ve all seen foundation drains drawn that way many times but the drawings are wrong.
“I know that’s the standard drawn detail and I’ve seen it a thousand times, but I think it’s a bad one, and there are several important [performance] reasons that no one ever builds it that way, at least not in my area,” he says.
The seam between the footing and the foundation wall is vulnerable, he adds, but it takes hydrostatic pressure to push water through the seam and into the basement. “Perforated pipe laid along the top of the footer and running to daylight ensures that the maximum head of water is just the thickness of the corrugation, or about 3/8 of an inch,” Morgan writes. “This is simply not sufficient to cause any penetration of the seam if a normal standard of care has been taken with the waterproofing application. This is the simplest, most foolproof and most reliable location. That’s why all the experienced builders that I know and regularly work with all prefer to do it that way.”
It’s more time and trouble to install the drain line next to the footing, Morgan says, plus it also requires more digging and a larger volume of backfill, “thus an enhanced path for water to reach the footing.”
“Backfill can never be consolidated to the degree of imperviousness of undisturbed soil,” Morgan writes. “According to well known foundation contractors, most codes now sensibly require that finish grade be sloped to a swale at least 6 feet away from the foundation wall. With a standard dig this places the swale well outside the backfill area and into the zone of undisturbed dirt: overdig brings the porous backfill closer to the swale and the large volume of stormwater it regularly contains.”
Keep the drain below the level of the slab
To GBA Senior Editor Martin Holladay, the correct location for the foundation drain is a pretty simple proposition: If you want to avoid problems, keep the level of the drain below the slab. “One thing is for sure,” he writes, “if the center of the 4-inch drain pipe is above the top of your slab, you are setting yourself up for potential problems.”
He tells Morgan water can reach the drain in more than one way. “It can trickle downwards from the surface, due to ponding under the eaves (as you propose),” Holladay says. “But during the spring, groundwater levels can rise from below, until the level of the groundwater is higher than the level of your slab. In that case, a footing drain pipe that is installed above the slab will work — but the slab will still get wet.”
David Meiland also would opt for a lower drain location. “I want the footing drain well below the slab, and I want it equal to or below bottom of footing so that the bearing soil under the foundation is less likely to be saturated,” Meiland writes. “I am lucky in that we rarely deal with expansive clay, but that would make it all the more important to drain the footing.”
Holladay’s point is well taken, Morgan replies, in areas where groundwater levels are periodically high. But he adds that even when the drain line is placed on top of the footing, it would still be 2 inches or so below the top of the slab.
“And I think there’s some value in having the the pipe right beside the vulnerable seam rather than a foot away where drainage paths could potentially become obstructed,” he adds. “Either way, Steven’s belt-and-suspenders approach should be fine.”
http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-install-foundation-drain
