How to Install a Foundation Drain
Does it really make a difference whether the perforated drain pipe goes on top of the footing or beside the footing?
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.
“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. “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.”
How much should the line be pitched?
A related question is whether the drain should be installed with a pitch, and Ron Keagle is willing to go first. “I would include some,” Keagle writes. “Once you commit to pitch, the tile elevation has to change as the tile runs along the foundation. I would regard the bottom of the footing as being the maximum acceptable depth for the bottom of the drain rock bed.”
At the lowest point of elevation, both the bottom of the drain rock bed and the bottom of the drain tile are even with the bottom of the footing. At the highest point, the bottom of the drain rock bed and the bottom of the tile might be about mid-footing height.
Knapp replies that the foundation drain should be level with the foundation and routed to daylight. “The idea seems to be to provide an easy way for the water to move away from the foundation,” he says. “Water always follows the path of least resistance, correct?”
Meiland says he’s rarely seen a foundation drain installed with a pitch, and that despite Keagle’s misgivings, standing water in a level drain isn’t likely to be a big problem. “With a level footing drain, there may be a bit of water standing in the bottom of the pipe, but it can’t really accumulate without flowing out one way or another. Hydrostatic pressure is low with this type of setup, and it’s a whole lot better than a bunch of wet native soil backfilled against a foundation.”
Should backfill be compacted?
Part of Keagle’s strategy is to backfill with sand, compacting as he goes in lifts of 6 inches to 8 inches.
Machine compacting? asks AJ Builder. Really? “Never in thirty years of building have I seen backfilling of a foundation machine compacted. How many homes have you GC’d and did you do this compacting every single time? Never had a wall of your foundation fail from the compacting?”
Keagle thinks compacting backfill isn’t a big deal. “Furthermore,” he adds, “it is the one thing that can make the most difference in water and moisture issues. It can be done without bowing the walls if you are careful, use fill sand, and work in lifts.”
Meiland doesn’t compact backfill, but he uses crushed drain rock that mostly compacts itself as soon as it’s dumped. “The reality is we’re going to to trample all over it while framing and siding, so it doesn’t matter,” he says. “I do compact the hell out of the same stuff when under a slab (vibe plate) and compact fill placed in a utility trench, which often has a lot of loose soil.”
Yet to Keagle, the best approach is not to use drain rock against the foundation exclusively because that would encourage surface water from filtering down to the drain tile. “I don’t want a free draining path from the surface grade down to the tile,” Keagle says. “I want a surface that is impermeable and pitched to drain away from the house. Ideally, the drain tile will never get wet.”
Our experts’ opinions
Here’s what GBA Technical Director Peter Yost had to say:
I remember discussing the location and other details of the perimeter foundation drains when Steve Baczek and Mike Guertin and I did all of the GBA details several years ago now. But I decided to check back in with them.
Mike Guertin: “I only set the drain pipe so that the top of the pipe is below the top of the footing when there is a basement or crawlspace on the inside of the foundation. I also make sure that the top of the pipe is below the bottom of the slab. Sometimes we have to pour our slabs so that the top is flush with the top of the footing. In those cases, the top of the pipe will be below the top of the footing, usually by 4 inches to account for the slab thickness.
“I have not seen anyone install a footing drain pipe above the level of the footing when there’s a basement or crawl space within the foundation.
“There have been a very few times when we installed foundation drains above the footings. This has only happened when we uncovered unsuitable soil at the proposed footing level so we had to dig several feet deeper. We backfilled on the inside and outside of the foundation up to the slab level. Perimeter pipe was installed about a foot below the slab level rather than at the bottom of the deep footing. There are always situations where SOPs need to be modified to account for site conditions.
“I use either 6- to 10-mil plastic as a capillary break or a specialty product like Delta Footing Barrier, or I coat the top of the footing with liquid membrane like Henry 787 before setting the foundation forms.
“I install a chamfer strip ripped from scraps of XPS or EPS along the foundation/footing intersection (cold joint) on the exterior and then apply another round of coating or sheet membrane over it. My thinking is that any water that runs down the foundation face may puddle on top of the footing and migrate inward. The chamfer kicks water out beyond the top of the footing. I picked up the idea from an old-timer who laid up CMU foundations. When he parge-coated the block, he buttered up the block to the footing and smoothed it off at 45 degrees or so.”
Steve Baczek: “I believe the best location for the drain tile is alongside the concrete footing. That being said, the bottom of the drain pipe should be about 2 inches above the bottom of the footing (NEVER below,) as placing it below may undermine the footing.
“I also think the drain pipe should be fully encased in a stone bed, with roughly a 4-inch wrap on the pipe, with the stone bed wrapped in a filter-type fabric to ensure the drain pipe remains clear of any accumulated dirt. I have also used a drain pipe that comes wrapped in a filter fabric sock that we then placed in the stone.”
And to wrap this up, I decided to check in with Pat Huelman at the University of Minnesota. Pat and UMN have done a ton of foundation research.
Pat Huelman: “In my opinion the perimeter drain tile should never be on top of the footing. It leaves too much water at the critical footing/foundation wall joint. In old textbooks you can find the use of a ‘dumbbell waterstop’ at this joint to prevent inward water migration – a practice that I have never seen.
“It is been a tough road to get capillary breaks between the footing and foundation wall, but I see progress being made on that front. It is important to recognize that the soil will frequently be totally saturated at the bottom of the drain tile, and there can be free water as well. So it just makes sense that the bottom of the perimeter tile should be below the capillary break.
“Some argue that you can put the capillary break under the footing and conceivably this can work. But following the logic above, this would require the drain to be lowered below the bottom of the footing and that makes the engineers nervous. Perhaps the footing could be placed on a bed of larger stones (that cannot support capillarity) that extend outward and the drain tile could be incorporated that region. In this case, the stone acts as the capillary break and the drain tile is once again below this break.
“Finally, I would use the foundation design handbook as your guide. Version 1 is still good stuff. But John Carmody and his group have revised it as on online version.”
Originally published on GreenBuildingAdvisor.com.
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