Why don't builders to this (basement seep prevention)?

[AJScott]

In my own 1930s Cape Cod, the vast majority of my basement seepage would originate at the point where the concrete slab met the concrete walls. There was no visible cracking, so I chalked it up to that meet-point being a weak link in the chain that hydraulic pressure found fit to remind one of weak links in below-grade basements. Seepage from that meet-point has also been pretty characteristic of a lot of others who have/have had seepage, so with that in mind, comes the question:

Wouldn't it be really easy for builders to adopt the practice of sinking a continuous band of stripping (a hardy rust-resistant metal, or some thick plastic like that used for PEX water pipe) into the foundation between the foundation pour and the wall pour so it provides a more-reliable barrier? Hear told, old-timer building contractors would sink a strip of copper when building their own homes exactly for this purpose.

My question, given the durability and low cost of PVC-type plastic these days, is why isn't this something builders adopt as normal practice when building homes with below-grade basements?

AJS

[Joseph Meehan]

If there is water on the outside, not being drained away, it will find its way through the concrete. Noting is going to stop it. You need to keep the water away from the foundation and that will keep you dry.

[HA HA Budys Here]

Well, you know builders and what their priorities are...

That aside, I'm racking my brain (and others) for a low-cost, simple (hopefully) preventitive solution for a house I'm about to build on the lot next door.

I'm subbing out the grading and foundation, sheetrock & roofing work, and doing the framing, sheathing, millwork, flooring plumbing electric & heating myself. I've never been involved in new housing until the framing was complete.

The basement will be a full walk-out basement in the front of the house, (actually a 2-car drive in) but 48" below grade on the other 3 sides. I want the option to finish it at a later date so I want to assure there are no water issues. I have no idea what builders are doing these days other than a coat of tar and I know those styrofoam panels are required by code here.

I'm sure there's standards to mitigate future water issues, like drain tile, but I'd like more than "typical."

[Art]

Search the web or this newsgroup on how to properly install drainage for a basement. If done right your basement will be dry as a bone. The key is in the details.

[Paul Franklin]

This is a pet peeve of mine as well. Builders generally don't properly waterproof basements because money spent there isn't immediately obvious the way a nice master bath or more square footage, or interior upgrades are. And most basement water problems don't show up until years later. Few clients care enough, are informed enough, or have the extra money to demand better.

When I build a new house, I will have them install a drainage system under the slab, along with a real water barrier. (If done right, this can also be used for radon mitigation, should that become an issue.)

The walls will be covered to all the way down over the footer with heavy duty waterproofing membrane, bonded to the wall. Outside of that goes drainage mat designed to create an easy path for ground water to flow down to the perimeter drain, which will be properly sized, sloped, and wrapped with filter fabric to prevent clogging. The drainage mat also protects the membrane from the backfill. Follow that with proper backfill (not slag or other material that breaks down too easily) up to 6-12 inches below final grade, again isolated from the earth with membrane to keep the soil out.

An option is exterior insulation, but I think the jury is still out as to the insect and rodent problems with the various foams. I think it's safer to be 100% sure to keep water out, then insulate on the inside.

By the way, when the walls are constructed of poured concrete, rather than concrete block, it is common practice to form a key in the footer (a wide depression running all the length of the footer). Then when the wall is poured, the concrete flows into the key, locking the wall to the footer. In addition to providing a mechanical connection to lock the wall to the footer, it also helps prevent water flow through that joint. But the best thing is still to keep water away from the joint altogether.

All of this add only a few thousand to the cost, a fraction of what it would cost to repair later.

Paul

[ConRes]

There are really 2 issues here. One is the proper perimeter drain system as suggested by several other posts. But I always like to have a backup for anything that has to do with water. The idea about using a keyway for locking the slab to the footer is alright in some applications - except where there is the possibility of movement without accounting for it somewhere. If you have a structural "hanging" slab on void form then that is a different thing altogether. I currently have this on a 22000+sf home with 6" of expansive soils. But we're sitting on 199 12" caissons with 12" grade beams between them. All grade beams and slab on 6" void form. Is what I do is install a bentonite tube at the intersection of the slab and wall. When water comes into contact with it - it expands and does not allow the water to flow through while still giving any movement a chance. I've also used this for interior concrete sump pits in large buildings.

[Chris Lewis]

According to HA HA Budys Here :

The key is "good drainage", full stop. When it comes down to it, no "sealing" technology (eg: membranes, epoxy, etc) is going to be bullet proof if the drainage is poor.

The CMHC and NRC have some rather good papers on the subject.

[AJScott]

Good drainage is indeed a key, but there's one thing nobody can prevent, and that's high water table in naturally bad ground -- at least "bad" in terms of nature conspiring against you. In my neighborhood, the dirt has a bit of clay in it, and since the whole town sits on a massive bed of limestone, it doesn't take much to get that ol' water table rising. Once it rises, it stays up for a good while -- which means the seepage can continue (especially thru cracks in the slab floor) for as long as 2-3 weeks after the rains stop if you get several inches of it at a crack.

AJS

[RB]

Since you can have a full walkout on one side I'd suggest a drain around the entire foundation. I have 4" perforated pipe in gravel along the footings on three sides of my house which drain to the surface on the fourth (low) side. The pipe trenches were covered with permeable polymer fabric before backfilling. These drains drip continuously and without them I suspect that I'd have seepage problems. It's not a big expense item when the foundation is exposed.

RB

HA HA Budys Here wrote:

[Chris Lewis]

According to AJScott :

I was going to expand a bit on what I said, but I had to go beddy byes ;-)

The obvious parts of "good drainage" is things like making sure that the ground slopes away from the foundation.

The more subtle part of it is ensuring that you have a good gravel bed, and drains around the footings, etc.

Even with a high water table and expansive clay and other yuckies, french drains etc. can do a great deal to keep the basement dry. Our basement stays absolutely dry even though the water table sometimes rises above the floor level. That I attribute directly to the french drains, proper attention to the footings, and keeping hydrostatic pressure _away_ from the foundation walls.

I strongly recommend browsing the NRC and CHMC web sites for this information. Some of the US sites (ie: Minnesota's housing stuff) references the NRC/CMHC research.

One thing that's worth reading is the standards for treated-wood foundations. More than anywhere else, proper site/foundation prep is _critical_ for success with treated-wood foundations.

This is not to suggest that you _do_ treated-wood foundatons, but, reading up on how they do it will give you lots of good information and ideas for damp-proofing traditional concrete basements.

[AJScott]

I dig the French Drain concept; too bad trenching down 4 feet to install one is quite the significant expense for people with existing homes. I'd have one put in in a heartbeat if I was having a new house built, tho-- along with having the outer foundation wall waterproofed top to bottom, too. Once you have an old house that seeps, keeping it from happening on a new house becomes more of a priority ;)

AJS

[HA HA Budys Here]

Thanks, yes I considered the preforated pipe circling the entire circumference.

[Chris Lewis]

According to AJScott :

French drains are on the _inside_ of the footings - underneath the floor just inside of the wall.

The problem isn't trenching down 4 feet, it's pick-axing the floor to stick the pipe in, the carting of dirt and gravel in and out of the house, and redoing the concrete.

It's done, it ain't cheap, but sometimes it's the only...

In new construction, the materials cost is just the cost of a of a couple rolls of perforated corrugated plastic drain pipe, plus a little fussing with the gravel bed just inside the footings.

A true nobrainer.

[AJScott]

I'm aware of the ones that are often built along basement walls and feed into a sump, and the drainage sump-pumped outdoors. It's been my impression (perhaps mistaken, eh?) that French drains can also be built on the outside of existing walls as well -- except the water is carried out to the back 40 of the property instead of being pumped. Granted, that sort of thing isn't *as* effective as interior-wall drains because you're catching less water since you're not addressing the water/hydro pressure on the inside of the footings, but you're still and all relieving a good bit of pressure/water from the foundation proper -- which can't be all bad, no?

AJS