Hi,
I have a 1950’s ranch with a cement block foundation with a mild water seepage problem. I’m considering a french drain system around the interior perimiter of the cement block walls before refinishing the basement. One contractor has warned me that the digging of the perimeter trench could destabilize my foundation walls because I have what he calls a monolithic pour in my basement. The basement floor and footers for the wall were supposedly poured at the same time. He tells me that he knows this because he can see that the bottom cinder block row is fully visible at the base of the walls.
Two other contractors dispute this.
Who should I believe? Will I need a structural engineer to settle the issue?
Replies
Even with a monolithic pour, it should be safe to install the drain if you keep it some distance away from the wall -- 18 inches or so. This will still work OK if the soil is sufficiently permeable.
However, if possible it's better to install the drain system on the outside.
happy?
Thanks for the reply.
An retrofit exterior system would be prohibitively expensive. My water seepage happens only after prolonged rains of over 3".
Regardless, be sure exterior drainage is good. Soil should slope away for at least 5 feet, and no standing water within 10 feet.
If ignorance is bliss why aren't more people
happy?
Also a good point.
Gutter drainage issues have been corrected. The site is well drained. Very rare to see standing water anywhere on property except with rainfall greater than 6" within 24 hours. In northern NJ this only occurs once every several years.
maybe you don't need a deep drain but only something 1" to 1+1/2" by 3" wide leading to a sump pit or drain. If there's not a large amount of water entering but just a bit of nuisance water say 1-3 gallons an hour, the small drain will take care of that.
The idea of the subfloor tile is to lower the water table below the basement, so that water doesn't seep in. This keeps the concrete dry, humidity down, and reduces mold problems, etc. Catching the water after it gets in has none of these benefits.
If ignorance is bliss why aren't more people
happy?
We have been using this system here for over 100 years since concrete floors have been poured inside the leaky stone foundations. As a matter of fact, it is called the "haligonian drain" and recommended in the "Foundation Problems" booklet of the Builders' Series put out by our national housing agency, Canada Mortgage and Housing Corp. in the mid 80's. Another version of the interior drain is installed/poured into over 50% of the new floor slabs in the city I used to work in. I have a 10 foot modified retrofit version behind the walls/under the built-up floor of my finished family room where I now type. Been there 6 years now; no problems showing.
This is not a constant moisture problem but only a few times per year.Is the problem from a high ground water table or just not open enough drainage (like crushed stone) along the block walls to allow the water to get to the water table or exterior tiles (if installed) fast enough. The water on its way down will take the path of least resistance and if that is sideways through cracks, then there it goes. I have seen hundreds of concrete shrinkage cracks that leak from 1' to 5 or 6 ' below grade. Putting in a new exterior perimeter drain will not help these leaks.
Story: I was asked by an employee relocation co. from Dallas to re-inspect a house in the small city where I used to run my home inspection firm (yeah way up in Moncton, New Brunswick; I was surprised also). The 7 year old house had a foundation wall leak and an engineering co. had been hired by their client to do the repair. By the time I got there, they had taken concrete core samples from walls and floor to determine strength by cracking the cores, the foundation drain tile had been dug up including moving a large wooden deck and the leak had been patched- Cost in the late 80's= $7,300. Upon talking to the excavation contractor, he informed that the original tile was in good shape and quite clean but they were given the job to replace all and did so.
The water had been entering at a normal shrinkage crack that had poor grading near it. The water actually came through the wall about 3' above the basement floor. It never even got near the drain to drain!! The solution was to seal the crack from the exterior for about $400- the going rate for such a crack at the time. $6,900 less than the amount spent. I have found that in basement wall leaks all is not as it seems; the first reponse is not necessarily the best.
Occasionally you'll get a situation where the water is "caught" on the way down, due to non-homogenous backfill. It's unusual, but, as you say, the fix is a simple waterproofing job from the outside, or, better still, simple regrading.Much more common is high water table. The solution there is to lower the water table, as it's impossible to perfectly seal any standard masonary foundation to be leak-free against water with any sort of "head". To lower the water table you either deflect water entering the soil or tile below the level of the floor. How far below depends on the permeability of the soil and hence the "slope" of the water table.I've seen several trench and baseboard "solutions". They do (mostly) keep liquid water out of the basement, but don't do anything for dampness.
If ignorance is bliss why aren't more people
happy?
The very first house I owned (a 200 year old post & beam Cape Cod; paid $9500 for it and 2 acres in 1980; it was my main learning project as I actually bought "a frame" and a good view) had a high water table. Even though it was on the crest of a hill, the local geology had the water table only 5' from surface at certain times of the year.The hand driven point well was only 17' deep and we drew water from the 13' level. So it was "trial by water".
When we raised the house and put in a new 8' foundation by excavating another 2.5', we found a spring about dead center of the interior basement which I encircled with 4" drain tile and ran out through the footing to the exterior drain tile. Ran 2 - 4" drains out to the municipal ditch to take ground water away. In wet springs, they would both run 1/2 to 2/3 full for a week or two.
Your Quote: "Occasionally you'll get a situation where the water is "caught" on the way down, due to non-homogenous backfill. It's unusual, but, as you say, the fix is a simple waterproofing job from the outside, or, better still, simple regrading.
Much more common is high water table."
In my home inspection business in the Canadian Atlantic provinces, what I see predominantly (95%+) for basement water leakage in newer homes (last 40 years or so) with drain tiles is nuisance water at normal concrete shrinkage cracks in foundations built just to code. This water enters from 1' to 5-6-7' below grade. This is what you refer to as "caught water" (a good term, by the way)
In older housing without drain tiles, it leaks all over the place with a lot coming up through cracks in slabs poured 50+ years ago without a layer of drainage material underneath or without an interior drainage system. These slabs more often than not are poured directly on bedrock!! Note: The local bedrock is seen as outcrops all over the city and may only be 2-4' down when you go to excavate for an 7-8' deep foundation
Edited 11/23/2005 8:45 am ET by experienced
Its not possible to know for sure if I have exterior foundation cracks without excavation. However, my house is next to the last house at the bottom of a hill with the slope of the hill leveling off in front of my property. The water entering my basement visibly weeps through the lowest cinderblock course after about a 3" rainfall without any ponding noticible on my property anywhere near the house. My guess is that the water table below ground does rise, enters the cinderblock wall either through small cracks in the block or the mortar joints, or maybe the block is just pourus from age and the chemical milieu underground.
My real question is whether or not excavating an interior trench to install a french drain system will cause structural damage in my situation where the wall footers and basement floor were originally poured at the same time in a so called monopour or monolithic pour.
A full excavated interior drain may not ensure drainage fast enough from the exterior under the slab/footing if the soil is not porous but clay/mostly clay. The exterior drain is the 100% solution but the 99% solution is the small 1-2" interior drain at the edge of the interior block to catch the water before it runs onto the floor. Here if it is only 1/32" -1/16" deep but covers quite a number of sq ft it becomes a large humidifier. Catching it and directing it to a sump pit/drain, prevents this at a much reduced cost.
Side Note: This inexpensive little interior drain cast into the floor slab/wall junction of homes may be the topic of my application for an external research grant from our national housing agency. I talked to one award winning small builder who has used it in every house he has built (350-400). He has had basement water problems in only 2 of his houses and those were extraordinary cases caused by local conditions or the homeowner.
Storey: How bad can it get???
Twenty years ago, I installed a heat recovery ventilator in a small house being built by a young couple trying to put as much sweat equity into the house as possible. And they wanted to do a more than good job on the house. (He was called on the carpet at the window manufacturer he worked for because he bought a competitor's windows as he knew they were better!!!) After the foundation was poured by the highest bid contractor, they did the dampproofing, drain tiles, crushed stone and backfill themselves.
For drain tiles, they installed not one but 2 runs of 4" along side the footings and then installed about 1+1/2' of crushed stone over the tiles. This constituted more than double what codes required here. I had been given a key to work at the place as I pleased since it was a year long project. One Saturday morning in cold late rainy November , I arrived at the house to see condensation on the windows and when I entered it felt very damp.
I went to the basement to install ducting and saw a 3' square hole smashed in the slab with a hole and a sump pump installed. The owner had come to work in the house Friday night to find 1/2-3/4" of water fully across the slab! How did it get there? There was no water installed yet! It came up through normal shrinkage cracks in and at the edge of the slab/wall intersection.
The scenario leading to failure: (1) The footings had been poured on pure clay; (2) no drain tile was installed through the footing to drain the inside of the footing to the exterior drain tile (I recommend every 6-8'); (3) the house was at the lower section of a long downhill slope (like yours); (4) with the wet fall, a spring came up inside the footings that acted as a dam and didn't let the water out to the doubled drain tile so it rose where it could- through the shrinkage cracks!!
Joining the inside of the footings to the outside tiles is not required by codes here but should be. In this case the young couple went way beyond code but still had to install a sump pump! I'm surprised that the foundation company didn't recognize the situation and add the $100 of extra tile- must not have run into it before.
Edited 11/26/2005 10:40 pm ET by experienced
"he can see that the bottom cinder block row is fully visible at the base of the walls..."
Normally a footer is poured, the foundation block is then laid up, and the floor is poured later. The floor usually "rests" on the inside edge of the footer, thus covering up a portion of the bottom block course. Since the block are not partially covered, it does seem to indicate that the block was laid on top of the floor...which could indicate a monolithic style pour. But usually,monolithic foundations are for single story buildings...and it appears your structure is at least 2 stories.However, it is possible that your house is on a monolithic pour.
It would be wise to contact a structural engineer since there seems to be a dispute among your solicited contractors.
Personally, I would not tear up the inside perimeter of your basement unless last resort. An outside dig is always the best way to go.
Davo
Thanks for the reply.
My house has just one level above the basement. It is a 1950's ranch.
I just might need a structural engineer's input unless I can find some other definitive source of information about this issue.
Any ideas about a good source of information about interior french drains?
PVAC22, Workers started demo today in my basement an found that I have a monolithic slab/footer. I am now worried I made the wrong choice and should have gone with the more expensive / destructive external system. What ended up happening for you?
Doubtful you’ll get a reply from a 20 year old post….
Well it was worth a try - you responded… ;)
April,
It doesn’t hurt to start a new post.
You might give it a try. Can’t hurt and there very well might be new ideas regarding your problem. Give it a good title and you might get lucky.