Moisture in stem wall slab foundation

[Randall Clark]

Home is three years old and has moisture stains on all four sides of the foundation. Some of the stains had elevated moisture (~25%). No signs of interior moisture in floors or walls (floor coverings are carpet and cheap engineered wood). Water meter did not indicate a leak, although that would be hard to determine if very slow.

I doubt this is a plumbing issue considering stains are all all four sides of home and no signs in the interior. Also, multiple houses on the street have similar staining.

My guess is possibly moisture wicking and/or wet from driving rains. 2018 was the highest amount of rain in this area ever recorded.

Any suggestions as to the cause(s) and if this will be of detriment?

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[Gunnar Alquist]

Home is three years old and has moisture stains on all four sides of the foundation. Some of the stains had elevated moisture (~25%). No signs of interior moisture in floors or walls (floor coverings are carpet and cheap engineered wood). Water meter did not indicate a leak, although that would be hard to determine if very slow.

I doubt this is a plumbing issue considering stains are all all four sides of home and no signs in the interior. Also, multiple houses on the street have similar staining.

My guess is possibly moisture wicking and/or wet from driving rains. 2018 was the highest amount of rain in this area ever recorded.

Any suggestions as to the cause(s) and if this will be of detriment?

Randall,

Certainly, this is something that you should put in the report. The problem that I have with this kind of condition is the uncertainty. If it were me, I would also mention that I could not determine the source of moisture. As far as a deferral, my best guess would be plumbing, but you already addressed that.

[tkaiser1]

Randall,
Given your comment that other houses in the neighborhood have similar observed moisture staining, and that the staining was observed on all four sides of the foundation, I would ask a few more questions, though this is not for the report: They all come down to one word: drainage, drainage, drainage!
Where I lived for over 30 years and started my inspection business - in the Willamette Valley in Oregon, drainage was everything, kind of like the 3 most important things in the success of a restaurant (location, location, location!)
1. Is the whole development located in a "bowl"?
2. Did the builder not do underground drainage appropriately?
3. Is there any outslope at the foundation, or does the ground slope towards the home? (6" drop in 10' is the standard I have seen applied since I've been in this business: not always possible, but a good target)
4. Is the gutter system properly terminated so that the runoff water is directed a suitable distance away from the foundation? 5-6 feet hopefully.

For the report, at least a part of Gunnar's comment is appropriate. (I could not determine the source of moisture)
If it's plumbing, I would be surprised; though if the whole development had the same plumbing contractor that used a bad batch of piping or other parts????

Best luck and have a great New Year!

Home is three years old and has moisture stains on all four sides of the foundation. Some of the stains had elevated moisture (~25%). No signs of interior moisture in floors or walls (floor coverings are carpet and cheap engineered wood). Water meter did not indicate a leak, although that would be hard to determine if very slow.

I doubt this is a plumbing issue considering stains are all all four sides of home and no signs in the interior. Also, multiple houses on the street have similar staining.

My guess is possibly moisture wicking and/or wet from driving rains. 2018 was the highest amount of rain in this area ever recorded.

Any suggestions as to the cause(s) and if this will be of detriment?

[Randall Clark]

1. Is the whole development located in a "bowl"?
2. Did the builder not do underground drainage appropriately?
3. Is there any outslope at the foundation, or does the ground slope towards the home? (6" drop in 10' is the standard I have seen applied since I've been in this business: not always possible, but a good target)
4. Is the gutter system properly terminated so that the runoff water is directed a suitable distance away from the foundation? 5-6 feet hopefully.


Best luck and have a great New Year!

Thanks, guys. To answer your questions, the houses I looked at are in a low area with houses above these across the street. Grading around this house is decent and gutters are installed and diverted a few feet away from the home, but that could be improved.

[Jerry Peck]

What stands out to me is the vertical rectangular stacked bond pattern ... and the "What is causing that?" question.

I would expect to see a horizontal rectangular running bond pattern on a stem wall.

If stacked bond was done (which is not as strong laterally), then a horizontal rectangular pattern instead of a vertical rectangular pattern.

I would mention that with a question of "why".

That could give the answer to the other "why" question.

[Gunnar Alquist]

Thanks, guys. To answer your questions, the houses I looked at are in a low area with houses above these across the street. Grading around this house is decent and gutters are installed and diverted a few feet away from the home, but that could be improved.

Ahh, new information.

Then I am more inclined to agree with tkaiser1's (he must have gotten a lot of crap as a kid with a name like that) answer. There is a development near me that was built on cut soil at the base of a hill. The homes were slab construction and the floor tiles in many of the homes began popping-off after the winter rains. The going theory was some kind of hydrostatic pressure building up under and in the slab was pushing-up under the tiles (I don't pretend to understand why pressure would build-up under the slab and not just vent-out through the surrounding yard). Since the home in your post is lower, I would suspect some kind of pressure pushing the moisture up into the fill behind the stemwall.

If stacked bond was done (which is not as strong laterally), then a horizontal rectangular pattern instead of a vertical rectangular pattern.

Jerry,

This statement (pulled-out of context) had me scratching my head a bit, until I remembered that I live in earthquake country and you don't. In my (earthquake prone) area, rebar is used horizontally and vertically to reinforce block (or concrete) walls. Pretty much any block or poured concrete wall will have horizontal bars (bond-beams, in the case of a block wall) and vertical steel if the wall is holding back a surcharge or over a certain height. Also, all of the block cells (in the case of a block wall) will be solid-filled with concrete.

Then I realized that N.C. (and Florida) is not considered to be in a seismic zone. One of the fun things about this site is learning about different construction needs from different parts of the country. Like - what's an ice dam and why don't we have basements? (sunny California)

[Jerry Peck]

Gunnar,

We (In Florida) also use reinforcement steel horizontally in footings, headers, bond beams (made from CMUs), and tie beams (formed and poured concrete) as it holds the roofs to the foundation when hurricanes try to lift houses by their roofs.

For the above to work, vertical steel is used in filled cells (of CMUs) and formed and poured concrete columns.

What I was referring to was laying up masonry walls using either stack bond or running bond patterns.

Running bond creates a stronger wall than stack bond does.

The crack pattern in the photo suggests a stack bond pattern behind the stucco (could just be a parge coating, but that's a rather large surface for parge coating.

[Gunnar Alquist]

Gunnar,

We (In Florida) also use reinforcement steel horizontally in footings, headers, bond beams (made from CMUs), and tie beams (formed and poured concrete) as it holds the roofs to the foundation when hurricanes try to lift houses by their roofs.

For the above to work, vertical steel is used in filled cells (of CMUs) and formed and poured concrete columns.

Ah ya, I guess I forgot about dem stiff winds youse guys gets down dere. Personally, I'll take earthquakes any day. I don't want to get blown to Oz (or Kansas).

Running bond creates a stronger wall than stack bond does.

Even with rebar? I knew that a running bond was better with brick or unreinforced block, but I assumed the steel would make up any difference between the two bond types as far as strength goes.

[Jerry Peck]

Think of it this way - you start out with two types of wall structures, running bond and stack bond, and one is stronger than the other.

Now add steel, which makes each stronger ... strongER ...

But the wall which started out strongER is still stronger.

It's like adding 5/8 plywood to 3/8 plywood versus adding 5/8 plywood to 5/8 plywood ... the 5/8 plywood reinforcement added (the steel) makes each basically the same amount stronger.

Like adding HJR to a masonry wall makes it stronger.

[Gunnar Alquist]

Think of it this way - you start out with two types of wall structures, running bond and stack bond, and one is stronger than the other. Now add steel, which makes each stronger ... strongER ... But the wall which started out strongER is still stronger. It's like adding 5/8 plywood to 3/8 plywood versus adding 5/8 plywood to 5/8 plywood ... the 5/8 plywood reinforcement added (the steel) makes each basically the same amount stronger.

I wonder what the effective difference in strength is, given the same amount of steel in a stacked bond vs a running bond.

Like adding HJR to a masonry wall makes it stronger.

HJR... Hepatojugular Reflux?

Oh, Horizontal Joint Reinforcement.

[Jerry Peck]

Long time members of this board will likely recall a case I had many years ago where the client had wood floors which were saturated with water.

I determined that the cause was areas of the plastic sheeting moisture barrier had been pulled back by the rake when the concrete for the slab was placed and rakes were used to 'pull the WWM back up from having been pushed down to the plastic' by the concrete (the WWM was supposed to be near the center of the concrete, not laying pressed against the plastic under the concrete).

The contractor (builder) finally agreed to remove the wood floor in the effected area and bore a 16" hole through the slab 'to prove me wrong' ... with the result being that the contractor proved that I was right ... there was no plastic sheeting under that hole. We could reach under the edges of that 16" hole (the largest bore cutter the drilling contractor had on his truck) and feel the plastic an inch or two back from the hole.

The reason I bring this back up is that top of that slab was about 8" or so above ground level, yet the hydrostatic pressure was sufficient to drive the moisture up through the slab in sufficient quantity to saturate the wood flooring (I could press my thumb to the woof flooring and water would squeeze out, it was that saturated) - so pushing the water high enough to cause the moisture being discussed should be easy to occur, especially after above normal rains.

(The builder ended up buying that house back and making my client whole - fortunately it was only a small house of about 4,000 sf and only around $1 million or so in cost ... from memory, a search may find those old posts where I would have more accurately described the house because it was a current issue, not a 20 year old memory.)

[Jerry Peck]

On a similar issue, is my following hypothesis reasonable and possible?

Conditions:

1) The house our daughter bought in NC had the basement flood (only about an inch or two deep, but that was with two portable sump pumps brought in to pump the water out as fast as it was coming in) last week after record rains, which also raised nearby rivers to record levels.

There had not been any previous flooding, and no water staining on the basement walls.

The water apparently was coming up around the perimeter of the floor slab where the slab met the walls.

2) When they bought the house last April, the radon level was slightly high, so a radon mitigation system was installed.

Conclusions:

We brought out granddaughter back up after winter break and I came up to help assess and address the flooding, and while here I noticed that the radon mitigation system which had been making a 'whoosing' sound (best description of a fan running and pulling air from under the slab and exhaust that air to the outdoors) - but the radon mitigation system is now making a 'gurgling' sound like it is drawing air through water.

Hypothesis:

The radon mitigation system is (intentionally) placing a negative pressure on the underside of the basement floor slab.

Prior to that, atmospheric air pressure was on the slab and potentially under the slab (getting between the slab and the walls).

The atmospheric pressure, as it does with unrestrained water tables, was keeping the water level below the slab slightly pushed down below the slab.

The radon mitigation system's negative pressure under the slab has allowed hydrostatic pressure to raise under the slab, and with record rains, that negative pressure from the radon mitigation system has allowed the water level to rise to the point that it came up through the 'space' between the floor slab and the walls.

....

Sound plausible?

[Gunnar Alquist]

Hi Jerry,

In my area of California there is no measurable radon (that I know of), so my knowledge of radon mitigation systems is essentially nonexistent. Ditto with basements. CA just doesn't have them as a rule. As a result, I am going to have to ask some pretty basic questions:

How does the fan system evacuate the radon from under the slab? Is a hole (or are multiple holes) drilled-into the slab? I assume something penetrates down below the level of the slab in order to draw out the contaminated air.

If a hole is drilled through the slab, is it possible that it penetrated through the vapor retarder/barrier and that is the source of water? In my area, the vapor retarder/barrier under the slab is not sealed at seams or penetrations, so it isn't going to prevent water from pushing-up from the soil. I assume this is typical throughout the U.S., but wanted to verify.

I have no idea what pressure differential a radon mitigation fan would be able to generate. That would require someone with at least some physics and math knowledge to be able to do the calculations. My online research (I didn't actually do any math, I just looked at a bunch of websites until I found an answer that seemed reasonable) indicates that it takes 0.2 psi differential to raise a column of water 6".

That is not a significant pressure drop and, depending on the strength/volume of the fan in the mitigation system, it seems to me your theory is plausible. But, by the same token, it might also be weather-related. If the barometric pressure in the area drops significantly, is it possible that would be enough to allow the water table to rise into the basement? Both examples appear to follow the same principle (to my undereducated little grey cells). The two pressure factors together with a higher water table caused by the "record rains" might also provide the needed oomph to draw water into the basement.

What if the next time there is a big rain and water enters the basement, the mitigation system is turned-off for a while to see if the water levels drop? Would this significantly increase the radon exposure to your family? (Do you like your son-in-law? If not, maybe your daughter and her kids could visit you during this experiment.)

[Jerry Peck]

The mitigation fan is inline in a 3"-4" pvc pipe which is through a hole in the slab, then the hole is sealed around to the pipe.

The house is 1972, NC didn't have building codes until the early/mid-1990s (based on what I've been told).

I've suggested putting in a sump and sump pump as there is not one now - but it's their house so that decision is theirs to make (remember the stairs?).

If I get to put in a sump and pump (possibly next summer when we go back up), I'll find out what is down there. And I'd need to seal a cover to the floor so as to not negate the radon mitigation system.

Think of how little pressure differential is needed to suck drink up a straw - there is a gauge on it and I think I recall the previous pressure drop of about 1"water column, it is now about 2" w.c. differential ... I presume because the water is additional resistance to air flow (the air has to be drawn through the water, not unlike trying to get the last out of a cup with ice in the bottom of the cup).

[Gunnar Alquist]

Jerry,

I forwarded this to my son. He is majoring in physics in college, so this should be easy-peasy for him. I'll let you know what he says.

[Jerry Peck]

Gunnar,

Thank you, we'll see what he says.

[Lon Henderson]

In the Denver area, in the spring of 2015, we had rainfall unseen for the lives of many homes here. Flat to negative drainage around homes that had not been corrected in 50 years was contributing water to basements that had never seen water. (I got a call from an attorney representing a client whose home I had inspected a few years earlier asking why I hadn't reported evidence of water in the basement. I said that the reason was that there wasn't any evidence of past water intrusion, but I had reported that the flat drainage apron around the house should be corrected. I never heard anymore about it)

Re: the OP, the moisture pattern in the CMU foundation looks like water wicking up the mortar joints. A short duration of that may not be a big deal if the wall dries quickly but chronically wet will deteriorate those mortar joints. Does it get cold enough there to freeze that wall? The saturated soil might liquefy and destabilize the foundation or differential drying might do the same. That CMU wall will not tolerate that well even with rebar reinforcement and worse if it is a stacked bond wall.

As far as Jerry's daughter's home, since I live in a radon area and see all kinds of mitigation systems, my two cents is that the sub-slab ventilation system didn't contribute to the flooding. The hydrostatic pressure from the soil saturation could easily force water into the basement. I concur with Jerry about adding a sump pit and pump but recommend a perimeter drain system tied to it. The radon mitigation can be moved to the pit. My best recommendation is expensive and for a one-off flood event, may not be worth it.

[Jerry Peck]

... but recommend a perimeter drain system tied to it. The radon mitigation can be moved to the pit. My best recommendation is expensive and for a one-off flood event, may not be worth it.

I did explain to them that, when wet basements are found, one solution is to install a perimeter drainage system and explained what one was and how it worked. Half of the basement is a finished room (which I suspect was a finished room from when the house was constructed in 1972 as one wall of the finished basement room is a brick masonry fireplace with a brick masonry fireplace outside going all the way up and past the roof - the grade level on that side of the house goes from full height basement wall at the front to about 1 foot above a walkout french door to a patio under a deck above ... I reasoned that if one "added" a fireplace after the house was constructed, that the fireplace would not be a masonry fireplace and chimney, it would be a manufactured fireplace with matching chimney, framed in to look like brick).

A perimeter drain system would not be practical for that finished room, or for the other 'finished room' (that is not "habitable space" as there is no window or door for natural light and ventilation - I am sure that this 'finished room' was done at some time after the house was built).

The radon system is on the right front corner, the logical place for the sump (so it is out of the way) is under the stair near the middle of the basement.

I agree that a perimeter system if not practical for a "one-off flood event" - definitely not worth it.

Put down some waterproof vinyl wood plank look flooring in the two rooms with laminate flooring (which now needs to be replaced) and install a good sump pump - if it floods again (it was about an inch or so deep with no sump pump, should be less with a sump pump), mop up the water, let the sump pump drain it back out, and don't keep anything on the floor which is valuable unless it is waterproof. The only thing damaged was the laminate wood floor - make it a waterproof vinyl wood plank look floor and the damage basically goes away for any future flood even (especially after adding a sump pump).

Basically, what is the best way to spend money to reduce the likelihood of another flood event: 1) put in a sump pump; 2) replace the floor with a waterproof vinyl wood plank look floor; 3) buy flood insurance (with flood insurance possibly not being needed after doing 1) and 2) ... ).

[Trent Tarter]

Are you sure that's the actual foundation/slab edge. It looks like it could possibly be rigid foam insulation with failing stucco coating. Does it feel like solid concrete?

[Gunnar Alquist]

Hi Jerry,

I have been in contact (back and forth) with my son. The answer that I received (if I understand correctly), is this:

Robin told me the problem with the original theory that the fan has raised the water level under the basement slab is primarily the weight of the water over the whole area of the basement. While it doesn't take much pressure to raise water contained within a straw a few inches (my example). A bit more pressure is required with a 2" pipe (I don't know the diameter of the radon pipe, so I assumed 2"). The problem is - the intake end would have to be immersed in water in order for it to raise any water into the pipe (this might account for the gurgling you noticed), otherwise it would just be sucking air as normal. If the fan intake is above the water, then it would be necessary for the fan to exert enough force to lift water the entire area of the basement. That's a lot of weight.

Think of it this way by moving to solids: If we tried to lift a 2" diameter by 4" thick piece of concrete, we might be able to do it with a 2" vacuum cleaner hose (as long as it was nicely sealed to the concrete). However, we would not be able to lift a basement-sized slab of concrete with a 2" vacuum hose. That would require a vacuum hose the same size as the slab and a vacuum motor large enough to exert the force necessary to counter the weight of the entire slab of concrete. So, instead of several ounces, it would be thousands of pounds.

He wasn't able to give me information on the effect of barometric pressure, but I might just be barking up the wrong tree here.

This seems to leave the hydrostatic pressure that Lon mentioned along with the heavier than normal rains.

Were other basements in the neighborhood flooded as well?

I like the idea of the sump pump to discharge the water from the basement the best. But, even if the lid were sealed, would the pump's discharge line to the exterior negate the effectiveness of the radon system by allowing air from the exterior back under the slab? Oh, wait. A check valve would prevent that. Ok, assuming your theory is correct, then the area under the slab is under a negative pressure. Would the sump pump be strong enough to counter the pressure-differential and evacuate the sump?

[Jerry Peck]

Yes, many other basements flooded too.

The vacuum and slab raising is a different issue as the slab is being held down by gravity, whereas the water table is held down by gravity and atmospheric pressure, and being as water weighs a lot more than air, a 1 foot column of water equals 0.43 psi, thus a pressure difference of 2 inches of water would equal about 0.07 psi, reducing the pressure by that amount, which would then calculate out to allow the water to raise that 2 inches.

It's not that the radon fan is 'sucking water up 2 inches', its that the radon fan is reducing the pressure against the water by 2 inches water column (as measured on the manometer) in the confined space below the slab (try this - blow up a balloon, put a 1 inch long piece of ScotchTM tape on one part of it, poke a straight pin through the tape ... the balloon will not 'pop', and the space around the pin is in close contact with the pin - 'sealed like the slab is sealed to the wall' - and the higher pressure inside the balloon will deflate the balloon).

Thus, at least it seems like, the lower pressure will not hold the water back as far in that contained space under the slab.

I figure if I seal the sump pump cover (tight fitting seal, not "sealed" with sealant) that whatever effect the radon mitigation pump has about 20 feet away will not be great on that cover. And the water in the sump will be hydraulically pumped out, not by moving air, but by being submerged in the water, thus pumping the water out should not be affected, other than by any effect of the radon mitigation system 'allowing' a slightly higher water level.

Anyway, that was/is my theory, the reduced pressure must have "some" effect on the under-slab conditions ... wouldn't it?

Besides, I don't see a negative side to installing a sump and sump pump ... do you?

Just for fun:
- It takes approximately 34 feet of water to give 14.7 psi.
- It takes all the air in the atmosphere above sea level to give 14.7 psi.

[Lon Henderson]

For those of you, who never see radon mitigation systems, the exhaust tubes are 4" in diameter. When integrated into a sump pit, the pit lids are sealed and there is a check valve on the sump discharge tube. Best practice for the mitigation system is to seal all the slab and foundation cracks and transitions in the basement, but that sealing won't keep water out.

BTW, in the Denver metro area, sump discharges are not allowed to go into the city sewer systems and must discharge out in the yard a minimum of 5' from the foundation. That may seem counter intuitive, but a lot of sumps adding water to the city system can overwhelm it.

[Gunnar Alquist]

For those of you, who never see radon mitigation systems, the exhaust tubes are 4" in diameter. When integrated into a sump pit, the pit lids are sealed and there is a check valve on the sump discharge tube. Best practice for the mitigation system is to seal all the slab and foundation cracks and transitions in the basement, but that sealing won't keep water out.

Thanks Lon. I have no concept of a number of the things that other parts of the country deal with (ice dams, sinkholes, radon mitigation, locust). But, we on the left coast have our own nifty challenges.

BTW, in the Denver metro area, sump discharges are not allowed to go into the city sewer systems and must discharge out in the yard a minimum of 5' from the foundation. That may seem counter intuitive, but a lot of sumps adding water to the city system can overwhelm it.

I thought that was true for most places. The sump should be just water, although probably not potable. I do know that in San Francisco, the storm drains share the sewers. I can't imagine the amount of water that adds to the system.