Wet basement leads to mold (C) Daniel FriedmanWet Basement Dryout / Prevention
How to Stop or Prevent Basement Leaks & Water Entry

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Wet basement prevention: this article discusses methods for preventing wet basements by attention to multiple best construction practices, including the basics of foundation d & roofing, poor site drainage, bentonite clay for basement waterproofing, foundation membranes to prevent leakage, foundation drain tiles, proper backfill, and proper finish grading.

A case study of a foundation collapse is described and we offer a few simple steps that might cure a wet basement without major work. A case study of a flooding basement, a rotted piano, and clogged gutters describes how home buyers were fooled into thinking they were buying a home built over a stream.

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How to Prevent a Wet Basement - causes and cures for basement water entry may be more basic and easy than you think

Foundation collapse (C) DanielFriedmanWet basement prevention steps discussed here include:

  • Steps to prevent a wet basement and offers a few simple cures for existing wet basements - photographs & explanation of of basement waterproofing methods & procedures, simple cures for wet basements without major work - steps that might work
  • Multiple defenses are the best guard against basement leaks: foundation d & roofing & correcting poor site drainage to stop basement leaks;
  • Using bentonite clay, foundation membranes & geotextiles, foundation drain tiles, proper backfill, proper finish grading to prevent basement or foundation leakage
  • Using geotextiles and foundation waterproofing barriers and drainage mats
  • Case report of foundation collapse due to settlement & wet soils; case report of the two foot piano - "I'm just washing the basement walls", said the home seller

Article contents

Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Wet Conditions Lead to Building Foundation Collapse

Wet basement or leaky basement prevention and cure  (C) Daniel FriedmanThis article begins with a pair of foundation collapse case studies, describing poor site drainage and wet soils that led to the foundation bursting into the home. Our photo (left) shows an example of severe foundation damage and ultimate collapse due to water and wet soils outside the foundation wall. Mr. Bliss points out several factors that led to two foundation catastrophes that he investigated:

  • The site was sloped, and each foundation collapse was on the uphill side of the building
  • The land was sloped so that water collected behind the foundation and soaked into the ground
  • While original site grading might have sloped away from the foundation it was inadequate, so as excavated soils settled around the building ground sloped in towards the foundation wall
  • Surface or perhaps roof runoff collecting close to the foundation saturated the soils, increasing the soil's mass and ultimately bursting in the foundation wall
  • No sub-surface foundation drainage system was installed, or if there had been one, it was no longer working
  • In-house basement wall waterproofing, if present, might actually have contributed to the foundation wall collapse by failing to provide another possible path for water (and its pressure) to escape from outside the foundation wall

Bliss notes that foundation leaks are said to cause more callbacks than any other problem in new home construction. Some building sites with good natural drainage are unlikely to result in a leaky basement even if the builder does not follow all of the best foundation water entry prevention steps during construction and site work. But others are a catastrophe waiting to happen - or as above, a catastrophe that has already happened.

Living With a Basement that Floods is Unnecessary and It's a Bad Idea

Wet basement leads to mold (C) Daniel FriedmanThe Two Tall Foot Piano

Living with periodic basement flooding - which some people manage, is not a great strategy, risking moisture-related damage such as toxic mold contamination (photo at left), building rot, termite damage, or even shorter roof shingle life.

The epitome of the "make it work" approach that I find troubling is wet basements.

People will install a $15,000. internal trench and drain system to "dry out" their chronically wet basement before first noticing that outside the gutters and downspouts spill right by the foundation wall, and inside we see water stains originating high on the basement wall, confirming that source of moisture. Instead we install a "fix" that first lets the water come into the building (where secondary problems of moisture will continue), then drain it away.

One of our clients (DJF) lived for thirty years in a house that flooded repeatedly in wet weather.

Many years ago I was hired to inspect a house for sale. The owners were told by their realtor they really ought to look into fixing their flooding basement before putting the home on the market. The basement had been chronically wet, flooding to several inches each time it rained, since before the owners bought the home 25 years before.

At my inspection the basement was full of 30 years of accumulated stuff, most of which was soaked and rotted. There was an upright piano that whose legs had rotted off - the piano had gradually settled down to the floor and just its top two feet remained.

"I'm Just Washing the Basement Walls" the home seller said.

When Mr. owner was looking at the home and liked it his wife wanted to come up from New York City to see it. They called the owner the next day. It was raining. "Today is not a very good day" the owner said. "I'm kind of busy". They insisted. Owner acquiesced.  On their arrival, in pouring rain, the buyers, naive city dwellers who'd never owned a house, found the owner, in the basement, standing in 24" of water. He was  wearing hip boots, and with a garden hose hooked up he was "spraying" the basement walls.

"What are you doing? " they asked.  --- I would have asked "What the hell are you doing???"

"I'm washing down my basement walls" was his explanation."I do this once in a while. Today seemed like a good time."

The buyers were not the least suspicious. They didn't think this was an odd rainy-day activity. They bought the house with the very clean basement walls.

The house is built over an old stream bed, he explained

Soon however, the new owners complained to the old owner that every time it rained their house had a basement flood. He explained that well the house was "... built over an old stream bed, and when it rains the stream rises into the basement - there's really nothing to be done about it."

Starting by noticing that basement leak stains originated high on the foundation walls I was sure that even if there was an underground stream, basement water was also coming off of the roof or from bad surface drainage. Outside I found that a concrete patio had been poured against the foundation wall, later settling and tipping towards the home.

Inside the basement I saw huge water streak stains running down the rear basement wall at each of three basement grade-level windows.

Outside I found a solid concrete patio that had been poured against the foundation wall and that sloped in towards the house.

Tall pines near the home made sure that the house gutters were constantly clogged with pine needles and clogged, overflowing onto that patio at every rainfall. Water ran across the patio and down through the basement windows and inside, down the foundation wall onto the floor. Interestingly I did not see other water entry marks elsewhere on the basement walls.

A simple experiment of cleaning the gutters and temporarily routing the downspouts well away from the home using above-ground flexible extensions was enough to immediately stop the basement water entry.

Fixing the in-slope patio was also needed to handle the heavy rainfall case.

Steps to Prevent Basement Water Entry - the Basics

Soil Testing Predicts Soil Drainage Ability

For new construction, start with a few soil tests to understand what the house is being built upon. Any civil engineering manual will include a soil guide describing soil types and their drainage characteristics. Watch out for clay soils and silts that are unstable and expand when wet or frozen.

Conduct a soil percolation test to get an accurate idea of how soils drain around the home. If a septic system is to be installed you're going to need soil perc tests anyway.

See Septic Soil PERC TESTS and PERC HOLE SPECIFICATIONS. Remember that if your perc tests are performed in the dry season you may get over-optimistic results. And if living space in the home is going to include below-grade areas, consulting with a soils engineer may be good insurance.

Water Entry Prevention Priorities

Inslope grade makes site drainage difficult (C) Daniel Friedman
  • Keep water away from the foundation. This means proper site drainage that assures that surface runoff and roof spillage are conducted away from the building.

    Most of the wet basements that we (DJF) have investigated were suffering from mishandling of roof drainage. Between gutter defects that spill large volumes of water close to the foundation and improper site grading, we estimate that 80 percent or more of basement water entry problems can be explained. \

Our photo (left) shows a home with an in-slope grade facing the house wall. It would have been relatively easy to install a swale draining hillside water and roof spillage around the left side of the home in this picture. Instead the owners suffered decades of wet basements until the wet conditions made the home sills so attractive to termites that major structural damage had occurred.

  • "Tar coating the foundation" - D & roofing: traditional d & roofing is a bituminous coating that is rolled, brushed, or sprayed (least effective) on the foundation wall. D & roofing is not waterproofing, and water will leak through a coated foundation wall wherever there is the combination of cracks or other openings and water.

    What d & roofing does is slow the flow of water vapor through the foundation wall by breaking the capillary flow of water through the masonry. Without d & roofing, water evaporating from the more dry indoor side of the wall actually pulls moisture through the foundation wall. But we're talking about moisture, not flowing water. If water never accumulates against the outside of the foundation wall, leaks and moisture won't be flowing through the wall.
  • Amateur foundation waterproofing: as extra insurance, some builders drape plastic against the d & roofing coated foundation wall. This is a very inexpensive step that certainly reduces foundation leaks, even if the plastic is somewhat damaged during backfill. However amateur waterproofing alone does nothing to prevent a foundation collapse if lots of saturated soil is pressing against the foundation exterior.

Effective Foundation Waterproofing Method Details

Foundation Drainage Mats & Geotextiles for Foundation Waterproofing

Foundation drainage mats and geotextiles (C) Carson Dunlop Associates

As Carson Dunlop Associates' sketch (above) and our photos (below) illustrate, proper installation of a combination of damp proofing, a drainage mat, and a geotextile to keep the drainage mat from becoming clogged by soil particles can be effective in keeping roof spillage and surface runoff from penetrating the building foundation walls.

Watch out: even this approach won't prevent basement or crawl space water entry if groundwater levels are so saturated that they are rising up beneath the basement floor slab.

Foundation waterproofing (C) Daniel Friedman Foundation waterproofing (C) Daniel Friedman

Sealing Foundation Walls: D & roofing vs Waterproofing

Details about water barrier coverings for foundations are at WATER BARRIERS, EXTERIOR BUILDING. Also see SEALERS, Basement Floor & Wall Moisture. Excepts are below.

Basement foundation sealer (C) Carson Dunlop Associates

True foundation waterproofing: for poorly drained soils, investigate true foundation waterproofing, such as heavy textured plastic or rubber membranes that are placed against the foundation wall to form a drainage layer to conduct roof spillage or ground water down the exterior of the foundation wall and into a foundation drain system to carry water safely away from the building.

Our foundation waterproofing system photographs above show (photo above right) and also Carson Dunlop Associates' sketch (left) show the use of a plastic membrane, protected by a geotextile to combine good water drainage down the foundation wall (and into the footing drains) with gravel backfill to nearly the top of grade (photo above left).

This basement waterproofing system was installed on a home that had suffered recurrent basement flooding due to a combination of in-slope grade at the rear and right side of the home combined with improperly installed and non-working footing drains, aggravated by wet soils in the area.

  • Bentonite clay waterproofing: bentonite clay can be pumped into soils around the building foundation wall - an old basement waterproofing method that in some installations works quite well to slow or stop foundation leaks. Watch out - by leaving water in soils near the foundation wall, the risk of foundation collapse may remain.

    Below at our Basement Waterproofing FAQs we give details & authoritative citations discussing the effectiveness of bentonite clay for building foundation waterproofing.
  • Use a basement waterproofing paint on the interior (or a d & roofing on the exterior) of porous masonry block foundation walls - we have had excellent results with Thoroseal™ but don't expect an indoor foundation waterproofing paint to hold back a flood. See BASEMENT WATERPROOFING for details.

See BASEMENT HEAT LOSS for a discussion of foundation and basement insulation methods. See POLYSTYRENE FOAM INSULATION for a guide to using this material in below-grade applications. See TERMITE SHIELDS vs TERMITICIDE for a discussion of avoiding insect damage when foam insulating board is used below or at ground level.

Details about water barrier coverings for foundations are at WATER BARRIERS, EXTERIOR BUILDING

Foundation Footing Drain Details to Prevent Basement Water Entry

Footing drain to daylight (C) Daniel Friedman

Foundation drainage - drain tiles also called footing drains and by some folks "French drains" (a mistake - see FRENCH DRAINS for DOWNSPOUTS),: water flows in the path of least resistance.

Perforated 4-inch PVC or flexible ABS are the least costly and most foolproof foundation drainage conduits. Foundation drains should pitch at least 4 inches in every 100 feet of length.

The top of the foundation drain, should be below the top of the finished basement or crawl space slab.

From the low corner of the building, the foundation drain should continue to daylight or, if permitted by local codes, to a storm sewer, so that water will drain away from the building without relying on an electrical sump pump or other magic.


Details about foundation drains or footing drains are found at FOOTING & FOUNDATION DRAINS

Foundation Drainage: backfill details

Footing Drain details (C) Daniel Friedman

Bury the footing drains in gravel, both under and above the drain tiles. Extend the gravel backfill at least two-thirds of the way to the top of finished grade.

Gravel helps water flow easily into the drain system instead of seeking a way into the building, and the removal of water outside the foundation wall also avoids foundation collapse later. The gravel size needs to be larger than the holes in the drain tiles.

Our sketch (left) shows a less than optimum footing drain installation because the artist placed soil too close to the drain pipe.

More details about foundation drains or footing drains are found at FOOTING & FOUNDATION DRAINS

Eliminate Exterior Sources of Water that Cause Building Leaks, Water Entry, Flooding

Details about eliminating exterior sources of foundation leaks and building water entry are at EXTERIOR WATER SOURCE ELIMINATION . Check and correct these sources of building water entry:

  • Roof gutters and downspouts - check to see that roof runoff is effectively disposed of away from the building, and that it is actually working - see GUTTERS & DOWNSPOUTS Also see FRENCH DRAINS.
  • Site grading and control of surface runoff - see Finish Grading [below]
  • Footing drains / foundation drains - check for presence of and check that water is flowing out of footing drains if water is in the basement. If the basement is wet and the footing drains are dry, they are not working. Details are at FOOTING & FOUNDATION DRAINS.
  • Plumbing leaks - don't mistake a hidden water supply or drain waste line leak for water entry. See LEAK TYPES, WATER SUPPLY or DRAIN PIPES
  • Repair foundation holes and cracks - see Seal Cracks in Concrete, How To
  • Storm drains - nearby storm drains should be un-blocked and not backing up onto the property.
  • Sump pumps - also check that if they are installed, the sump pump(s) are working and that water is pumped to a location that does not drain back into the building. Some properties use an outdoor catch basin and sump pump to dispose of roof runoff or surface runoff. See SUMP PUMPS GUIDE

Finish Grading to Prevent Foundation Leaks & Water Entry

Swale used to control surface runoff (C) Carson Dunlop AssociatesDetails about proper site grading to handle surface runoff and groundwater, including swales (shown below) are at GRADING, DRAINAGE & SITE WORK. Excerpts are below.

  • At grade, the main object is to get water away from the foundation as quickly as possible. Finish grade should slope away from the building for at least 10 to 15 feet, and should not contain low spots that will make water ponds.

Swales: if one or more sides of the building face an upwards sloping hill, slope the finish grade away from the building for at least 10-15 feet, and then shape the finish grade at that point into a swale that itself continues to carry water around to the downhill side of the building.

A swale, illustrated by Carson Dunlop Associates' sketch at left, is a nice word for a "gentle ditch" - it does not have to be deep.

  • If the building roof system includes gutters and downspouts, do not tie the downspouts into the footing drains - you will simply overload the footing drain system and risk future basement water entry. We want to see gutters extended to release roof drainage no less than 10 feet from the home, more is better, and even more important, from the point at which water leaves the downspout end, it should continue to flow away from the building, not back towards it.

Simple Retrofit Repairs Can Cure Basement Leaks

Foundation crack leak (C) Daniel Friedman
  • First identify where water is entering the basement or crawl space. See BASEMENT LEAKS, INSPECT FOR for examples of common leak points that might be hidden behind finish walls or paneling.

    Our photo (left) shows a previously-hidden foundation wall crack that was sending water into a finished basement and under the raised wood basement floor from the time the home had been built until severe flooding in the basement motivated the owners to remove wet walls and find this leak.

  • Patch leaky foundation wall penetrations or holes:

    Mr. Bliss reports stopping a substantial basement water leak (a couple of gallons per rainstorm) by chipping out around a waste pipe where it passed through the foundation wall, and patching that area with hydraulic cement.



Find and Clear Clogged Footing Drains

Details about installing, finding, and repairing foundation drainage systems are at FOOTING & FOUNDATION DRAINS. Excerpts are below.

Clogged footing drain (C) Daniel Friedman Clogged footing drain (C) Daniel Friedman

Find the end of the footing drain system that used to drain to daylight - it may have become buried with mud or covered by backfill. Clear it open and check the end for water flow in wet weather. Our photo (above left) shows the footing drain that was excavated and removed at the home of a client whose house suffered recurrent flooding. The old footing drain was totally impacted with mud.

The photo at above right shows the ends of three new footing drains that were installed and carried to daylight. We remained a little nervous about just what the builder used for backfill - notice that silty mud coming out of the new drains? They may not have a long life

Use an Intercept Drain at In-Sloping Driveways and Walks

Intercept drain at garage (C) Daniel Friedman

Add an intercept drain at the entry of garages into which a paved driveway is sending water whenever it rains.

If you are planning to re-pave a driveway, that's a great time to go to the trouble of cutting in and installing a drain that prevents water from flowing into the garage.

Connect the intercept drain to a storm drain or to a new buried drain line that slopes to daylight well away from the building.

Don't connect the intercept drain to the building footing drain system - you may overload the footing drains and cause basement leaks.

  • Add a splashblock to route surface runoff around a chimney whose side formed a water trap against the foundation wall

Basement Perimeter Drain Systems for Basement De-Watering: Installing & Inspecting "French Drains"

Details about interior perimeter drain systems such as B-Dri™ and Beaver Basement™ are found at PERIMETER DRAIN SYSTEMS. Excerpts are below.

Some people call an interior perimeter drain used for basement or crawl space de-watering a "French Drain". Whatever you want to call it, an interior perimeter drainage system cut into the basement or crawl space floor can effectively stop basement or crawl space flooding - or can it?

Wet basement cure: interior drain system Carson Dunlop Associates


An interior drainage system can indeed work to keep water off of the floors, and properly installed it can also resist sending problematic high moisture levels into the building as well.

But first you should check and repair obvious outdoor water entry sources that send water through the building foundation walls.

Carson Dunlop Associates' sketch (left) shows t approaches to installing an interior drainage system


Our home page for diagnosing and curing foundation leaks and wet basements or crawl spaces is WATER ENTRY in BUILDINGS. Readers should also see BASEMENT LEAKS, INSPECT FOR and BASEMENT WATERPROOFING.

Readers needing more extensive guidance on preventing or fixing basement leaks and moisture should see BASEMENT LEAKS, INSPECT FOR, or if your building includes areas over crawl spaces, see CRAWL SPACE DRYOUT - home.

If your building has been flooded, see FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. The original wet basement prevention article (linked-to below) has been adapted and expanded in this document.

The article above paraphrases, quotes-from, updates, expands and comments on an original article original article (see links just above)"The Almost-Too-Tight-House" by Steven Bliss.

Keeping Basements Dry: Wet Basement Prevention - part 1 [PDF form], use your browser's back button to return to this page &
Keeping Basements Dry
: Wet Basement Prevention & Cures
- part 2

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