WET BASEMENT AND CRAWL SPACE
PROBLEMS, CAUSES, AND REMEDIES --TIPS FOR
HOMEOWNERS, AND HOME BUYERS
DR. BRUCE A. TSCHANTZ, P.E.
PROFESSOR OF CIVIL ENGINEERING
UNIVERSITY OF TENNESSEE
KNOXVILLE, TENN 37996
The primary purpose of this booklet is to
inform the homeowner, home buyer, and home builder about the
cause of wet basements and crawl spaces plus effective measures
for preventing or correcting problems. This information will
enable the public to build, select, or repair homes wisely. The
booklet is also intended to provide a common basis for
communication among the homeowner or buyer, realtor,
builder/contractor, inspector/regulator, insurer, mortgager, and
WET BASEMENTS OR
CRAWL SPACES--SOURCES OF IMMEDIATE AND LONG-TERM PROBLEMS.
CAUSE OF WET
BASEMENTS AND CRAWL SPACES
REMEDYING WET BASEMENTS AND CRAWL SPACES
TIPS FOR HOME BUYERS,
HOMEOWNERS, AND HOME BUILDERS
WET BASEMENTS OR
CRAWL SPACES--SOURCES OF IMMEDIATE AND LONG-TERM PROBLEMS.
Standing water inside and/or seepage into
residential crawl spaces and basements can cause frustrating
problems for the homeowner. These problems can be both immediate
and long term. For example, standing water and mud inside crawl
spaces make it very difficult and messy to gain access under the
house for inspecting, maintaining, and servicing electrical
circuits, drains and water lines, heating and air conditioning,
and other utilities. Wet basements and crawl spaces are sources
of high humidity, which can produce surface condensation, mildew
and fungi, musty odors, and an unhealthful environment. Such
moisture can cause deterioration of floor joists, beams,
subflooring, insulation, and electrical-mechanical systems.
Prolonged water around the footer and foundation wall can soften
the soil and weaken its bearing capacity, increasing the
possibility of wall settlement and cracking. Serious seepage
under the foundation footer may erode soil away and cause the
wall to drop or crack. Excessive moisture can eventually
penetrate the subflooring and buckle the flooring or cause
warping, making doors and cabinets difficult to close or open.
Since crawl space or basement dampness always moves toward the
drier upstairs areas, higher humidity will result in costlier
heating and air conditioning bills. In the case of crawl spaces,
if the underflooring insulation collects moisture, or sags from
excessive wetness, the heating and air conditioning costs are
driven even higher. Finally, wet basements and crawl spaces
reduce the value of the house--at least by the amount that would
be required to repair the damage and to eliminate the cause of
the problem. Some homeowners are reluctant to discuss or admit
their water problem, for fear that the publicity of an actual or
even a perceived problem would reduce the value of their
investment. Homeowners, in such situations, should immediately
seek professional assistance in assessing the source and extent
of the problem and in finding a remedy.
CAUSE OF WET
BASEMENTS AND CRAWL SPACES
Most wet basements or crawl spaces are caused
by surface water which is not adequately drained away from the
foundation wall. Sources of this water include the following:
- Roof water if no guttering is present
- Roof water if the guttering leaks or
overflows because of clogging from leaves and bird nests
- Roof water if the downspouts (leaders) are
clogged or do not have sufficient means at their outlets
to drain water away from the foundation wall. Frequently,
a downspout ends at the corner of the house without a
splash pad (splash block) or elbow (shoe), leaving roof
water to concentrate at that point and seep into the soil
next to the foundation wall. A typical 2000 square foot
roof can produce almost 1250 gallons of water during just
1 inch of rainfall. If the rainfall is steady and
prolonged, the opportunity for this roof water to soak
into the ground next to the foundation wall is high.
- Excessive watering of flower beds and
shrubbery around the foundation wall. Once the upper soil
layer or mulch bed air spaces are filled with water, the
excess water either runs off or seeps into the ground
next to the wall. Prolonged and excessive watering can
contribute a large amount of water to crawl spaces or
- Rainwater runoff from the adjacent lawn,
walks, or driveway areas if the landscaping forces water
to drain toward the house instead of away. If surface
runoff is directed toward the foundation wall, this water
will pond and eventually soak into the soil, thus
becoming a potential source of basement or crawl space
water. Downspout splash pads are not very effective if
they drain onto a backward-draining slope toward the
Water or dampness problems in basements or
crawl spaces are sometimes caused by other factors:
- Subsurface or groundwater may be
intercepted or dammed up by a basement or foundation
wall. Houses which are built downslope on or at the base
of hillsides are particularly vulnerable since there is
greater opportunity for surface water to soak into the
soil to become groundwater and because groundwater flows
downhill by gravity. Foundation walls act like dams and
can intercept and trap this subsurface water, causing
pressure build-up on the outside and forcing water
through joints and cracks in basement walls or seepage
under the footer.
- Nearby springs may have been filled in or
covered up by the developer. Unless the springs were
properly drained away from the lot or subdivision, such
water will eventually seep into the surrounding fill,
become a pool of groundwater, and eventually force itself
laterally and upwardly into basements and crawl spaces.
- Nearby creek(s) may overflow during storm
runoff and either directly flood basement or crawl space
areas, or contribute to the groundwater, which may become
sufficiently high to cause seepage into the basement or crawl
space area. Homeowners may not experience the effects of
groundwater seepage or overflowing creeks for months or
years after purchasing a house because of drought or
infrequent out-of-bank flooding. However, when such
conditions do occur, they may come suddenly without
warning and cause serious problems after the warranty
period has expired.
- Improperly installed, clogged, collapsed,
or leaky drains may not allow downspout water or
foundation wall water to escape. Perimeter, footer, or
foundation drains are installed around the exterior of a
house below the basement floor to intercept and dispose
of subsurface water to eliminate groundwater build-up and
seepage under the house. If these drains are improperly
installed or become clogged with silt or roots, they will
not operate as intended. Sometimes an otherwise good
perimeter drain gets covered up at its end(s) during the
final backfilling or landscaping stages of construction
and the intercepted water has no place to go but to build
up behind the foundation wall and eventually to seep into
the basement or crawl space.
- Underground drains leading away from
downspouts may not have sufficient slope to carry water
away; may empty into the adjacent lawn and get
covered with topsoil; may become crushed during
landscaping or become pinched at their outlets; or may
lead to an area which ponds and backs up water. The end
result in all these cases is leakage or overflowing at the
bottom of the downspout and water penetrating the
- Soil continuously draws water up from
subsurface groundwater sources in a crawl space like a
blotter by a mechanism known as capillary attraction. The
finer the soil (e.g. clays), the more aggressive the
capillary pumping action. As the water rises to the
surface, it evaporates into the crawl space. Such ground
moisture can be a significant source of dampness and
humidity under a house, even without standing water. The
presence of capillary water is often indicated by a
whitish residue, left on the ground surface of the crawl
spaces, resulting from evaporation of water containing
minerals and salts. Lack of a moisture barrier, such as
plastic sheet, will allow this capillary action and
evaporation to contribute almost unlimited moisture
sources to crawl space areas. Figure 1 illustrates how
surface water and moisture can enter a crawl space area.
Figure 1. Typical paths of water and moisture into a crawl
- Closed, inadequate, ineffective, or no
crawl space venting around foundation walls will force
the buildup of humidity in the space beneath a house.
Given the combination of high humidity and low
temperature, condensation can form on heating/AC ducts,
joists, underflooring, and insulation. This environment,
together with likely darkness, encourages mildew and
other fungi to form.
- Damp or wet basements and crawl spaces may
be caused by ruptured water or sanitary lines either just
outside the wall or under the house. If a crawl space is
unusually wet and muddy, inside leaks may be difficult to
find and repair. Outside pipe leaks may be even more
difficult to find, since water may appear several feet
away from the actual leak. Old field drains under a house
may also be a source of unwanted water.
REMEDYING WET BASEMENTS AND CRAWL SPACES
- Most new home construction complaints
arise from inadequate site drainage and "water
problems." Proper drainage of surface water is a
primary element in preventing wet basements, damp crawl
spaces, eroded banks, muddy yards, and possible failure
of a foundation system.
The Council of American Building Officials
(CABO) code for 1 and 2 family dwellings requires that
"Lots shall be provided with adequate drainage and shall
be graded so as to drain surface after away from foundation
walls" R-301.3). Many local governments have adopted
this code or similar building codes such as the Standard
Building Code (Southern Building Code Congress International,
Inc.--SBCCI); Basic/National Building Code (Building
Officials and Code Administrators--BOCA); and Uniform
Building Code (International Conference of Building
Officials--ICBO) as a standard for permitting and inspecting
new home construction or for alteration of existing homes.
The Federal Housing Administration (FHA) of
the Housing and Urban Development (HUD) Agency generally
relies on the code adopted by local ordinance. In case there
is no code, FHA/HUD uses the CABO code for new construction.
Generally, surface water drainage should be
directed from all sides of the house and off the lot in a
manner that will
- Minimize possibility of dampness in
basements and crawl spaces
- Prevent standing or ponding water on
- Prevent soil erosion
- Adversely affect the supporting
foundation soil behavior.
Walks, driveways, retaining walls and other
landscape improvements should be constructed so as not to
interfere with drainage. Walks should not be used as drainage
Site grading plans should specify minimum
slopes from the house (usually 2 to 5%), depending on
location, type of soils, frost depth, and soil moisture, to
ensure water drainage for some specified distance (usually 6
to 25 feet) away from supporting foundations. In cases where
minimum slopes or distances cannot be attained, paved gutters
or other drainage structures acceptable to the Building
Inspector may need to be installed.
Maximum slopes are usually specified to
prevent erosion or unstable banks around the house and yard.
Roof water should be directed to a
downspout and away from the foundation wall toward a suitable
ditch, swale, or drainage pipe to prevent ponding or backflow
as shown in Figure 2. All drainage structures should be
properly connected to adequate outlets that are protected,
where necessary, by recorded permanent easement.
Figure 2. Correct installation of downspout
shoe and splash
block at foundation wall. Note that ground surface slopes
away from house.
House plans and landscaping should be
developed to prevent "dead" drainage areas around
the foundation wall--areas where rainfall has no place to
flow away except by ponding and soaking into the soil near
the foundation wall. Areas bounded by the front
entrance/sidewalk/garage/driveway are especially vulnerable
to trapped pockets of surface water.
Adjacent creeks, drainage swales, rivers,
and lakes should be checked for historic or calculated flood
hazard levels. Usually, first-floor or habitable space floor
elevations are required to be above the level of a 100-year
return frequency flood; however, more frequent flooding may
be allowed in crawl spaces and certain basements.
- Another vital step in preventing water in
basements and crawl spaces is to intercept outside
subsurface or groundwater with a perimeter drain at the
footer base level around all sides of the house where the
exterior ground surface is higher than the inside floor
or crawl space level. While foundation drains are clearly
necessary for houses with basements or potentially
habitable living space below exterior ground surface,
they may also be necessary in crawl spaces where water,
soil, and/or earth floor elevation conditions warrant
their use. The drains should always be installed at or
below the level of the area that is to be protected (i.e.
basement floor or crawl space ground surface) and should
discharge by gravity to a positive outfall such as an
approved drainage ditch or swale, or into a storm system.
In some cases, sump pits and pumping may be required.
- Another means of preventing subsurface
moisture penetration and dampness in a basement or crawl
space is the use of waterproofing of foundation walls of
below-grade habitable rooms and damp-proofing of
basements and other foundation walls, where necessary, to
protect crawl spaces. Specifications for water proofing
and damp-proofing foundation walls can be found in
adopted local building codes.
- Excessive moisture may be prevented form
entering a basement from the walls by the use of a vapor
barrier installed on the warm side (in winter) of the
insulated walls. Building codes usually specify the
material(s), maximum vapor transmission rate, venting,
etc., appropriate for construction.
- Excessive moisture vapor can be prevented
from entering a crawl space area with the use of an
effective and correctly installed vapor barrier over the
ground surface. Approved types of polyethylene or asphalt
saturated felt materials, thickness, and joint/seam
construction are specified in local building codes. Torn
pieces, poor or non-overlapping joints, missing sections,
or improperly sealed corners and edges at the walls,
fireplaces, and interior piers must be avoided to produce
an effective vapor barrier.
- Necessary in crawl space areas under
houses without basements is the installation of adequate
wall ventilation openings around the foundation walls.
The purpose of these openings is to provide cross
ventilation for preventing and relieving the buildup of
water vapor inside the crawl space. Most building codes
specify minimum vent opening areas (usually about 1
square foot of net opening for each 150 square feet of
crawl space), opening location or arrangement,
corrosion-resistant wire mesh screen, and any reduction
in ventilation opening area allowance if an approved
vapor barrier is used.
- In case water does get inside a crawl
space, it must be able to drain out. The crawl space
ground surface should slope or drain to a common low
point, where water can drain by gravity through the
foundation wall or under the footer to an outside
drainage ditch or swale. Outside surface water should not
be allowed to back up into the crawl space through this
In older houses where any of the above
defensive or primary moisture and water control methods are
missing, measures should be taken to install appropriate
drainage facilities, vapor barriers, or ventilation openings.
Installing any of these elements after a house has been built
will be more costly than while the house is constructed.
In some cases it may be necessary to use
secondary measures to alleviate a wet basement or crawl space
problem--either along with the above measures or alone,
depending on the situation. Additional measures include the
- Installing trenches or
gravel/perforated pipe French drains around the
inside perimeter of the foundation wall and from
other central low areas of the crawl space to
intercept water which manages to enter under the
house. This drain system should converge at a common
low point in the crawl space and be piped under the
footer or through the foundation wall, as
appropriate, to a ditch or swale outside, where water
can freely drain away without backing up or allowing
outside water to enter.
- In cases where basements collect
seepage of surface water, either (1) a basement floor
drain should be installed at the low point(s) and
piped outside to a convenient ditch or swale, or (2)
a sump pit or collection chamber should be dug below
the basement floor, lined, and a sump pump installed
to remove the collected water to a convenient
location away from the foundation wall. The sump pump
can be actuated automatically by a float system or be
manually operated as needed. A sump system could also
be installed in a crawl space if gravity drainage is
- In case the yard area slopes toward
the house and surface water collects or ponds near
the foundation wall, a V-ditch or swale should be
constructed around the house to allow surface
drainage from both the foundation wall and the other
yard areas to an adequate ditch or storm drain. Such
cases often exist where the front street is higher
than the first floor of the house or when the house
is built on the side of a hill. Figure 3 illustrates
how this problem can be solved.
Figure 3. Correction of reversed slope drainage problem using
swale or ditch
- If flower bed and shrubbery watering
around the foundation wall is producing a significant
source of water for the basement or crawl space, and
the flower bed or shrubbery cannot be relocated,
consider the installation of heavy plastic sheet
under the flower bed so that any water which soaks
deeply into the soil is intercepted and carried
safely away by gravity at least six to eight feet
from the house to a gravel collection drain or swale.
TIPS FOR HOME BUYERS,
HOMEOWNERS, AND HOME BUILDERS
"A teaspoon of prevention is worth a
gallon of cure" certainly applies to new home
builders--at least in avoiding water problems in basements or
crawl spaces. Buyers of new or older homes should be cautious
about drainage. The best time to sign a contract is on a
New home buying/building
The following tips are suggested to avoid water
problems when building or buying a newly-built home:
- Work with an engineer or architect, to
help you locate the new house on the lot and at an
elevation which would minimize potential surface or
groundwater drainage problems and save drainage costs.
- If a flowing stream or dry ditch borders
your lot, check with local planning agency authorities or
a hydrologic engineer for potential flooding, whether in
a designated 100-year flood hazard zone or in an area
where lesser but more frequent flooding could occur or
- Work with a reputable home builder who can
give you reference names or locations for houses that he
has built. If the house you are looking at is already
built, find out who constructed it and ask your realtor
for references to homes built by the same builder. Visit
these sites and check for patterns of any drainage
- Assume that the local building inspector
will not check new home construction for landscaping or
site drainage. This matter is usually left up to the
builder and buyer to resolve. However, if you dont
feel competent to make your own inspection, hire an
engineer or architect to help you check slopes,
foundation wall water proofing/damp-proofing, underground
drains, general surface and roof water drainage, and
general quality of construction. If you suspect a
potential problem, ask the local building inspector for
- Check to make sure that the perimeter
foundation drain, basement drain(s), or crawl space drain
has an unobstructed outlet to a ditch or swale which
leads away from the house.
- If you are considering the purchase of a
newly built house, pay special attention around the
outside and the basement or crawl space for
- backsloping lawns and landscaping
toward foundation walls.
- backsloping driveways (toward garage),
stoops, walks or patios which force surface water
toward the foundation wall. If necessary, use a level
to check the slope direction.
- a very flat lot with little
opportunity for drainage away from the house and lot.
- standing water inside
crawl space--check next to foundation walls.
- pattern of wet concrete blocks on
inside basement walls below grade; check for whitish
salt deposits on inside foundation walls as a result
of leaching from moisture seepage and evaporation.
- potential or observed surface drainage
from the street or from neighbors roof toward
your foundation wall or garage.
- downspouts which drain
to the foundation wall, where water has no place to
escape away from the house.
- depressions around the
foundation that can collect surface water and cause
seepage into the ground.
- If you have specific questions about
construction or drainage matters, refer to applicable
building codes and inspection/permit requirements; local
stormwater ordinances; local subdivision and zoning
ordinances; and local flood damage protection ordinances
and maps required by the National Flood Insurance
Administration (NFIA). These sources are available at
your local planning office, building inspectors
office, or library.
- Above all, dont be timid about
asking the realtor or builder questions. Document your
questions or concerns in writing.
Older home buying tips
The following recommendations are made to the
prospective older home purchaser:
- Visit the house during or shortly after a
prolonged or heavy storm. Check for water in the basement
or crawl space. Ask the realtor about any known water
problems--inside or outside--by the previous owner.
- Check with the neighbors to see if the
house that you are looking at has had a history of
drainage problems or wet/damp basement or crawl space.
- For houses with basements, carefully check
for stain signs of standing water around the walls or
seepage coming through the walls, especially along the
wall(s) having the highest outside ground level. Look for
whitish salt deposits on inside foundation walls left
from moisture seepage and evaporation. For houses with
crawl spaces use a flashlight and check for current or
previous water ponding, mud, mildew, condensation on
various surfaces, or sagging or wet insulation.
- Check the basement and crawl space for
musty odors and signs of mildew, condensation on
surfaces, and unusually heavy rust deposits. Try to
determine the source of the moisture--foundation wall
seepage; capillary moisture from the crawl space ground;
water or sanitary pipe leakage; poor crawl space
ventilation; or combination of problems. Check the inside
corners for cracks and separation of blocks at the joints
from foundation settlement.
- Dont accept a realtors or
builders suggestion that water seepage into
basement or crawl spaces is normal and should not be a
concern. Either ask that the source of the problem be
eliminated or look for another house!
Tips for homeowners who
currently have water problems
- Check to see if the house is in
warranty--if so, discuss the problem with the builder and
ask him to correct it. If he refuses, contact your
attorney for advice.
- If the warranty period has expired, have
an expert in drainage engineering determine the cause of
the problem, whether or not it is serious, and if so,
what can be done to correct it.
- Check to see if all surface and subsurface
drains are functioning. Use a hose to see if roof and
gutter water is being directed away from the foundation
wall. Refrain from heavy watering of flower beds and
shrubbery next to the house. Observe the path of all
surface drainage around the house during prolonged or
heavy rainfall. Watch for ponding near the walls. Check
to see if the foundation drain is operating by looking
for evidence of seepage from the ends.
Make use of local organizations, governmental
offices and experts if you have concerns about drainage around
your house or about wet basement or crawl space. Check with
people from these organizations for professional services or
- Local/county Soil Conservation Service
- County hydrologist or engineer
- Local building inspector or code
- Local planning office staff
- Agricultural Extension Service
- Professional engineer or hydrologist
- University Civil Engineering or
Agricultural Engineering departments who have experts in
drainage, foundation, and structures
- Local Home Builders Association
- Local remodelers or
- Local Better Business Bureau
- Professional/commercial house inspectors.
Some problems may be solved simply by repairing
an obvious gutter or downspout leak, while other problems may
have causes which are difficult to identify and very expensive to
fix. Severe crawl space water problems may cost several thousand
dollars to remedy. Always use a competent professional
to help you find and eliminate the source of your problem.
Dont settle on quick and cheap fixes which merely deal with
the symptoms of the problems.
Above all, remember that in many home
purchases, its BUYER BEWARE!
OTHER WET BASEMENT INFORMATION WEB PAGES (under construction)
Examples of Wet Crawl Space Problems
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