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The causes of basement moisture problemsare mainly external, but they can beaddressed by providing diversion of wateraway from the building, drainage along the

basement perimeter, protection of the foun-

dation walls against moisture, as well as byeffective and durable grading near the base-ment and over the entire lot. (see Figure 1).

Well-considered design of foundationsand their interface with surrounding soils,combined with effective site drainage, caneliminate most of the moisture problemsfound in basements.

Moisture in Basements Basement moisture problems can be caused

by water penetration (leakage), dampness in

the form of water and water vapour migratingfrom the soil through the basement enclo-sure, and sump pump failures and sewer

backups; they can also be caused by internalsources (humidity and plumbing leaks).

Leakage occurs when there is an accumu-lation of water outside a basement wall orfloor that is adjacent to a crack or defect.Surface water that is not directed awayfrom the building can enter the basementthrough unintentional openings (e.g., jointcracks) at or below grade. By providingadequate drainage around the perimeter of the basement, this situation can be avoided.

Alternatively, the basement can be water-proofed (see sidebar, p. 2) to resist hydrosta-tic pressures and thus control leakage. Afluctuating water table that rises above the

basement floor level may cause leakageunless the basement is waterproofed or the

By M.C. Swinton and T.J. Kesik Proper site grading and foundation drainage strategies are required in orderto prevent water damage to basements and their contents. This Updatereviews current construction practices used for basements, discusses someof the key issues and deficiencies that lead to problems, and provides

practical suggestions for improving drainage and construction.

Site Grading and Drainage to AchieveHigh-Performance Basements

Construction Technology Update No. 69

Low-permeanceapron or surface

Figure 1. Key elements involved in providing moisture protection for basements

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groundwater level around the basementis lowered (to below floor level) by theoperation of a sump pump.

Achieving complete basement protec-

tion requires that appropriate controlmeasures be provided for each form of moisture. There are five major consider-ations:1. Site grading and drainage provide the

first line of defence against exteriorwater entering the basement.

2. Foundation drainage (weeping tileand/or a granular drainage layer),combined with effective drainagearound the basement walls andappropriate moisture protection of the below-grade basement envelopesurfaces, provide the second line of

defence.3. Incorporating special design detailsand building in redundancy or factorsof safety are ways of addressing weaklinks in these two lines of defence.Preventing leaks at window wellsand sump pump failures are twoexamples of this approach.

4. Selecting the proper materials is an inte-gral part of achieving complete basementprotection (see Construction TechnologyUpdate No. 70 Options in the Selectionof Materials for Basement Constructionfor additional information).

5. Eliminating and/or controlling indoorsources of moisture is the final step inproviding adequate moisture protectionin basements.The first three of these considerations

are discussed in the following sections.

Site Grading and DrainageImproper grading of the lot can result inpoor surface drainage, ponding or floodingaround the basement wall, foundationsettlement or other damage, and basement

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dampness and other undesirable effects.It is one of the main generators of customercomplaints, call-backs, warranty repairs andlegal proceedings following the completionand occupancy of a dwelling.

The two most common types of lotgrading are:

Back to front . With this type of grading, therear lot line is the high point. First of all anelevated apron must be created around thehouse. This allows the surface drainage toflow towards the back and then forward tothe street at the edges of the property. Back-to-front grading is typical of properties on ahillside or mountainside.Split . With this type, the house is the highpoint and the lot is graded so that surfacedrainage flows forward to the street and

Waterproofing: Treatment of the surface or struc-ture to prevent the passage of water through the

basement envelope under hydrostatic pressures.Dampproofing: Treatment of a surface or installa-tion of a technology to resist the passage of mois-ture caused by differences in moisture content,vapour pressure and temperature across basement

envelope components.Note: Most foundations in new Canadian housesfeature footing drainage systems with complemen-tary dampproofing elements in the wall. Thisapproach normally precludes the need for water-proofing. The basement systems discussed in thisUpdate use drained approaches, without water-proofing.

Figure 2. Split site-grading and drainage plan

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backward toward the rear lot line, which isthen generally drained by a swale and catch

basin system (see Figure 2). This is the morecommon type and is typical of housingdevelopments on relatively flat land.

Most surface drainage problems occuralong the side lot lines between twohouses, especially where the houses areclose together. In these areas, a well-definedswale between the houses should be formed,which allows surface water to be divertedfrom the house and directed to the swaleand then forward or backward, dependingon the type of lot-grading approach used.

Downspouts from the roof should dis-charge at grade onto a splash pad at least1 m (3 ft.) away from the building; they

should not be directed toward adjacentproperty but rather toward swales estab-lished between the properties and withinthe property boundary.

In addition, the following guidelinesshould be followed: Minimum height of top of

foundation wall abovegrade: 200 mm (8 in.)

Minimum slope away fromhouse for at least 1.5 m(5 ft.): 5%, i.e., approxi-mately 19 mm per 300 mm(3/4 in. per ft.).

Minimum slope on rest of lot: 1.5%, i.e., approxi-mately 6 mm per 300 mm(1/4 in. per ft.)

Minimum elevation of lot,at house, above street level:450 mm (1.5 ft.)

Minimum depth of swales:150 mm (6 in.)

Surface drainage should bedirected away from windowwells, exterior stairwellsand decks

Planned elevations at the peri meter of the lot should not be altered as a conse-quence of gardening, landscaping orfencing projects.Site grading and drainage require careful

planning, as this work will not be executeduntil the building is nearly completed.However, moisture-protection measuresmust be considered and addressed duringthe construction process.

Foundation Drainage and Moisture Protection To deal with groundwater and surfacewater that have not been directed awayfrom the building by site grading, certainmeasures must be taken to provide ade-quate foundation drainage and moistureprotection.Foundation DrainageThe most common approach to foundationdrainage is illustrated in Figure 3(a). It usesweeping tile (pipe that is continuous andperforated, which can be either rigid or

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Patios next to the house basement. This is a common landscape feature and one that invites problems.In time, patios almost always end up sloping toward the house. The 5% recommended slope couldpose problems for interlock patios and related walkways next to the house. These require additionalplanning consideration. A lesser slope may have to be used as a compromise, for reasons of practicalityand safety. Gradual and consistent sloping away from the house (with a slope of 1%), using laser tech-nology, in combination with thorough tamping, should allow acceptable outward water flow to beachieved during heavy rain events. Such patios can provide a more consistent surface for water runoff than other landscaping, hence the exception to the 5% rule.

Place drainpipe/weeping tile on a layer of free-draining granular material/mineralfibre insulation to minimize the possibilityof it becoming plugged with silt, thencover with granular material

Figure 3(a). Best practice associated with foundation drainage design

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flexible) installed around the perimeter of thefoundation wall footing and covered with agranular material (gravel) prior to backfilling.

It is important to keep in mind thatalthough Figure 3(a) depicts the most common

best practice, this approach is neverthelessnot intended to drain large quantities of water away from the footings. The follow-ing situations require some additional plan-ning to promote effective footing drainage: The travel distances for water through

the drainpipe and gravel (from the back of the house to the storm sewer or sump)

can sometimes be relatively long, result-ing in large flow resistances and low water flow rates.

Consider an additional line connectionto the storm sewer or sump so that bothsides of the house can be drained directly.

The corrugated design of the drainpipe,needed to provide strength and flexibil-ity, introduces friction into the pipe,slowing the water flow. As well, the many corners associated with complex floor

plans and attached garages introducebends in the drainpipe resulting in even

more flow resistance.Use a straight run of unperforated pipein areas where water is not expected todrain from above, such as under attachedgarages, as shown Figure 3(b); extend theperforated drainpipe on the side of thehouse with the garage so that it intersectswith the drainpipe running parallel to

the front of the house. This configurationeliminates two corners when the garageis inset into the house.

House footings are generally level, so thechannel next to the footing is alsobasically level. This channel is formed by the edge of the footing, the ground and the face of the excavation, and providesa space in which the water can accumu-late and then flow toward the pipesconnecting to the storm sewer or sump(see Figure 3(a)). However, in low-perme-ability soils such as clays, the water tends to sit there, even when there aredrainpipes and sufficient gravel. Sitting water tends to deposit silt, decreasing theavailable flow area in the pipe and

gravel, and under some conditions,totally blocking both.

As indicated in Figure 3(a), place theweeping tile or drainpipe on a layer of free-draining material. This can extendthe life of the drainpipe, as the silt getsdeposited in the granular material under-neath (not inside) the pipe.

When the water has to travel a relatively long distance, sloping (standard) drain-

pipe with random perforations will not provide the desired result. The corruga-tions create additional friction and the

random perforations prevent the build-up of water pressure, or head, inside the pipe. This head is necessary to drive thewater down the pipe and gravel toward the storm sewer or sump.

There are now some proprietary drainagesystems available on the market that fea-ture side slots and smooth rectangularchannels, with no perforations at the bot-tom. Sloping such channels toward thestorm sewer or sump connections canimprove water evacuation rates.Even when all of these best practices are

followed, great emphasis still needs to beput on keeping the water out of the founda-tion drainage system. Footing drainage isthe weakest link in the system, and thisneeds to be recognized in the overalldrainage plan.

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Front faceof the house

Connection to lateralsand sewer (unperforated)

Use a length of unperforatedpipe that is sturdy enough towithstand the loads duringbackfilling of the excavatedgarage space

Perforated drainpipenext to the footing

Extend the perforateddrainpipe on the side

of the house where thegarage is so that itintersects the drainpiperunning parallel to thefront of the house

Garage

Living space

Figure 3(b). Drainpipe type and configuration needed to achieve optimal flow

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Moisture ProtectionThe basement walls below grade are sur-rounded by soil containing moisture. Byproviding an adequate slope away from the

building, as well as an impermeable cap,one can minimize the amount of waterdirected toward the below-grade basementwalls though not eliminate it altogether.Two approaches are commonly used for

moisture protection of the below-grade basement walls, as depicted in Figure 4.The first approach involves using a

drainage membrane or exterior insulation tocontrol water migration through cracks andpores of basement structures. The secondapproach uses free-draining granular back-fill to drain bulk water and dampproofingto control migration through the pores of the concrete.

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Both approaches provide a vertical path(air space) that allows the water along thesoil/wall interface to drain, and both are

backed up by a second water-resistantsurface (i.e., dampproofing membrane orcoating) that drains any water down tothe footing.

Each of these approaches has advantages.For example, in many parts of Canada,granular backfill materials are readily avail-able and inexpensive. However, drainagemembranes can also be used in unfavourableground and weather conditions, withoutincurring the cost of removing excavatedsoil from the site. Both approaches provideeffective perimeter basement wall drainage.

Regardless of the approach taken, oncethe basement moisture-protection systemhas been installed, attention must be paidto the grading and capping of the apron(see Figure 2 in Site Grading section) sur-rounding the building to ensure that wateris directed away from the grade/basementwall interface. Grading and capping of theperimeter drainage system ensures that thissystem does not become overloaded suchthat its carrying capacity is exceeded(see Figure 5).

There are different ways of capping: oneoption involves using a low-permeability,fine-grained cohesive soil such as clay.

Another more effective capping techniqueuses a membrane or low-permeability insu-lation board placed just below ground andsloped away from the wall. In all cases, theslope must be maintained after soil settle-ment and the capping must extend beyondthe line of excavation and backfill.

Grading and capping are the final stepsin completing the foundation drainagesystem, tying it into the site grading anddrainage system to control surface water.

Special Design and Construction Measures There are numerous situations encounteredin the construction of basements wherespecial methods, materials or equipmentmay be required. Such measures includewindow wells and sump pumps.

Figure 4. Two options for effective drainage of below-grade basement walls

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Window WellsWindow wells are not a preferred basementconstruction practice, as they are lowerthan grade, attract snow and surface water,and can quickly overload the drainpipe sys-tem below. They rely on positive drainagefrom the granular layer below the windowthrough a drainpipe to the footings. Thishelps avoid moisture problems at the foun-dation wall in the vicinity of the well andthe wetting of the window itself. Figure 6provides details for achieving properwindow well performance.

The performance of the window well can be improved by providing sufficient dis-tance from the top of the granular layer tothe bottom of the window opening to avoidaccumulation of water and melting snowthat could rise above the bottom of thewindow opening. Because the window isnot designed to resist the build-up of water,this additional vertical space provides ameasure of added protection.

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Woven or sheet polyethyleneor geotextile membraneprevents plant growth betweenpavers and serves as cappingmembrane

Avoid long lengths of pavers or aprons that cannot be easily adjusted.

To maintain a positive slope around the house foundation: Initially provide an exaggerated grade to compensate for

long-term settlement; or Compact backfill in several lifts to reduce settlement.

.

Install paved surfaces with a positive slope over a sand layer.

Use discrete units to facilitate future adjustments of grade.

Consider the impact of future landscaping on the grading.

Figure 5. Two ways of providing effective gradingand capping of the basement perimeter walls

Figure 6. Critical considerations in the design and constructionof window wells

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Summary The site grading and foundationdrainage practices highlighted in thisUpdate and summarized below caneliminate most basement performanceproblems in Canadian housing: Plan and design site grading and

foundation drainage to anticipateall sources of moisture;

Divert water away from the base-ment wall and provide an effectivedrainage layer around the perime-ter of the basement. This will pre-vent the accumulation of groundwater that can cause hydrostaticpressure on the basement wallsand at the footings;

Design and construct windowwells properly to ensure acceptableperformance under adverse weatherconditions.

Size and locate sumps and sumppumps to effectively control thegroundwater level over the entire

basement area.Performance expectations for basements

in Canadian housing have become higher because of the increased use of basementsas living spaces. This Update presents keymeasures that need to be considered to helpmeet these expectations.

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Figure 7. Important factors for sizing and locating sumps and sump pumps

Performance Guidelines for Basement Envelope Systems

and Materials The Guidelines were developed underthe guidance of the project steering com-mittee, which was comprised of industryassociation and government agency rep-resentatives. This committee ensuredthat the Guidelines reflect the best col-lective knowledge of Canadian industryand related public and private agencies.

The complete report is now available athttp://irc.nrc-cnrc.gc.ca/pubs/rr/rr199/index_e.html

Sump PumpsUnder some conditions sump pumps must

be installed; for example, when there is afluctuating water table or when the founda-tion drainage system is lower than adrainage receptor, making gravity drainageof the foundation impossible.

Sumps and sump pumps should conformto building code requirements; however,several issues not explicitly addressed bycode requirements should be carefully con-sidered (see Figure 7):Area and configuration of the basement .For very large basements, or basements withunusual shapes (e.g., L-shaped, J-shaped orU-shaped), more than one sump may berequired to achieve effective drainage.Low-permeability soils . The minimumrequired depth of the sump (750 mm or30 in.) may not result in a sufficient waterflow rate to the sump to maintain waterlevels at the extremities of the basement

below the basement slab or floor assembly;this minimum may need to be exceeded toachieve an adequate flow rate.Basements used as living spaces . It isadvisable to provide an emergency back-uppower supply to the sump pump in caseextreme weather causes a power failure.A pump alarm is also recommended toindicate pump failure and/or the need formaintenance.

Review local practices for sump pumpsfor guidance on these issues.

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Construction Technology Updates is a series of technical articles containing practical information distilled from recent construction research.

For more information, contact Institute for Research in Construction,National Research Council of Canada, Ottawa K1A 0R6Telephone: (613) 993 2607; Facsimile: (613) 952 7673; Internet: http://irc nrc cnrc gc ca

2008National Research Council of Canada

June 2008ISSN 1206-1220

Further Reading Source and On-Site Controls for Municipal Drainage Systems . InfraGuide, Innovationsand Best Practices: Storm and Wastewater(No. 3). National Guide to SustainableMunicipal Infrastructure. Ottawa, 2003.http://sustainablecommunities.fcm.ca/Infraguide/

National Housing Code of Canada 1998 andIllustrated Guide. Canadian Commission onBuilding and Fire Codes, National ResearchCouncil Canada. Ottawa, 1998.

Performance Guidelines for BasementEnvelope Systems and Materials: FinalResearch Report, Institute for Research in

Construction, National Research Council.Ottawa, 2005.

National Building Code of Canada. CanadianCommission on Building and Fire Codes,National Research Council Canada.Ottawa, 2005.

M.C. Swinton is a Principal Research Officer inthe Building Envelope program at the National Research Council Institute for Research inConstruction.

T.J. Kesik is a professor of building science in the

Faculty of Architecture, Landscape and Design at the University of Toronto.