The problems

Wet are nasty places, home to mold and mildew. Wet basements lead to moisture problems in the house and even in the attic. Saturated soil next to the house causes seepage through cracks in walls. that can't get away from the house will find its way in.

Details for a Dry Foundation Think of the ground around the house as the roof that keeps the foundation dry

by William B. Rose The solutions

Details make the difference between a wet and a dry foundation. Every house doesn't need every detail shown here, but used together, they'd certainly keep a basement dry. Pay attention to soil types, the grade of the lot and the climate, then pick the details that seem appropriate to the situation.

As a research architect at the Building Re- crawlspace was low, even with the bottom of That path happened to be under the addition's search Council of the University of Illinois, I am the footing. The soil along the edge of the foot- footing. By following that path, the runoff had paid to solve some of the more nagging prob- ing was in small, rounded clumps, unlike the washed away the ground under the footing and lems that houses have. Frequently, I visit trou- grainy, gritty surface of the rest of the floor. I dug caused the addition to settle. bled houses, and the most common problem I away at the dirt clumps, and my fingers hit air. I encounter is poor drainage away from the foun- dug a little more and found a space that re- Bad drainage can cause a raft of prob- dation. This problem became worse as wetlands minded me of a prison escape . In all, lems—I call my studies of the zone where the were developed; I know what to expect when 10 ft. of the footing was undermined. house meets the ground "building periodon- the name of the development is Frog Hollow. Water from a downspout draining too near the tics." Proper preventative care of this area can I was once asked to look at a house that had corner of the house and the addition was the avoid a variety of problems, some less obvious settling problems. There was an addition, built culprit. The water was taking the path of least and a lot more serious than a damp cellar. over a crawlspace, that was moving down rela- resistance to the footing drain and For example, a common problem in base- tive to the main house. The dirt floor of the in the basement of the main part of the house. ments, particularly those with block walls, is in- the foundation wall (drawing left). The soil near- est the surface is the first to freeze, and as the Saturated soil leads to frost heaving. In a common cold weather continues, deeper soil freezes. This scenario, water from downspouts has nowhere to go but next to the saturated soil expands by 8% as it freezes, and it foundation. This results in damage to a garage slab from soil freezing. exerts a tremendous force that lifts the soil frozen to the wall above. The wall lifts and cracks the slab. Moisture damage around foundations isn't lim- ited to masonry problems. In 1947, Ralph Brit- ton, the government researcher whose work led to the current attic-ventilation standards, showed that water vapor traveling upward from damp foundations caused most attic moisture problems. He concluded that if attics were iso- lated from wet foundations, the standard 1:300 venting ratio could be reduced to 1:3,000.

First, pinpoint the trouble spots—Let's take a walk around an imaginary house and study the sources of its foundation water problems (draw- ing p. 98). We easily spot the first one: The front gutters are clogged. Been clogged for so long, in fact, that saplings are sprouting in the compost- ed leaves. Rainwater overflows these gutters, causing the ground below to settle. A small crater has developed, and its contents have nowhere to drain but down and into the cellar. Keeping water away is the solution. Proper detailing can be as simple as sloping the ground away from the house and using The gutters at the back of the house, however, downspout extensions to deposit runoff away from the backfill onto are clean. The leader feeds into an underground undisturbed soil. drain that goes... where? Walking downhill, we find an outlet pipe at about the right elevation to be a footing drain. Composting leaves and gran- ules from asphalt shingles clog the corrugated pipe. Might the water that should flow from this pipe be ending up in the basement? A concrete patio, probably poured on a sand or gravel base, extends off the back of the house. A shallow depression next to the patio's edge collects a pool of runoff. This collected water will drain down the path of least resis- tance-through the gravel, under the patio and down the foundation wall. Going into the basement, we find leaks in places that confirm our observations. There is al- so a leak where the sewer line exits the house, indicating that water is flowing into the house through the sand in which the sewer line is laid. Timing can provide clues to the source of leaks. If they occur in a matter of hours following a rain, the problem is surface water. If leaks fol- low a day or so after a rain, a rising water table is ward buckling. This usually shows up as a hori- soil. Over time, the wall is ratcheted inward and likely the cause. zontal crack one or two blocks below grade, or eventually buckles. You avoid this problem by at windowsills, stepping up or down at the cor- keeping the soil next to the foundation dry. Know your soil—As I write this article, I am sit- ners. A study I did with the Illinois State Geolog- Slabs suffer from water problems, too. Garage ting in the middle of the Midwest. The soil here ical Survey revealed the cause. Clay soils shrink floors, for example, commonly crack at outside is Drummer silty-clay loam, great for agriculture, during dry spells, forming a crevice between the corners near where gutters drain. This cracking murder on construction. The available water ca- soil and the foundation wall. Wind and light may be due to upward expansion of water di- pacity is about 20%. This means that if I had rains carry dirt into this crevice. Then, when sea- rectly below the corner. It can also be due to 5 cu. in. of dry soil, adding I cu. in. of water sonal rains come, the soil swells back to its orig- adhesion lifting of the perimeter wall, a situa- would saturate it. Being clay, the soil will swell as inal dimension, plus the increment of added tion occurring when saturated soil freezes fast to I add water. The permeability is about I in./hr. This means that any layer of rain will need an hour to get through a horizontal layer of soil 1 in. Moisture in the ground leads to moisture in the thick. That's really slow. From these numbers, I house. Ground sloping to the house and no capillary break between can estimate that a 1-in. rain will saturate 5 in. of the slab and the earth give water nowhere to go but up. Moisture from soil and take five hours for full penetration. saturated soils will diffuse upward and warp floorboards, cause peeling That's useful information. It is from the U. S. paint and possibly rot the framing. Department of Agriculture's county-soil survey, available from your county cooperative exten- sion service agent. It allows a builder to estimate how much vertical water penetration there will be and how much of the rain runoff must be treated as sheet flow on the surface. This infor- mation matters a lot more on flat lots than on sloped ones, but it can still be important on the uphill side of sloping sites. The perc test for septic systems is also a good predictor of how the soil drains. If your soils have a good percolation rate, say, 10 in./hr. to 15 in./hr., to below the bottom of your footings, Good detailing isn't much harder you may not have to do much to ensure a dry than bad. Ground that slopes away from foundation. Install gutters and downspouts and the house and the gravel drains under the make sure the first 10 ft. of ground around your slab lead to a dry house. house slopes away at something like 1 ft. in 10.

First lines of defense—What if your percola- tion rate is considerably less than 10 in./hr.? First, don't build on the lowest part of the lot, because that is where the water will go. Gutters and downspouts are at the heart of rainwater management, the heart of moisture control in buildings. Deposit rainwater from gut- ters onto splash blocks and onto undisturbed soil so that the water runs away from the house. Most modern houses are damp-proofed (FHB #95, pp. 48-53)—that is, the exterior of the base- ment wall receives a bitumen coating. This pro- vides a considerable amount of water protec- tion. But water can enter through cracks consisting of solid pipe leading to daylight. If light of my opening story. I'm trying to solve resulting from utility penetrations, concrete cur- you use pipe (as opposed to just gravel) in a problems here, not create them. ing, settlement, swelling soils, seismic activity footing drain, it should be slightly pitched to- or other causes. Think of damp-proofing as a ward the outlet, or at least not pitched back- Good backfilling and grading are crucial— secondary defense against water. ward. It should have surface clean-outs every Proper backfill procedures go a long way to- 50 ft. Discharge by gravity flow is preferable to a ward eliminating water problems. At the risk of Dealing with a rising water table—Footing sump pump. Sump pumps are a weak link, like- sounding simplistic, be sure the ground slopes drains have been used for decades to intercept ly to fail when most needed, but a gravity drain away from the house. You'd be amazed how rising . Rising groundwater usually may not be possible if the footing drains are many builders get this wrong. I recommend a isn't the major problem for foundations. Surface deeper than any possible discharge point. slope of 12 in. in the first 10 ft. as a minimum water is much more likely to cause trouble if it Gravel alone is probably just as good as gravel (drawing above). Builders commonly don't al- isn't led away from the foundation. Still, footing with pipes in it. A continuous gravel base that low enough extra soil for settling, and they al- drains should be installed. They don't cost much leads to a sump pump or to a daylight drain of most never compact the backfill. Lightly com- when you're excavating, anyway, and they're solid 4-in. plastic pipe will handle most rising pacted backfill may settle 5% of its height or the devil to retrofit if you find later that you have groundwater. The gravel is the main water route, more, often resulting in a situation where the a high water table. so pipe used as a collector is not critical. In fact, grade pitches toward the house. When backfill- Footing drains should consist of a foot or so I believe that most pipe is placed by people who ing, include a correction for settlement. There of gravel around the outside of the foundation. don't really know what water is supposed to go really aren't any hard-and-fast rules. Deep, light- Use a filter fabric over the gravel to keep it from where. Drain pipe here symbolizes good prac- ly compacted backfill needs a big correction. clogging with silt. Filter fabric comes in several tice while making a doubtful contribution. Shallow, well-compacted fill might require none. weights; the lightest is just fine for residential Never connect the downspouts to the footing Remember, too much slope near the house use. Footing drains can have 4-in. perforated drains, even if the drains run to daylight and not doesn't create water problems; too little does. plastic pipe with the holes pointing down. They to a sump pump. Putting that volume of water Compact the backfill as tightly as possible must lead to a sump pump or a gravity drain closer to the footings makes no sense at all in without damaging the foundation walls. Brace Flashing the house to the ground is an innovative means to a dry foundation. This "ground roof" keeps A metal bar holds the EPDM to the water away from foundations. Use it around the entire house or house. Called a termination bar, it just in trouble spots. It has the added benefit of insulating the more commonly holds EPDM to roof parapets. Masonry fasteners hold it in foundation. place, and polyurethane caulk seals the top of the joint.

them well, using trusslike assemblies of heavy base serves as a capillary break between the soil you do this, install clean-outs at least every 50 ft. framing lumber spanning from wall to wall. and the underside of the slab. Extend the gravel and keep the gutters clean. Otherwise, the pipe Have the first-floor deck on, and fill all sides base to a footing drain to carry water away. It is can become clogged with leaves. I don't use evenly. Block walls require more caution than important to remember that a capillary break corrugated pipe for drainage because it is more poured concrete. Compact the backfill in 1-ft. works only as long as it remains unflooded. difficult to ensure smooth, straight runs. It clogs lifts using a hand compactor, commonly called more easily and is more difficult to clean out. a jitterbug, or careful pressure from a backhoe. Swales and French drains—Other means of Concrete patios, stoops, driveways and side- Because intersecting walls brace each other, the transporting water away from the house besides walks abutting the foundation present problems. soil at outside corners can be compacted with sheet flow (when the surface is effectively run- It is important to design them so that the gravel less risk than in the middle of a long wall. For a ning water) are the and the . base beneath drains outward, a perfect use for a minimum, compact these corners well, and be A swale is a small valley formed by two sloped French drain. You may find that the driveway is sure that all the downspouts drain near them. soil surfaces. Swales must be pitched, or they one of the most convenient sites for a French Take special care where utility lines enter the become ponds. A swale should be located away drain. Driveways usually pitch away from the house. They are frequently laid in sand that pro- from the building, and it is often used to divert house, and a French drain can be integrated vides a direct path for water to reach the foun- sheet flow coming from uphill (drawing p. 99). with the driveway so that it will not call atten- dation. Be sure the soil under the utilities is well A French drain is a trench filled with rock or tion to itself. compacted, and cement and damp-proof the gravel that collects water and transports it later- utility penetrations. Then use an impervious ally (top drawing, facing page). I prepare the Where should the water go?—To my knowl- earth such as clay soil, or mix a bag of portland bottom of the trench so that it is smooth and edge, municipalities no longer provide storm- cement with the soil you have, to fill around the carefully pitched toward the outlet. Mix dry ce- sewer service for new residential runoff. In my utility penetration. Tamp well (drawing p. 99). ment with the soil in the bottom of the trench to area, they do not receive the output from sump make it less permeable, and fill the trench with pumps. They receive and treat storm water to Slab construction needs good detailing pri- whatever clean gravel is locally available. I hes- keep streets open, and that's about it. or to the pour—Getting water away from slab itate to use road stone, a blend of gravel and If there isn't enough elevation difference be- foundations is just as critical as with basements stone dust, because water passes through it tween the house and a point on the lot where a or crawlspaces. Remember, there are retrofit slowly. If the gravel is to be exposed, I try to cap pitched drain can come to daylight, then an- draining and venting options that can, to a de- it with an attractive rounded stone. If the drain is other solution is needed. Theoretically, if the gree, rescue a damp basement or crawlspace. to be covered, I provide graduated layers of pipe never pitches back, you don't need more There are none that work on slabs. smaller stone toward the surface, then perhaps than the diameter of the pipe in elevation dif- Good preparation of the ground surface is crit- filter fabric before the sod covering. ference. Practically, more is better, and in. per ical prior to slab placement (drawing p. 5). Lev- I sometimes use 4-in. smooth-wall perforated ft. is a good number to shoot for. el the center, slope down to the excavation for plastic pipe in a French drain, particularly if I But say you don't have even that much pitch. the thickened edge of the slab, and compact expect it to carry a big volume of water, say For hundreds of years, cisterns and dry the soil well. Pour the slab on 6-mil polyethyl- runoff from the gutters. There are fittings that collected water below grade. Often, there was ene over at least a 4-in. tamped gravel base. This connect downspouts directly into this pipe. If an overflow toward a leach field. Such design is still feasible, and indeed it is useful for garden ir- rigation where fresh water is scarce. Some mu- French drains collect and transport water. A French drain nicipalities require new subdivisions to handle can play an important role in draining slabs, gutters and ground-roof runoff with on-site dry wells, rather than feed- systems of water. ing it to a common . Usually a 1,000-gal. precast-concrete dry well (drawing bottom right) or commercially available plastic drainage structures are buried somewhere on site. Water from the gutters is piped in and stored until it can soak into the surrounding soil. The likelihood of success with either one of these systems depends on the perc rate of the soil and sufficient storage capacity to handle the maxi- mum likely runoff. It also depends on how big the design rainfall is. If you are not required to treat runoff in a spe- cific manner, then take advantage of natural drainage courses on your lot. Get the water away from the house responsibly. If several downspouts connect to a French drain, enough water may flow from it to cause erosion prob- lems. Place rocks under the end of the pipe and in the outwash area to spread the flow out and reduce erosion. Don't flood the neighbors' base- ments to spare your own.

Flashing the intersection of the ground Dry wells avoid draining and the foundation—I call that zone where runoff on the surface. Required in some municipalities, the house meets the soil the "ground roof" be- dry wells give runoff time to soak cause the soil surface must shed rainwater away into the ground. Their success is from the foundation and the soil below. During highly dependent on the how well dry spells, I commonly see a -in. crevice be- the surrounding soil drains. tween the soil and the foundation. If that gap appeared on a roof, wouldn't we flash it? In se- vere cases or in old houses with hopelessly porous foundations, I have flashed this gap in the "ground roof" with EPDM roofing membrane metal strip called a termination bar, more com- Consider your building site to be unique, (drawing facing page). Polyethylene sheets and monly used to attach EPDM to roof parapets, and then plan accordingly—There are so bitumen membranes would work, but they de- clamps the membrane to the foundation at many soil classifications, foundation types and grade more easily when backfilled. grade level. I attach the termination bar to the climate variables that assembling general rules Ideally, I would flash a house as it was being foundation with expanding nail-in fasteners, al- is challenging. If there is a general rule, it is this built. In reality, I've done it only as a retrofit. The loy or plastic sleeves that slide into holes drilled one: Design the soil surface that goes around frost-protected shallow footing described in FHB into the foundation and then expand as a nail is the building to act as a roof. The overall aim is #107 (pp.76-81) would lend itself well to a driven in. I run a bead of cutoff mastic, a high- preventing the soil that is in contact with the ground-flashing system. I dig down 8 in. at the quality polyurethane caulk used for water- building from being saturated with water. This foundation wall and extend outward 40 in., slop- proofing termination bars on roofs, on this joint "ground roof" should ensure that rainwater ing the excavation 1 in. in 10. The hard work is and backfill. moves quickly and effectively away from the the digging. Having done it, I should get as The flashing could extend farther outward building. Downspout discharge, grading, flash- much benefit as possible, so I take this opportu- from the building at the downspout locations. In ing, drains and soil treatment at the surface all nity to insulate the foundation. Slice through existing buildings, you can often get away with play major roles in keeping the ground in con- one side of a 4x8 sheet of 1-in. rigid foam, 8 in. flashing only the trouble spots, usually inside tact with the building dry. from the edge, and fold along the cut. The re- comers with downspouts. The "ground roof" Basements, crawlspaces and slabs all have sulting piece fits neatly into the 8-in. by 40-in. need not be as watertight as a house roof. After their own peculiarities. With thoughtful design excavation. In the South I suggest high-density all, moisture control in soils is always a matter of of the area where the house and the ground in- mineral wool because it is less hospitable than playing the percentages. tersect, any foundation can be dry. Well, maybe foam to termites. In new construction, compact In soils with an average percolation rate, flash- any foundation that doesn't have provisions for the backfill under the flashing well. Otherwise, ing by itself is enough to keep the water away boat docking. settling could tear the EPDM from the wall or from the foundation. If your perc rate is slow, in- cause it to pitch toward the house. stall a French drain near the outboard edge of William B. Rose is research architect at the Build- After placing the insulation, I roll out the the flashing. Shallow plantings can go right on ing Research Council at the University of Illinois EPDM, letting it hang over the end of the foam. A top of the EPDM. Urbana/Champaign.