The Scoop on French Drains [Foundation Drainage]
1. The first thing to appreciate is that contractors will sell their services based on what they are able to do. They are not independent professionals capable of doing a proper diagnosis without self-interest. Many will try to convince homeowners of the need to do a new French drain and waterproofing on the foundation because that is what they do and it is easy to sell. Most basements are damp so the assumption is that installing a new French drain, doing waterproofing, will solve that problem. You would be wrong to make that assumption.
2. Historically, basements in Québec and Ontario were never intended to be inhabited. Copybook architecture prevailed based on European [French and British] examples. Our frost lines go down to 4’6″ or more. Foundations have to be built to that depth; not so for the European buildings. Having the need to excavate to that depth for the foundations, local contractors had the idea of excavating the complete footprint of the building to the same depth thereby creating a basement in the process. However, going back about 100 years or so, foundations were built of stone [not poured concrete as at present] and stone foundations are difficult to waterproof. Inevitably, over time the mortar joints become powdery and voids develop between the stones, even when the foundations are 3 feet wide. All basements originally started out to serve utilitarian functions – the hot water furnace and the domestic hot water boiler using a fossil fuel, the entry for municipal services such as water and the exit pipe for sewage, the distribution panel for electricity etc. They also became places to store seasonal materials such as double hung storm windows, or the screens that replace them in summer, winter tires, snow shovels, toboggans, ice skates etc. It was never intended to be inhabited. Dampness prevailed. The occasional infiltration was tolerated.
3. Starting in the 50s and 60s, owners started to renovate basements to provide additional living areas. Teenage kids would be sent down there to play floor hockey or to have band practice. Adults created recreational facilities with bars for entertaining. Cedar closets became popular for storing winter clothing especially fur coats. After the war, there were lots of displaced people who immigrated to Canada and many landed up living in basements because housing was scarce. In more affluent homes, the live in maid or nanny were given a room in the basement. That’s when homeowners began noticing that basements could be damp and subject to occasional infiltration. Although there were municipalities which developed building codes more than 100 years ago [Westmount recently celebrated the 100th anniversary of their construction code which was overseen by PAC, the Planning Advisory Committee and the City of Montréal also had construction regulations which were among the oldest in the country if not the continent] it is important to appreciate that basements were not intended to be living areas. Those early regulations had more to do with visual appearances than health and safety [still the case today even where historic preservation is involved]. Those that chose to renovate their basements and create living areas frequently did it without getting a building permit — one is required when there is a transformation of use.
4. When French drains [which are called “foundation drainage” elsewhere in North America] first became regulated, the term used was “Where necessary foundation drainage should be installed”. The discretion was left to the builder. There were those who believed that considerations of soil mechanics precluded the need for French drains. For example, clay soil is by definition a fine particle solution; about one third the island of Montréal has clay soil. To maintain bearing capacity for the foundations which are “floating” in this clay soil, it is necessary to keep the soil damp or stable – not too dry, not too wet [e.g. Hampstead, Plateau Mont Royal, lower Westmount are all in clay areas]. A poorly installed French drain can do more harm than good [just as heating a crawl space in the Plateau can cause the soil to dry out and shrink]. In other areas, the soil is completely sandy and there is no need to attempt to collect surface or sub-surface water. Houses located on the sides of mountains, which is especially true of country houses in the Laurentians or the Eastern townships [or city houses in Westmount and Outremont] are much better off with a diversion trench – an upside down U to divert surface or subsurface runoff around the upper slope than to allow that water to reach a French drain at the base of the foundation.
5. French drains are also difficult to do well because they are intended to collect water around the footings [at the base of the foundations] and then discharge that water effectively. If the French drain only manages to collect water without discharging it completely, then that is counterproductive. To discharge water based on a principle of gravity means you have to have continuous slopes in the pipes [the French drains] to the discharge point. That was very difficult to do and originally the connections [discharge points] were made outside underground directly to the sewage outlet pipe from the house which was headed to the municipal collector pipe under the street [which could be as much as 10 feet down]. Eventually builders made the connections inside the house because it was one way of checking if the French drain was really working. The building code evolved as well so that the pipe connecting the exterior French drain to the interior collector basin in the basement had to be a solid pipe with a downward slope – to make sure that the collected water was able to be evacuated. French drains have holes in them and should not be used to transfer water.
6. The early French drains up to the late 50s consisted of terra-cotta or clay tiles with spaces left between them to be able to collect subsurface water [including water draining down from the surface]. The gaps between the pipes were covered in tar paper. Normal lifespan was about 20 years. In many cases the drains were put under the house and not around the foundations. Those located in clay soil became easily displaced over time. Doing a video of the pipes to determine the extent to which they are blocked by mud or other debris is immaterial if, in fact, pipes are displaced because it is then impossible to really transfer the collected water and dispose of it completely/effectively. In the 70s contractors began using a 4 inch diameter corrugated PVC pipes with holes in them to be able to collect the water. The normal lifespans of those types of pipes was also relatively short because granular material was able to seep through to the pipes and it was very difficult to achieve a continuous slope for drainage all the way to the discharge point. Weak points were corners of buildings because it is difficult to contain water in a pipe with holes in it going through a right angle turn. Most contractors simply installed those corrugated PVC pipes without ever achieving any slopes so the pipes only managed collect water which is counterproductive in terms of relieving dampness or infiltration at the basement level. Finally in the 80s French drains were sold with geotextile coverings; looking like a women’s nylon stockings. It prevented granular material from entering the French drain. However, the slopes for drainage were still as poor as ever so that French drains were still not discharging the collected water completely/effectively.
7. French drains have become one of the most frequent claims for latent defects. They are rarely properly assessed at the time of a pre-purchase inspection and are an easy mark for a claim. It is very rare for a building inspector to say that “it is unlikely that the French drains are really working”; rather, they will say “get them checked by an expert” – which homeowners rarely do until after the purchase. That becomes the basis for a latent defect claim. At that point no-one seems to appreciate that some houses may not need them, that the house has already proven itself by virtue of the passage of time. Besides, even when they are present, they may not work very well. For those houses that have some level of dampness in the basement and possibly the occasional infiltration, my recommendation is to give priority to repairing or improving defects other than the French drains. 1] Houses located on the sides of mountains should be installing diversion trenches instead, they are a lot less expensive than a French drain.2] Roof gutters to be effective need downpipes at every roof valley and at every corner of the house. Most houses don’t have enough downpipes. Really good drainage slopes are required for them to be able to work during freeze thaw cycles [we can have as many as 60 in one day] because the water has to be discharged quickly. The lack of slopes for drainage is the same problem that affects French drains; only the French drains are unusable all winter under frozen ground whereas the roof gutters can be made to work during freeze/thaw cycles. Some homeowners and real estate agents complain that downpipes look ugly and the gutter contractors are only too compliant in that regard [to provide the lowest price]. To be effective, the downpipes have to have subsurface drainage going to dry wells at the four corners of the property reaching down to the frost line. Four inch corrugated pipes are used, similar to those used for French drains only without holes in them. Properly done they are as effective as a French drain to manage drainage/chronic water infiltration problems in a basement, especially when there is a very large roof involved on a relatively small lot. The worst mistake that builders make is to connect roof gutter downpipes to a French drain so that in the end, neither of them work properly. 3] One has to monitor the use of underground irrigation systems/sprinklers. Most of the time homeowners will over-water their lawns or flower gardens which are next to the foundation resulting in a saturated foundation and a water buildup in the French drain – damp concrete foundations and water collecting and stagnating in the French drain. 4] It is important to have backwater valves in the basement because storm sewer backups have become much more frequent. Those who experienced the downpour on July 14, 1987 that filled up the Decarie Expressway have to realize that downpours have since become much more intense [due to climate change] and so we get as much rain but in a shorter periods of time. The storm sewers, many of them more than 100 years old, are incapable of coping with the intensity. Within the last year or so I have had two latent defect cases where the problem had to do with storm sewer backups and the claim is for the cost of replacing the French drains. They are examples of misdiagnosis, contractors offering to do what it is they are able to do. In one of those cases, there was absolutely no water coming through the newly installed French drain on the day of a very heavy downpour while the city has improved the storm sewers in the area. 5] The highest priority in regard to drainage should be to repair all foundation cracks and to have positive surface drainage [away from the foundations].
8. Most property owners who have older buildings with stone foundations would love to leave the stones exposed on the inside of the basement. They are easily convinced of its feasibility by contractors who specialize in waterproofing the outside of foundations and installing new French drains. That is a mistake. Stone foundations will continue to be damp even if the outside face is made completely waterproof simply because moisture leaches down from the masonry wall above the stone foundations. Sometimes the two are monolithic or continuous; masonry from the roof down to the footings [once again the influence of European copy book architecture]. Most lay people don’t realize that in the 20th century masonry walls are based on the rain screen principle where there is a cavity behind the masonry cladding, with weep holes at the base of the wall just above the foundation and through wall flashing directing any infiltration or moisture in the cavity to the weep holes. Without those features, the dampness will leach down into the basement regardless of the waterproofing on the foundation or the introduction of new French drains. The only basements I have seen where there are exposed stone foundations on the inside which are not damp are those that have been properly insulated with polyurethane [filling all the cavities in the mortar joints], some have the Delta MS membrane on the inside face prior to being insulated with polyurethane, and then a new stone wall is built inside of the actual foundation. It is also essential to have ventilation [part of the central HVAC system in the building] in a basement which is going to be lived in and used for purposes such as recreation, exercise/workouts, or for bedrooms. It is stagnant air which allows basements to have that smell of mildew, that permits fungal growth to occur and levels of bacteria and mold spores to get out of hand.
9. In summary, get a proper diagnosis done in regard to dampness or infiltration in your basement by an independent professional without being seduced by contractors whose specialty is waterproofing and French drains.