Surface and Subsurface Water

Minimizing the Potential for Damage to Buildings

Requirements in the National Building Code of Canada 2005

 

By: Ken Rauch

Introduction

The Bible says that the Lord “… sendeth rain on the just and on the unjust …”. This hasn’t changed much over the past few thousand years. The National Building Code of Canada (maybe not the Bible, but still a thick, heavy book), starts from this first principle, and goes on to explain what to do with the stuff after it hits. The answer is that, except where its retention is desirable, direct it from the assembly it first contacts to a location away from the building where it becomes someone else’s problem.

Drainage is the key, and means, for our purposes, “conduction of water in contact with a surface, under the force of gravity, to some planned-for location”. The word “drainage’ isn’t defined in the NBC, so I had to do it for them. After minutes of research I came up with the foregoing—you’re welcome.

Drainage Requirements in the NBC

Drainage requirements in the NBC deal with water from rain and melting snow that impinges on horizontal or sloped building assemblies or on the ground near buildings, and subsurface groundwater. With a couple of exceptions, water must be drained to a sewer, drainage ditch or dry well and be directed away from the location of a water supply, well or septic tank disposal field. Where groundwater levels are higher than a floor-on-ground, and could cause hydrostatic pressure, a concrete slab is required that is designed to resist such pressures. In such cases, gravity drainage is impossible. This is, of course, a much more challenging prospect than placing the floor above the elevation of the groundwater table in the first place and relying on drainage, but sometimes it’s unavoidable. That reminds me of touring the cavernous crawlspace beneath a wing of the Manitoba Institute of Technology—now Red River Community College, and being told of the huge pumps that had been installed to lower the local water table to solve a flooding problem. Apparently it worked so well that neighbouring houses began to subside along with the famous Winnipeg clay as it was dewatered. I’m not sure why I remembered that tour. It was almost 40 years ago. And, in case you’re wondering, I was a student then—we had lots of other things to do for entertainment—really!

Drainage requirements are found, mainly, in two Parts of the NBC. Part 5, “Environmental Separation” applies to all buildings, excluding housing and small buildings which are the focus of Part 9. There is some cross-referencing between these two Parts with respect to drainage.

Minimize Drainage loads

A good way to reduce the probability that a drainage system will be overwhelmed is to reduce the volume of water it must handle. The most obvious stratagem is to slope the finished grade away from the building. In the NBC 2005, backfill is required to be graded to prevent drainage towards the foundation after settling.

Water from roofs and other horizontal or sloping surfaces can be directed to downspouts and, if permitted by local authorities, discharged directly into storm sewers. If, as is often the case, discharge into a sewer is not permitted, downspout extensions must be provided to carry rainwater away from the building in a manner which will prevent soil erosion. From this point, our properly-sloped grade will do the job. It should be said that, while the NBC does not require downspouts or eaves trough to be installed, local regulations might.

Water that finds its way into a window well must be drained to the footing level or “other suitable location” (sewer, drainage ditch or dry well).

Runoff water from a driveway, that could pond on the driveway or enter a garage (such as where a garage is located beneath part of the living area of a house), must be intercepted by a catch basin that can facilitate drainage to one of the “suitable locations” mentioned previously.

An exception to the requirements for drainage and surface runoff is made if the building is “… specifically designed to accommodate the accumulation of water at the building or the ingress of water.” An example might be that reflecting pool or moat around your castle (you know who you are).

Drainage System Choices

Once water has reached the elevation of the footings (as some must), a system of tile and pipes waits to whisk it away—or perhaps not. The NBC provides two options to the drainage tile system. If it can be shown that a drainage system is unnecessary, one need not be installed (good luck with that). The second option is a granular drainage layer.

Granular Drainage Layers

The NBC permits installation of a granular drainage layer that extends beneath the whole building—and beyond. Granular drainage layers are most-often used beneath wood-frame foundation systems, and consist of a layer of coarse, clean granular material laid on undisturbed or compacted soil. The drainage layer must extend beneath the footings (compaction, compaction, and compaction!) and must extend beyond the perimeter of the foundation. The bottom of the excavation must be graded so that water is drained to a sump. Water from the sump is drained or pumped to a sewer, drainage ditch or dry well. Granular drainage layers are most-often placed beneath wood-frame foundations.

Surface-mounted manufactured homes are required, by the Canadian Standards Association standard, “Z240.10.1-07, Site preparation, foundation, and anchorage of manufactured homes”, to be installed on a granular drainage layer similar to its counterpart in the NBC.

Drainage Tile Systems

The most commonly-installed foundation drainage system in Canada is, undoubtedly, the drain-tile-around-the-footing system. The NBC provides detailed minimum requirements for this ubiquitous system.

Unless the drainage layer option is taken, exterior foundation walls must be drained by minimum 100mm diameter drainage tile or pipe laid around the exterior of the foundation. Tile and pipe are permitted to be of clay, concrete, asbestos-cement, corrugated steel pipe, vitrified clay pipe or plastic pipe. Some of these permitted materials might seem to be obsolete (I prefer “classic”). It’s true that most pipe used these days is of plastic. It’s with some fondness that I remember those individual, concrete drainage tiles with the little squares of sheathing paper laid over the joints, carefully mitred at the corners with a framing hammer. A good percentage of those tiles broke before or during installation, and became part of the backfill. But the system worked until it became clogged with clay or silt. If you don’t appreciate my Winnipeg reminiscences, you are free to skip them—or write your own article!

Drainage tile and pipe must be laid on undisturbed or well-compacted soil with the top of the tiles or pipe below bottom of the floor slab or ground cover of a crawl space (more on crawl spaces in a bit). The top and sides of drain tiles or pipe must be covered with at least 150mm of crushed stone or other coarse clean granular material.

Water intercepted by the perimeter drainage tiles must make its way, by means of pipes and/or the granular layer beneath a slab-on-ground (not to be confused with the granular drainage layer mentioned previously), to a sewer, drainage ditch or dry well.

If a sump pit is provided to hold water temporarily, it must be at least 750mm deep, 0.25m² in area, and provided with a cover designed to resist removal by children. I could tell you about fishing out of the sump pit the little Dalmatian puppies that slept in our basement when I was a kid, but I won’t. An automatic sump pump must be provided to discharge water from the sump pit if gravity drainage isn’t feasible.

Crawl Spaces

Drainage of crawl spaces is pretty-well the same as for foundations enclosing a basement. Ingress of water into crawlspaces must be controlled by grading or drainage, except where it can be shown to be unnecessary. Drainage for crawl space walls must conform to the same requirements as other foundation walls. Drainage of the ground cover or floor-on-ground in crawl spaces must conform to the requirements found in the Section on floors-on-ground (see next paragraph), and drains must conform to the provisions for other foundations

Floors-on-ground

Ingress of water beneath floors-on-ground (for example, basement floor slabs) must be prevented by grading or drainage, unless it can be shown to be unnecessary. Such floors that do not support superstructure loads must be supported on a layer of clean, coarse granular material at least 100mm thick. This granular layer aids drainage and provides a convenient space to install drainage pipes and sub-slab plumbing pipes, but according to the NBC, is primarily intended to facilitate sub-slab depressurization for radon-gas-control purposes.

As previously mentioned, if groundwater levels beneath a floor-on-ground could result in hydrostatic pressure, it must be a cast-in-place concrete slab designed to resist such pressures.

Where floor drains are installed, the floor surface must be sloped so that water is unlikely to pond on the floor. Floor drains meeting the requirements of Section 9.31., Plumbing Facilities, are required in basements in a dwelling unit provided gravity drainage to a sewer, drainage ditch or dry well is possible and in garbage rooms, incinerator rooms and boiler rooms serving more than one dwelling unit.

Dry Wells

Dry wells, as a destination for drainage water (until the water can percolate into the surrounding soil) are permitted only if the natural groundwater level is below the bottom of the dry well. To prevent migration of water from a dry well into the foundation drainage system, they must be at least 5m from the building foundation and located so drainage is away from the building.

Underground Ducts and Plenums

Underground ducts are vulnerable to water ingress which could result in entry of water vapour, other soil gases and mould and mildew spores into living space. These ducts must be constructed to facilitate drainage and cleaning at all low points. Connection to a sewer is not permitted.

Conclusion

Minimizing damage to building assemblies by surface and subsurface water that is intent on invading a building is a challenge that can be met by a strategy of surface drainage away from the building, removal with the aid of gravity or sump pump by tiles and pipes to a safe location or, in some cases, physical resistance. The National Building Code of Canada, which is the basis for all provincial and territorial building codes, outlines minimum requirements for drainage in Part 9 for Housing and Small Buildings and in Part 5 for all other buildings.

About the Author

Ken Rauch is a building technologist with 23 years of service with Canada Mortgage and Housing Corporation in various technical and building code related positions in Manitoba and at CMHC’s National Office in Ottawa. Since his retirement in 1997, he has assisted the Canadian Codes Centre, NRC with an Intent Analysis of the requirements National Building Code in preparation for the Objective-Based 2005 NBC and has contributed several code-related articles to this Forum. He makes his home in Cobourg, Ontario. Ken can be contacted by e-mail at ken.rauch@sympatico.ca