A Sandpoint, Idaho General Contractor
Blog Header Photo

Online Blog > Previous Entry 9/01/2014 > 10/20/2014 - Practical Concepts of good water drainage > Next Entry 11/18/2014

Subscribe to our RSS feed to automatically receive new blog entries

There are certain things we do in construction that we seemingly do a lot of and somehow all at once on several projects at a time. This year, there are two types of things it seems like we practiced often. The first was that we did a lot of concrete, broom finished sidewalks and exterior concrete flatwork. The second is the subject of this blog entry.

North Idaho has a challenging climate in one respect particularly - because of the seasons and the rainfall, snowfall and frozen ground, it is very important to plan for good water management around structures. The first aspect of this is pretty simple: We build the buildings high enough so that we can establish a finished grade that slopes away from the building. The minimum grade we desire is a 6 inch slope in 10 feet. Also, we make sure none of our building materials, except concrete, is within about 8 to 10 inches of finished grade.

The next aspect we examine is the subgrade at the building foundation. Of course, we establish the bottom of the foundation at least 12 inches below the 50 year average frost depth. Around here, that means we want to see the bottom of concrete footers at least 32" below grade. Then, on heated and enclosed structures with a concrete stem wall, we apply an asphalt emulsion coating to the concrete itself. This gets applied from the base of the footer up to just under finish grade:

Asphalt Emulsion applied to foundation

Next in the process is installation of a perforated drain at the base of the footer. We like to use PVC Schedule 2729 four inch diameter pipe for its durability and cost effectiveness. We make it level with the base of the footer, perforations facing down at the 4:00 and 8:00 positions, and then it is all primed and glued together and is positively drained to either a drywell or to daylight in solid walled pipe once it departs from the vicinity of the footer. The perforated pipe at the footer is surrounded by washed round rock of about 1 1/2 inch diameter or less (1 1/2" minus), and we use a screen type barrier of fabric between the washed rock and the dirt backfill. That way, no dirt builds up in the perforated pipe over time.

Here is a photo of the pipe used recently at the back side of a retaining wall as it is getting covered with the washed rock and before it gets the application of the filter fabric:

Perforated Drain pipe in washed rock

Now, once the backfill is complete and the final grade drains away from the building, there is still the issue of water coming off the roof and from other places at particular times of year, rain during mid-winter when the ground is frozen particularly comes to mind. Even though that water will run away from the building, it won't penetrate the ground if the ground is frozen.

There are two ways to handle the water. First, on asphalt composition roofs we can use rain gutters, downspouts and then drop those downspouts onto grade, but an even dryer solution is to drop those downspouts into a solid walled, below grade drainpipe system using the same PVC four inch pipe previously mentioned. Rain gutters are expensive, and they won't work for metal roofs though, since snow shedding will take them out.

So, for these situations, we frequently install an at-grade drainage system that becomes part of the landscaping. It consists of digging trenches at grade, sloped the direction we want to then lay in perforated PVC pipe. This pipe will get covered with a decorative, washed drain rock (and the pipe will be protected from silt infiltration with a filter fabric). Here is an example of this installation at a roof eave:

At-grade perforated pipe drain system

This pipe can also be run to daylight downhill in the landscape, or it can be run into a drywell. It can be made to look decorative by incorporating flexible pipe and laid in a curved trench that mimics a dry creek bed. Here is an example of that type of installation incorporated into the landscaping as it flows away from a home:

dry creek drain

Of course, there are other options, like swales, but our use of drain pipe lately got us thinking about the use of drain pipe in particular for this blog entry.