Perimeter drainage and site drainage
Parksville Excavating installs perimeter drains, drain rock and filter cloth at the footing and ties the line into a sump or a daylight outlet. Most of the 1,126 millimetres recorded each year at the Coombs station falls between October and March, so we size your drain for the wet half of the year rather than for an annual average.
Your perimeter drain has to keep water off the footing for the next twenty years, so we spend the money on the rock and the outlet rather than on the pipe itself.
What goes in at the footing
We set perforated pipe at the footing, below the level of the slab, on a bed of clean drain rock rather than on the native material. Filter cloth wraps the rock so fines can't migrate in and gradually pack the voids that make the whole assembly work. We set the fall with an instrument, not by eye, because a drain laid flat holds water and a drain laid steeply is no better than the outlet it runs to.
We put cleanouts in at the corners so we can scope and flush the line later without excavating. That decision costs very little while the trench is open and gets expensive to revisit once the backfill is in.
Where the water actually goes
An outlet that drains downhill on its own — a daylight outlet — is the better answer wherever the grade allows it, because it has no pump to fail. Where the fall isn't there, we run the line to a sump, and the sump becomes a piece of equipment you have to maintain.
We look at the ground before we look at the house. French Creek sits on unconfined deltaic sand and gravel with no confining layer above the water table, which drains readily but leaves very little margin. Inland around Errington the fractured Haslam shale takes water slowly, so the same detail behaves differently a few kilometres apart.
Discharge near a watercourse brings a second set of rules. The Englishman River, French Creek and the Little Qualicum River are all designated sensitive streams under the Sensitive Streams Designation and Licensing Regulation, and the Regional District of Nanaimo carries a Freshwater and Fish Habitat development permit area covering mapped and unmapped riparian assessment areas.
What keeps it working
Perimeter drains fail slowly, as fines work into the rock over years until the water has nowhere to go. We stop that with filter cloth, clean rock and an outlet that actually runs, and we decide all three while the trench is still open.
We treat surface water separately. Roof leaders stay off the perimeter drain and run to their own line, because a drain sized for groundwater doesn't want a roof's worth of water arriving in one November afternoon.
The drainage process
Read the grade — We read where the fall is, where the water arrives from, and whether a daylight outlet is reachable.
Trench and bed — We cut to the footing, place the bed, and set the fall with an instrument.
Pipe, rock and cloth — We lay perforated line and clean drain rock, wrap it in filter cloth, and set cleanouts at the corners.
Backfill — We place free-draining material against the wall, compacted in layers.
In practice
- Situation
- A Nanoose Bay rebuild on a sloping lot, replacing a 1970s perimeter drain that had never had cleanouts.
- Complication
- The water table in the bedrock units there moves between one and eight metres through the season, so the old line sat below standing water for part of the year and dry for the rest.
- Resolution
- We set the new line above the seasonal low with cleanouts at all four corners, discharging downhill on its own rather than to a sump.
People also ask
How long does a perimeter drain last?
The pipe outlasts everything around it — it's the drain rock that ages, as fines work into the voids over time. That's why the filter cloth and the rock we choose matter more than the pipe spec.
Does a perimeter drain need a sump?
Only where the grade won't give a daylight outlet — a gravity discharge has nothing to fail and nothing to run, so we use it wherever the fall exists.
Can roof water go into the same line?
We keep it separate. A perimeter drain is sized for groundwater arriving slowly, and roof leaders deliver a very different volume in a very short time.
Related: Land clearing and site prep · Foundation digs and backfill · Driveways and parking pads · Ground conditions on Oceanside · Qualicum Beach · Coombs and Errington · French Creek · Nanoose Bay