Permeable Paving

Permeable Paving

Most of Toronto's natural surfaces have been covered with hard (impermeable) surfaces like roads, sidewalks, driveways and buildings, which don't allow water to pass through them. When it rains or snows, the water can't soak into the ground, instead it runs off the surface and into the sewers, picking up pollutants along the way.

Increasing the amount of permeable, or absorbent, surface on our properties is one of the most important ways that we can restore the natural water cycle in the city.

This includes:

1. Reducing the paved and hard surfaces on your property,
2. Increasing the permeability of the remaining hard surfaces.

1) Reduce the paved and hard surfaces
What is it?
Reduce the impermeable landscaping on your property by removing all or parts of the paved or concrete patio, extra driveway space or walkways. Concrete patios can be replaced with a wooden deck over soil, walkways can be narrowed, and driveways can be reduced to just two tracks for the tires (with gravel or vegetation filling the gaps).

 

How does it work?
Reducing the impermeable surfaces allows more infiltration space for rainfall, reducing runoff. Gaps between paving stones or driveway tire tracks can be filled with gravel and/or turf or vegetation. This vegetation and gravel absorbs the water on the spot, filtering and cleaning it as it infiltrates.

What are the Benefits?

• Can enhance property attractiveness by increasing the amount of green space.
• Reduces the amount of runoff and the heating of rainfall as it flows across hot pavement (decreasing the Fast, Hot and Dirty effect)
• Vegetation in driveways can help filter and assimilate toxins from leaks in cars.

A few tips...

• Traditional paving stones often have cement or impermeable liners underneath (to prevent weeds from growing between stones). To allow infiltration, the liner as well as the paving stones need to be removed.

Costs:
Can be very inexpensive for simply remove paving stones or small areas of concrete, but costs increase if paved area needs to be removed by professionals. More extensive work may be required to remove an impermeable membrane laid under paving stones.

Maintenance:
Reduced paving areas requires virtually no maintenance. The maintenance of the green space will increase accordingly, and the maintenance of paved areas (including snow clearing and de-icing) will decrease.


2) Increase the permeability of paved areas
What is it?
Special paving stones can be installed that are designed to include gaps and spaces in the centre of or between stones. These gaps are filled with gravel or vegetation and allow for water infiltration. Specially designed permeable paving is different from conventional paving in two main ways: 1) Specific layers of soil and stones underneath the paving stones allow the infiltration of water, and 2) the cells in between stones are open and filled with either gravel or grass.

 

How does it work?
The spaces between the stones are known as infiltration cells, and these allow stormwater to filter through various layers and into the subsoil. There are 4 layers in a properly designed system:

1. Stone Layer: Paving stones form the top layer, laid with spaces that are filled with grass or gravel.
2. Bedding Layer: Below the stones, crushed aggregate (gravel or brick) stabilizes the stones and allows water infiltration and purification.
3. Base Layer: Crushed stone further filters water to remove contaminants and suspended solids.
4. Subgrade: This bottom layer is the natural soil of the area, a subgrade of sandy loam soil works best.

 

What are the Benefits?

• Permeable paving slows down or eliminates the flow of runoff and promotes infiltration on your property.
• Grass paving can trap rust, gas and other contaminants leaking from cars.
• Permeable paving can be more attractive than conventional paving as it allows more vegetation and interesting patterns, but offers the same function and stability.
• Permeable paving slows down or eliminates the flow of runoff and promotes infiltration on your property.

A few Tips...

• Permeable paving is only effective if installed properly. Choose a knowledgeable store that can help you if you plan to self-install, or experienced professionals.
• When installing the paving system, don't lay an impermeable liner down, which is typical of traditional stone interlocking. Non- woven filter cloths or geo-textured membranes are an adequate substitution. They and can be found at your local home improvement store.
• Ensure that the subgrade is suitable for infiltration. Review Infiltration Rates of Soil for how to identify proper soil infiltration rates.

• Permeable paving is ideal in low traffic areas such as driveways, laneways, walkways and patios.

Costs:

The cost of installing permeable paving can vary based on self installation, which type of paving type you choose (gravel or grass), the design style (simple vs. complex patterns). Gravel-filled paving will cost about $5 - 15 a square foot if a professional installs it for you.

Porous Asphalt and Concrete
Porous asphalt and concrete are very similar to their traditional counterparts. The difference is that large aggregate pieces are included in the mixing instead of smaller materials creating void spaces for water infiltration. It is generally not suitable for Toronto's climate as in the colder months, water trapped in the voids freezes, cracking the paving. Porous asphalt and concrete also easily clogs with sediment.

 

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