Hello! My daughter and her husband bought a house with a fully paved (with interlock) backyard. For several obvious reasons, they are pulling up more than half of the bricks and want to convert the bare area to sod. We know that sod requires several inches of soil but there is conflicting information on exactly how many inches. Search results vary from one inch to 6! If you can clarify, I would appreciate it. But more importantly, I would like to know if the sod will be healthy if there is any screening under the soil. We think there may be gravel under the screening. Before we put down the soil, will the gravel need to be removed and then the soil currently under the gravel be rototilled? If you have any other advice, we would be grateful. None of the info on the internet is directed at people who are replacing interlocking brick with grass. Thank you.
Thanks for contacting us about removing hardscaping (interlocking brick) and replacing it with lawn.
You have asked about soil depth for installing the lawn. You also asked about removal of gravel stones underlying limestone screenings. I have found these resources which appear to answer both questions. They suggest a minimum depth of four inches of soil is required and that the soil be free of stones; however, the recommendation for tilling is 7-9 inches. (Tilling can bring up dormant weed seeds, but it may also create a looser underlying structure for grass roots to grow into). I will answer the question of limestone screenings separately below.
Ontario Ministry of Agriculture – Sod Production (although this site is about the production of sod, it would apply equally to installing it). Note that the Ministry is reorganizing its websites; if the link no longer works just do a new search.
Prior to seeding fields must be cultivated and tilled to 20-25 cm [i.e. 7-9 inches]. Remove stones at seedbed preparation to a minimum depth of 4 inches. [Section B – Agronomy – Establishment]
Michigan State University – Nine Steps for Establishing a New Lawn Using Sod
If available topsoil is not adequate to level the soil surface or provide an adequate root zone (at least 4 inches), additional topsoil can be purchased. Specify that the purchased topsoil be screened to eliminate weedy perennial grasses and stones. [Steps for Successful Lawn Establishment – Step 3 – Preparing the Soil]
It is likely that six inches of topsoil is even better. See this from Landscape Ontario – Sowing Grass Seed:
The area to be seeded should be as level as possible, with a gentle slope away from buildings. If possible, avoid steep slopes or terraces where water may erode or wash away topsoil. The entire surface should have a layer of good topsoil about 12 to 15 cm [4-6 inches] deep. It may be advisable to mix peat moss or composted manure into your topsoil for best results.
and also from Landscape Ontario – How to Maintain a Healthy Lawn:
- Grass grows best in a moist, fertile soil that is not waterlogged.
- Sandy soil and heavy clay both need humus to improve the texture.
- A deep dense root system is most important to support top growth in grass.
- A minimum of 4 in. (10 cm) of soil is needed.
- More soil = deeper roots.
You also asked about limestone screenings. Usually these are applied to a depth of about two inches over the gravel layer, so they should be removed for two reasons.
First, the screenings would count as “stones” to be removed.
Second, even if they are often small, limestone screenings are very alkaline, i.e. high in pH (which stands for potential Hydrogen) that is available to plants to help them absorb nutrients such as potassium, phosphorous, nitrogen, calcium etc. Generally, lawn does best with a neutral or slightly acidic pH. The difference between acidity (low pH) and alkalinity (high pH) and the importance of pH generally for lawns and other plants is explained here: Oregon State University – Keeping pH in the right range is essential.
The best time to establish a new lawn is in the early spring, and in the fall from late August to late September. Whether your family chooses to use sod or seed is up to them, but the two resources above suggest that sod is preferable, as it will establish more quickly and will need a bit less care. If using seed, either for initial installation or for later overseeding to maintain the lawn, here is a useful calculator: Wildflower Farm – How Much Seed do I Need.
If there is still a fair amount of gravel left, you may wish to consider placing some loosely woven landscape fabric on top of it before putting down topsoil so that the topsoil does not wash into the gravel below. The reason for having it be loosely-woven is to allow oxygen to reach and be uptaken by the roots.
Another consideration is whether the area being contemplated for lawn installation is sunny or shady. Most traditional lawns do better in full sun; however, in part that is because a lot of traditional lawns use grasses that are native to the southern US. These lawns also require regular applications of fertilizer. With climate change and heavier rainfalls, fertilizers can run off into our streams, rivers and lakes and threaten aquatic wildlife.
Too much hardscaping can create a heat island effect around one’s home because the hard surfaces often absorb heat during the day and then reflect it back at nighttime. For this reason, the City of Toronto has by-laws about the ratio of hardscaping vs softscaping. If you are interested in more information about this, check out these links:
City of Toronto – Zoning – front yard back yard landscaping requirements;
City’s landscaping plan helps battle the heat island effect
However, as noted above, lawns – especially traditional lawns – have their own environmental impacts. I would therefore urge your family to consider lawns that are comprised of native grasses and/or other plants that will need less fertilizer and other maintenance (including watering) and also be able to absorb heavy rainfall better. The above resources and the ones below will hopefully help you with a way forward. Also do a search for “ontario importance of native grasses for lawns” to assist you in making decisions.
If you would like to find a landscape contractor that specializes in lawn, please contact Landscape Ontario – Landscape Ontario – Find Your Professional. Make sure that the contractor uses good-quality topsoil to a sufficient depth all over – I suggest you test the depth in several places to ensure sufficiency before the sod is laid or the seed is sown.
Also, I hope your family can make room for some pollinator-friendly plants as they transition their interlocking brick to a greener space.
Good luck!
Ontario Ministry of Agriculture – Lawn Establishment
Ontario Ministry of Agriculture – Lawn Maintenance
Oregon State University – How to Grow and Maintain a Healthy Lawn
Psychology Today – The Strange Psychology of the American Lawn
Ontario Nature – Reimagining Lawns
City of Guelph – Ground Covers and Lawn Alternatives
Wildflower Farm – Diagnose Lawn Problems