Green Roof Construction: Mistakes to Avoid

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A green roof is a radical improvement on your home. Properly implemented, it can increase the longevity of your shelter, insulate you from neighborhood noises, support your local ecology, and dramatically reduce your heating and cooling costs.

When planning your green roof construction, it's crucial to know what mistakes to avoid. It's a major investment and a big change, so little problems that go unaddressed at the start can cause major headaches down the road. As the old saw goes, an ounce of prevention is worth a pound of cure.

Mistake #1 - Not Knowing How Heavy Your Green Roof Can Be

Before you start a garden on your roof, you have to know the maximum dead load and live load of your structure. Otherwise, you could accidentally add too much weight, endangering the building and any residents. Dirt, water, and growing plants all add significant weight, which needs to be calculated carefully. Remember when you're crunching the numbers that your roof will sometimes get stuck supporting excess water from a rain event, or snow if you live in a cold climate.

If you don't know how much weight your building can bear, get in contact with the planners if possible, or get your building assessed. Shooting from the hip with a guess will save you an upfront cost, but it could have catastrophic consequences later.

Mistake #2 - Skimping on a Drainage System

Your green roof needs a reliable drainage system. If too much water gets stuck in the soil, your plants will struggle and your structure will suffer. Conversely, soil that dries out too quickly will erode more quickly and have trouble supporting healthy greenery. Invest in a carefully planned drainage solution, which will either feature coarse, natural materials or a layer of synthetics. This might seem expensive, but think of it in terms of preventing the extra cost later.

a roof with moss on it

Mistake #3 - Not Considering Context

Depending on the location of your house, the slope of your land, the shape of your roof, and the local weather conditions, various plants may or may not thrive on your new garden in the sky. Some very shady roofs may have trouble supporting any life at all. If you're not an expert on what plants like which conditions, make a trip to your local nursery, or at least conduct some web research.

Water will also behave very differently on steep grades and flat surfaces, so make sure you plan to build in some sloping if your roof is very flat, or some way to even things out if it's very steep.

Mistake #4 - Underinvesting

Purchasing low-quality materials to cut the expense of this project is a bad idea in the long run. If you can't afford to do this right, you definitely can't afford the costs of doing it wrong. Save up your resources and only start construction when you're sure you can foot the bill, which is likely to run into the tens of thousands of dollars (though doing the work yourself will still be far cheaper than hiring a contractor). If you're not sure about a specific material, try asking for opinions our forums.

Mistake #5 - Not Priming the Roof

Waterproofing the surface beneath your green roof is critical to avoid leaks, and you won't get another crack at that process once you've covered your roof in soil. You'll need to prime thoroughly, and follow up with a secondary layer of 15-milliliter polyethylene. Next, you'll add heavy plastic sheets like those made by Delta Floraxx, which will start setting you up to meet the water retention needs of your plants while protecting the structure beneath.

Waterproofing is a time-intensive phase of the process, but if you try to play fast and loose with this, you're virtually guaranteeing yourself a nightmare situation later on.

a green roof with colorful moss and skylights

Mistake #6 - Not Thinking of It Like a Garden

A mineral-rich soil with 25 percent compost will help start your roof plants off on the right root, as it were. The species and varietals you choose will dictate the exact balance you'll want, but like all gardens, your new elevated Eden will need dirt that has mulch, drainage, and fertilizer. Yes, you may want to invite some earthworms to be your new upstairs neighbors.

And not only will you need to start your garden carefully, you'll need to think of ways to access it and manage it later. Plants will grow and need tending to, soil will shift around, Frisbees will get stuck up there. Make sure your plans include a way for you to get to and work in your roof garden, and don't forget to factor your own weight into the load bearing calculations.