How to Build Your Own Green Roof

Our Roof

We started researching a way to build our green roof when we first conceived of putting a green roof on top of the garage.  The foremost important thing to a green roof is to engineer it correctly to hold the loads.  Since the green roof, once installed is not going to be removed, dead load considerations need to be discussed with the structural engineer so the resulting structure is strong enough for the green roof.

We opted to use a modular tray system for a number of reasons.  First, the medium would be shallower which means lighter loads (about 3.4-4 inches).  Second, a modular system is easily rearranged.  Not that you would do this all the time, but it’s almost like rearranging furniture.  Third, the perceived notion of fewer leakage concerns.  Well, we think the less direct contact the roof membrane has to the growing medium which will carry moisture, the longer that roof membrane is going to last.

Additionally, for a membrane roof to last a long time, it needs to be properly slopped for drainage.  Here’s Ruff Roofers installing tapered foam insulation that would create a slope to drain water into the scuppers.

Tapered Insulation

After that a TPO (Thermoplastic Polyolefin) is welded on top.  This material is preferable to EPDM because it is largely a single sheet rather than strips and the seam strength is said to be 3-4 times stronger than an EPDM roof.

Here’s the TPO.

TPO Roof

We chose a white TPO to reduce solar heat gain and keep the garage and the membrane itself cooler.  Additionally, to be extra cautious, Ruff Roofers added another sheet of TPO on top of the welded layer.

Then we started finalizing fun stuff on the roof.

Roof Layout

Our flat roof totals about 700 sqft, quite a large space to cover.  We selected two things to install up there: Green Roof Trays and Decking Tiles.  When we started out, both items are almost prohibitively expensive.  However, I realized that more than half of the cost of the pre-planted green roof trays we had considered was shipping.  I decided to find a local installer.  However, the contractors that do this really charge an arm and an leg for it.  We decided to see how much we can save by building our own green roof modules.  After searching high and low, we came up with a solution which I think brings the price point low enough that almost anyone can do it.

Source Locally

The community of green roof professionals is a small one.  This means everyone knows everyone and they’ve probably all been involved in projects together.  It also makes it easier to find different components, you simply ask.  We knew we needed the trays, the growing medium and the plants.  We want to avoid paying for shipping because none of these parts are very expensive but shipping kills the budget.

After spending a few hours googling and reading green roof forums, I found greenroofoutfitters.com.  Mike supplies both pre-planted trays as well as empty trays in South Carolina.  Mike told me where I can pick up his empty modules about 1 hour away from me.  Then I found Waynes Wholesale Nursery, a local supplier of green roof plants about 15 mins away in Fairfax.  The growing medium took a lot longer, since dirt is the heaviest part, this is definitely not something we want to ship.  After talking to people in about 5 different states in the region, someone pointed me to a supplier in Springfield Virginia that carries it.  We were excited that it is actually just in our own backyard.  So, now, we just need to put it all together.  The total cost of the green roof system ended up being about 1/4 of the quotes we received.  This also means we can now afford to build a larger green roof!

Picking up dirt

Eric laying out the trays and geo fabric

Growing medium

We had to move the growing medium one bucket at a time using a rope.  Here’s Eric shoveling them.   My back still aches.

Sedum

Wayne helped me select 8 different sedums that will work well for this site.  Can’t wait to plant them.