We installed the living roof on the cottage over the course of several days that were spread over many months. I was going publish a post showing the living roof fully planted and growing lushly but I’ll have to make a follow up post for that. Here I’ll cover the structural design considerations for supporting the weight of a living roof, building a retaining boundary to contain the soil and vegetation, installing the various synthetic layers to waterproof the roof and provide for drainage and finally infilling with growing media or in our case a very thick layer of wood chips.
Roof Loading Calculation
Designing a structure to accommodate the weight of a living roof doesn’t seem to get discussed much apart from the very wise suggestion to contact a structural engineer. Just a note before we get started I am not a structural engineer. The calculations shown here could be completely wrong and I could currently be buried under my collapsed roof as you’re reading this. Hopefully, I caught the disaster on camera and someone will publish the footage posthumously for me as a cautionary tale.
So if we divide the total load capacity of each beam (6350 lbs) by 60 sqft the roof can support ~105 lbs per square foot. We’ll be conservative and say the weight of the roofing materials like the plywood, rafters, water proof membranes, insulation, drainage mat, and the beams themselves weigh 20 lbs per square foot leaving around 80 pounds per square foot for our media (i.e. soil and vegetation).
The saturated weight of a 6” deep layer of media ranges from 39-46 pounds per square foot. 10” of media would take our beams right to their limit at 66-77 pounds per square foot. For the cottage roof I’m aiming for a 6-8” deep layer.
It’s also important to look how the weight of the roof is transferred down from the beams to the foundation.
When I was planning my window and door locations I tried avoid positioning them directly below a beam.
Another point that doesn’t seem to get discussed much is how to contain the media on the roof. First I installed metal roofing on the overhang. The weight of the living roof would have required knee braces which would have interfered with my window layout.
After that I built a parapet on the front and back of the cottage using pressure treated 2x8s fastened down with angle brackets. On the ends I used a 2×6 supported by heavy duty angle brackets. This allowed a 2 inch gap for water to drain off the roof. I got our local metal fabricator to make me a variety of trim to cover the parapet and tie it in with the rest of the roof.
Chapter 3: Prepping the roof for media
With the parapet in place it was time put in the various waterproof membranes, insulation panels and drainage mats. For this step, I called in some friends to help.
We fastened 1x4s to the parpet to trap the various layers in place. Later, I installed metal trim over the 1x4s.
Mulching the Roof
The pictures above are from September 2022. We didn’t get back to the roof until April 2023. The geotextile fabric was uv stable so I wasn’t to worried. However, it was only a matter of time before the appraisal district noticed me with their satellite imagery.
I was dreading moving 7 yards or so of dirt onto the roof. I had rented mini skid steer to build the berm for the cottage but it didn’t have the reach to get dirt onto the roof. There’s an arborist in our area who will occasionally drop off wood chips. One week in March he dropped off a load and then another arborist through ChipDrop gave us another load. So, I thought why not just put a thick layer of mulch on the roof and let it break down its my own.
Before, adding mulch I put down a length of 4 inch drain pipe at the low end of each side of the roof. The pipe has holes drilled in it and is covered with gravel. Hopefully, this will keep the mulch and/or soil from blocking the outflow.
To get the mulch up to the roof I originally used a ladder as an inclined plane and made a sled that I could load wood chips on and pull up to the roof. However, the brute force method of lifting buckets of mulch to Jean won out. I took up probably took us a total of ten hours over the span of several days to completely mulch the roof.
Here are some pictures of the roof taken last week.