“Living” Roof Install

August 30, 2023
Earth Berm Cottage, Roof

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.

The heavy lifting of the roof as it where is being done by these three middle beams. The weight of the roof on the ends is supported by the end walls. The end beams also have knee braces not shown here.
The living roof will only extend beyond the exterior walls about a foot so I won’t calculate loading for the front roof overhang.
This is the plan view of the cottage. The measurement 11’4” is the beam’s unsupported span between the front and back walls.
I used the timber tool box beam loading calculator to determine the maximum weight each beam could support. The most important numbers are outlined here. Through some trial and error I came up with 6350 pounds for the weight the beam is supporting. There’s the unsupported span of 11’ 4” or 136 inches. Each beam is composed of three 2x12s sandwiched together making the total beam 4.5 inches wide and 11.25 tall or deep.
Supporting a total of 6350 pounds the beam passes as shown here.
Each beam supports a portion of the roof encompassing half the distance to the next beam. In this front view showing the framing of the front of the cottage you can see each beam supports about a 4 and half foot wide section of roof.
Lets round up and say each section of roof supported by a beam is 5 feet wide by 12 feet long or 60 sqft.

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.

I also calculated rafter (2x6s on 16” centers) loading and the beam would fail long before the rafters did.

It’s also important to look how the weight of the roof is transferred down from the beams to the foundation.

Here’s the framing layout out the front of the cottage again showing the window and door locations.
Here, the arrows indicate how the load is transferred to the foundation. The left and center beams are positioned directly over a couple of studs. The right beam is over a couple of windows so the weight is transferred to the header which is supported by jack studs on either side.
More more reinforcement I opted to install two individual windows rather than a double window panel unit so I could a few studs in between so there is a direct path between the beam and the foundation. It gave me peace of mind that there is no weight bearing on the windows. Note, I also added a couple of cripple studs below the windows.

When I was planning my window and door locations I tried avoid positioning them directly below a beam.

Media Retainment

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.

Here is the retaining wall with the brackets circled.
I used this lighter duty bracket on the front and back walls
This heavier duty bracket holds the 2×6 on the end suspended.

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.

Here’s an exploded view of the various layers. Starting with the ¾” pressure treated plywood roof deck I put on a layer of synthetic roof underlayment. This is just typical roll material that goes on most conventional roofs. It primarily served to weatherproof the roof until I can install the living roof.

For the waterproof membrane I used a 20 mil pond liner.
We used spring clamps to temporarily fasten the pond linter to the parapet.
For insulation we used 1.5 inch thick Rockwool comfort board. Besides insulating the cottage it also protects the waterproof membrane from punctures.
Over the insulation we put on 6 mil plastic sheeting. This layer will shed the bulk of the water. Anything that makes past the plastic sheeting will be shed by the pond liner.
The drainage mat has a series of little indentations or cups. It creates a gap that allows excess water to run down the plastic sheeting. I also read that the cups are designed to trap and retain some water.
The final layer of is geotextile fabric. This keeps the media from filling up all the gaps created by the drainage mat.
It only took us a few hours to install all the layers. Having the front portion of the as a staging area was invaluable. It would have been much more awkward working it all from ladders.

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.

I did plant sedums back in April and they seem to be doing well especially considering the summer we’ve had