Building the Cob and Green Roof Garden Wall – Part 5: The Green Roof


From the earliest conception of our garden wall, we dreamed of topping it with a green roof. However until we actually started building the roof we had no idea how we were going to go about it. We first needed some sort of truss system since we wanted the roof to provide a protective eave for the wall. Again, we turned to Sketchup to model the dimensions of these trusses and settled on a pitch of 4 in 12 and a horizontal eve span of roughly 6 1/2″. We built the trusses out of dunnage from the lumber yard garbage bin, a GREAT source for scrap 2x4s, and fitted them along the top of the wall, tacking them in to place with blobs of cob. We did our best to tilt each one to rest roughly 90 degrees to the tangent of the curve at each point.

Trusses being tacked in to place with cob blobs.


Once they were all sitting more or less how we wanted them, we cobbed them in to place firmly.


Trusses firmly cobbed in to place. The dark splotches are manure plaster straightening coat as described in Part 2.


The T junction buttress wall proved to be a real pickle to sort out. Sketchup saved the day again!




Once all the trusses were firmly in place, we plastered the wall in a lime scratch coat as we did with the free standing fireplace. We plastered the top of the wall too, between the trusses.


Applying the lime plaster scratch coat.


Then the slatting began. We used thin strips ripped from 1×4’s and pallet wood to connect the trusses together using staples, thus forming the eves.




We now needed a way to attach the siding to the roof. We came up with a face plate solution that attaches to the ends of the trusses. We took a photo of this but it, along with many other of the roofing photos, got deleted from Jen’s phone when it went on the fritz. Here is a doodle of the face plates as we made them, from scrap wood of course.


These face plates proved a great system for attaching the siding. The siding was made of strips ripped from 8′ lengths of 1×4 (the first brand new material to make its way in to the wall!). Even though these strips were quite small (3/4″ x 3/8″) they were still too rigid to bend to conform to the curves of the roof. To overcome this, we made a DIY wood steamer using a Coleman propane stove, a length of metal down pipe, and an old gas can with a flexible out spout. It worked GREAT and made for nice rubbery lengths of wood that were easily stapled on to the face plates to keep the curvy magic of the wall roof. Here is a link to a video of how we set up our wood steamer. I worked perfectly!







We stained the roof in exterior stain.

Next came the drains. Our roof is drained at every low point on both sides of the wall. We first removed a few of the roof slats to create openings where the drains would slot in. Then we filled the space left by removing the slats with 1/8″ galvanized mesh stapled in to place. We cut a circular hole in the mesh large enough to just barely accommodate the drain fittings.


Our drains are made from ABS adapters that we found at Home Depot. Their official name in the home depot catalog is here.


We had to make some adaptations to these fittings to make them work as we wanted them to. We first cut the top cuff off the male adapter. We then cut the lower rim off the female adapter so that the female part could be threaded right up tight against the rim of the male piece. See doodle below (the photos got deleted).

Drain hardware_starwatermark

We then took plastic yogurt container lids, cut the rim off, and cut a hole in them to tightly thread around the male adapter. We placed the yogurt container lids over the wire mesh holes and then covered the entire roof in vapour barrier plastic (reclaimed from a construction job) which we tacked in to place on to the tops of the face plates with staples. The yogurt containers re-enforced the mesh and better supported the drain assemblies. Vapour barrier is reasonably tough plastic but we certainly wouldn’t use it in a roof that absolutely could not tolerate any leaks, such as a house roof. Likewise, if we had serious concerns about the plastic getting punctures in it, we would have first laid down soft padding (such as spent carpet underlay or straw) below the plastic to protect it from puncture from the wooden roof. However, since the purpose of this roof, from our perspective, is simply to prevent driving, mechanically erosive Vancouver rain from chipping our wall away, we were willing to accept the possibility that some bits of moisture might seep through the roof and reach the wall. Cob, we have learned, can tolerate getting surprisingly wet as long as it is able to get dry afterwards.


We then pressed the male drain piece first through a slit cut in the vapour barrier, then the yogurt lid, then the wire mesh, and finally threaded on the female drain piece from below the roof to smush it all tightly together.


Once in place, each drain was covered in a square of scrap landscaping fabric which was then covered in a layer of gravel to tack it in to the place. This is to prevent the soil topping from washing away through the drains.

We tested the drain fittings by pouring water on the roof and they worked well. We watched the drains in action to see where the drip fell from each one. Just above the point of dripping on each drain, we drilled a small hole through the drain hardware and threaded a small zip tie through the hole. This allowed us to attach rain chains to each drain to prevent water from the roof dripping against the wall.


Prior to putting the soil on the roof, we placed a square of landscaping fabric over the top of each drain opening and then covered the fabric in grape sized gravel to tack it in to place. This was to prevent the soil from washing through the drain openings.

The first layer we placed on the roof was wood chips which we made from running fallen tree branches through the wood chipper. We hope that these chips will provide a layer of drainage and an added mechanism for the roof to hold on to water over the summer months.

Then came the soil layer. We mixed our own soil using 2 parts rotted horse manure/hog fuel previously picked from the hog fuel horse paddock, 2 parts perlite, and 1 part sand. Earlier in the spring we had trialled this soil mixture by making succulent planter boxes which thrived over the summer months with extremely minimal waterings. This made for a nice fluffy soil that we soaked heavily in water prior to splatting it on to the roof.


The prototype roof planter boxes worked great!


Our soil roof covering, including the wood chips is roughly 4 inches deep (1.5″ deeper than our prototype planter boxes).


Time to plant! We planted mostly sedums and hen & chicks, and other drought and freeze tolerant evergreens. Our roof was planted in early October, just before the barrage of West Coast rains.


We left the vapour barrier slightly proud of the roof edging. In fact, we didn’t trim the plastic until after the soil was in place to ensure we allowed plenty of wiggle room for the plastic to shift under the weight of the soil.






We are now well in to the rainy season in our part of the world. The roof drains are working phenomenally well and drain at a spectacular rate when it’s pouring. The movement of water down the chains is mesmerizing, much like a kinetic piece of art. We have even had a snap of freezing weather and a brief period of snow which the wall has tolerated without missing a beat.

In the spring of 2015 we will continue to work on the rest of the wall with the hope of the having the entire structure completed by winter 2015. The first part of the all will spend the winter of 2014 in only a lime scratch coat but we will hopefully apply a finishing plaster coat to it in the spring. We have yet to decide what that finish will look like. Maybe tadelakt?

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