The Sustainable House, Part IX: Roof Membranes and Related Ramblings About Boxes

Just prior to the Christmas break, the crew was fortunate enough to get the plywood roof decks down. Progress, for January month, remained steady. The weather has been a blessing.

We had a late start last summer because my contractor had previous commitments. This wasn’t a serious problem in my view, as I knew that once he got going he would be dedicated to the project, and quite frankly, I was in no great rush. I had only one burning concern: that the roof membrane would be put on in the middle of the winter.

But here we were in January, and the crew were pushing to get the place watertight…

It’s important to understand the type of roof membrane I worked into the design. It’s not your typical shingled roof. Many of you will recognize the residential variant of the system, commonly referred to as “Torch On”, but that is an oversimplification. The membrane is properly called modified bitumen. The modification is a chemical one: styrene-butadiene-styrene or SBS, a synthetic rubber, is added to a common (and ancient) roofing material, bitumen (or asphalt). This co-polymer modification makes the bitumen far more flexible and durable, even in extremely cold temperatures. The bitumen is then processed into a sheet material by impregnation into a woven scrim reinforcement of fibreglass or polyester. The product is prepared in roll form, typically one metre in width.

SBS modified bitumen roofing membranes have about a forty-year history in Canada, and have undergone vast improvements over the years to become the commercial product of choice with over half of all new commercial roofs using the product/system. The market share in the US is not quite one quarter. This is due to a less severe climate and a wider range of available roofing expertise.

From an environmental perspective, virtually all roof membranes are problematic, and it will be interesting to see how this unfolds in the coming years. Fundamentally, however, an environmentally responsible choice should be towards a product that doesn’t end up in the landfill as quickly as the alternatives. A modified bitumen roof, properly designed and installed, should be serviceable for thirty to fifty years.

Now this is starting to get a bit technical and boring and I promised my readers I would not do that. But there are a few points that need to be covered.

First of all – and this isn’t the least bit technical – architects love boxes. I went to architecture school at a time when boldness of form and pureness of geometry was all the rage. The famous “less is more” adage eschewed decoration and historical bric-a-brac. We were coming out of the modernist era of steel and glass and concrete, even though some architects who pushed back against the proverbial Bauhaus School were gaining recognition.

My influences were many. Some would say L’ecole de Bozo Arts – having never been able to take architectural theory too seriously … or myself for that matter.

So boxes need flat roofs, except that well-designed roofs are never flat. They always slope to drains. Two percent grade is typically enough to drain away precipitation. We also employ parapets. These are low walls that contain the roof on all sides. Aside from capturing precipitation and funneling it to a drain, they rather conveniently convey the impression that we have created a box – even though we haven’t really done so at all. So we are in fact, cheating. And so what? It’s all about the architectural wow factor. We are aesthetic con-artists.

My sustainable house is just a bunch of boxes. You may say that’s just for aesthetics, but please let me dig into this. Bear with me. You’ll see where I’m going.

Many of you will start your dream home with an ideal steeped in Newfoundland heritage. The saltbox. The biscuit box. And that’s all very well, but these styles most often come with basements/crawl spaces and peaked (traditional) roofs. Going out on a limb here, it is my contention that these are bloody problematic. I have spent a lifetime trying to stitch together air and vapour barrier continuity in housing with these ridiculous, unoccupied and unneeded spaces. They were all well and dandy for a completely different set of reasons in the 18^th and 19^th centuries when there was no insulation.

But honestly, the painstaking efforts needed to achieve airtightness in traditional housing forms is completely ludicrous. Attics and crawlspaces are the bane of sensible construction. They will be the source of your problems. I’m shouting here now even though I’ve not jammed my Caps Lock.

Keep it simple. This means that if you don’t enclose spaces you don’t live in, you’ll be far better off. An insulated slab-on-grade is the bottom of the box. A “flat” roof is the top of the box and you can just spray the be-jeezus out of it with foam on the underside. Insulation, air and vapour barrier done. One shot.

The walls (over a foot thick) simply connect the lower and upper elements. No soffits. No roof vents. No scarfing in little strips of poly vapour barrier as you go along. This may be hard to grasp now, but as we progress, I hope that my point becomes clear.

Now back to the roof. Typical modified bitumen roof installations are two-ply. This builds in redundancy. Joints are staggered. Imperfections don’t result in leaks. There’s some forgiveness involved. The base sheet looks like rolled shiny bitumen on both the top and bottom side.

The cap sheet has granules on the top side to protect the bitumen from ultra-violet (UV) degradation – the same as shingles.

The mod-bit roofing is rolled out and melted into place with heat from propane roofing torches. The melting of the bitumen fuses the membrane into place upon cooling. Most commercial buildings don’t have wood decks, so this is not a problem (with proper cautionary measures). In the early days, we managed to burn down a few (well more than a few actually) wood frame buildings by torching mod-bit membranes onto wood decks. This is no longer the practice.

These days, the base sheet of the two-ply system (for installation on wood decks) comes factory attached to non-combustible insulation. There is a three inch selvedge edge on each “tile” that allows for positive overlap onto the previous tile. The three-foot by eight-foot tiles look like this:

Aside from preventing wood deck exposure to “hot work”, this method adds insulation value and allows the membrane freedom of thermal movement that would not be realized if the base sheet was fused directly to the deck. The tiles are placed side-by-side in successive courses. The top edge and sides are screwed to the deck with large washers. The selvedge edge of the next course is heat welded by torch flame onto the top edge of the previous course, creating a watertight membrane while burying the screws and washers into the installation. Side joints are typically covered with an eight-inch-wide strip of base sheet material which is also “torched on”. Flashings, or strips of base sheet required to waterproof the intersection of the horizontal base with vertical walls and/or parapets, are glued in place with a roofing adhesive. This also circumvents any torch flame on combustible wood back-up. Once the flashings are complete, the roof should be watertight.

At some locations, additional thicknesses of sloped, non-combustible, rigid insulation will be added to ensure a positive slope to drains. I have chosen not to employ internal drains, but to direct water “overboard” by means of a scupper (which will ultimately be connected to a downspout – see photo below). These require that the roof surface in the immediate vicinity be purposely sloped.

The decision I have made with the contractor and the roofing crew is to hold off torching on the second layer or cap sheet until the spring. This will allow us to progress construction on the exterior wall finishes and make sure that the base sheet is completely watertight. When the warmer weather arrives, the base sheet can be inspected for any deficiencies that might have occurred as a result of cold weather application conditions. Only then will the cap sheet be installed – 100% redundancy. The base sheet can be temporarily exposed for a period not exceeding six months. Remember, it has no protection from UV exposure.

The photo above shows great care and attention to flashing around the clerestory windows. It is important to note that the roof will be insulated beneath the deck when the envelope is fully enclosed and temporary interior heat has been introduced. This will be a foamed-in-place urethane. We are done for now.

Except for one thing – and I hope I haven’t lost you yet. As you may recall, about two fifths of this house will be buried into the existing hill. The roofing for this portion of the structure is a different beast.

The interior spaces of the buried portion of the house are largely utility plus a mud / laundry room and an artist’s studio. There will be no ceiling. The corrugated steel deck, beams, conduit and ductwork will be exposed and sprayed with a “dry fog” paint system. In other words, it will look like your local Sobeys or Dominion.

Here, the insulation sits on top of the deck. I don’t want any screws (visually unappealing and thermal bridging) so I have introduced the roofers to a fully glued-down system. An exterior grade gypsum board is glued onto the deck first. This provides a flat working surface. This is followed by a light gauge layer of modified bitumen that will act as a vapour barrier (keeping moist interior air out of the roof assembly).

Next, a layer of eight-inch-thick rigid expanded polystyrene insulation (EPS) is glued down, followed by a second layer of eight-inch EPS. The joints are staggered to ensure thermal continuity. The EPS we are using has an R-value of 4.26 per inch, so that’s a total of just over 68 – slightly less than the main living spaces. When we add in other system components (plus the 12-inch minimum of topsoil) we are approaching R-70.

Non-flammable protection board is glued down over the rigid insulation so that the base sheet of modified bitumen can be torched on. Again, having achieved an initial watertight enclosure, we are going to wait until the warm weather to install the mod-bit cap sheet. There are a variety of other layers to produce what is commonly called a “green roof”. More on this later.

Note that this portion of roof slopes to the outside edge with no drains or parapets. The water will be run into the ground and eventually find its way to the weeping tile at the base of the foundation. The edge of the roof has been turned down and temporarily secured with screws and washers, awaiting the completion of the EPS insulation on the walls together with backfill and eventually, topsoil and grass.

Kudos to those of you who have stuck with me on this. Feel free to respond with questions.

To recap, I have tried to introduce some thoughts for debate, primarily the notion that the “box” is easiest to effectively insulate and make air-tight. The roofing discussion seemed to be a great opportunity to throw this out there.