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A Discussion On Flat Roofing: Environmental Challenges In Building New Or Renovating

Contemporary home design often features the trend of flat roofing. Bruce Borden and Phil Gilmore talk about some of the challenges brought on by global warming and the extreme weather for this trend that homeowners looking to build new or renovate should consider. 

Bruce Borden is a partner in Walden Homes and has been renovating and building new homes in Toronto for nearly 25 years.

Phil Gilmore is the owner of Raymore Exteriors Corporation and has worked with Walden Homes. 

Bruce: Phil, in neighbourhoods where we are building, we see that contemporary new homes are a growing trend in Toronto. Since horizontal elements are commonly found in contemporary architecture, modern homes generally include some element of flat roofing. Would you agree that contemporary architecture we are seeing in these houses have introduced more flat roofs?  

Phil: Yes, I would agree. What's interesting is that as the owner of a roofing company, we are pretty much at the intersection of architecture and residential roofing. Whether that be flat roof, low-sloped roofs that combine or intersecting with steeper-sloped roofs with some combination of different materials. We are definitely doing more and more flat roofs.

Bruce:  Can you discuss how contemporary architecture and flat roofing affects the work that you’ve been seeing over the last few years? What are some of the changes that you’ve seen in residential home building trends over the past five or six years?

Phil: Well, if the house is a modern design we are dealing with predominantly flat roofs – like, the house that we did on Coldstream Avenue. These types of homes usually have a series of flats. To me, this is a contemporary look. You know, that style. Would you agree?

Bruce: That’s right. Yes. That is what I mean when I think of contemporary.

Phil: It’s a very contemporary look and of course, very popular. And, you know, if it’s done right, flat roofs can be long-lasting roofs that have some pretty nice curb appeal.

Details and Water Management

Bruce: So, let's talk about getting these flat roofs done right.

Phil: Well, it always comes down to the detail work. When you've got a good size flat roof there is always a large area and you've got to have a way to manage the water. Water management is a detail that you have to get right.

Bruce: Can you explain what your mean by managing the water?

Phil: Sure. On a typical sloped roof we have a large soffit and fascia with eavestrough running around the perimeter of the roof. The eaves are there to collect the water from the roof. On a typical sloped roof that sheds water, the eavestrough and downspouts are the water management tools. On flat roofs, water is collected in a completely different way.

Bruce: Yes, the flat roofs we build are designed with some slope so that the water does not collect. The roofs are not literally flat. The roofs slope towards one side where the water is directed drain into a downspout.

Phil: That's right. On a flat roof we typically have a parapet wall that we have to work with. Architects like the parapet wall because you get a very nice finish on the elevation. However, we have to have a way to get the water through that wall. This is done by way of a scupper, or as an alternative, an internal drain.

Bruce: Which means that all the water is drained through a limited number of outlets. This makes it all the more important that the flat roofing system is detailed properly because of the high volumes of water getting through a very small area.

Phil: Yes, the flashing detail work is critical. You know, in Toronto, the weather has changed so much in the last 10 years. Our winters – well, this winter is an exception – but, moving forward, how much of an exception is it really going to be? We get large accumulations of ice and snow. Things expand and contract at different rates: the steel, the wood, and the membranes. You have to use a certain type of material with a high tensile strength that can withstand the expansion and contraction, whether it's -40 or +40.

The Right Crew

Bruce: Is this the reason that your flat roofing installers are a different crew from the normally shingling crews?

Phil: That's right. The flat roofing crews are all certified. They know how to work the membranes so that they don't lose adhesion or open up to the elements. As you can appreciate, if there's a leak on a flat roof you're going to know it in a big sort of way versus a shingled sloped roof that sheds water. So, there is a fear with flat roofs because if you have an installer that's not up to speed on how to do this work, there's trouble. But you know if they're done right, they're fantastic.

The trick is the detail work. A key is to make sure that when you have a flat roof running into a vertical surface, like a second storey structure, the membrane needs to run up that wall high enough so drifting snow and driving rain cannot migrate and get in. Then you have to counter flashback onto the roof and make sure those areas all the way around the perimeter are tight. Any protrusions such as a drain, an exhaust vent for a bathroom below, a gravity pipe – they need to have the proper types of flashings and they need to be the proper types of stacks and then they need to be flashed in accordingly as well.

Bruce: With a well-installed shingled roof, we expect to get 20 or more years out of that roof. When we talk about flat roofs, is there such a thing as long lasting? What is that in years and what can our homeowners expect?

Phil: Okay. Well, that's a good question. In the Beaches, we did our first two-ply flat roof system close to 20 years ago. Two years ago, a gentleman called and said, "You did my roof. I think it was 18 years ago." He asked, "Can you inspect it? We are thinking about renovating the third floor." We went up there and you know what? It looked great! That was two years ago. So it's 20 years old and it is still going strong. So, I can tell you with 20 years' experience, a two-ply, modified bitumen system should last 20 years plus.

Bruce: Are there things that can affect the lifespan?

Phil: The materials are warrantied for 20 years. Labour, on a good roof should be ten years, but those systems can last much longer than 20 years if there's good drainage. You don't want significant ponding on there all the time. You need a granular cap sheet. They prevent the UV rays from breaking down the membrane.

The Right Materials

Bruce: What other things impact the longevity of the modified bitumen flat roof?

Phil: In layman's terms, the thickness of the rubber. So, when I'm saying, it's a 250 gran cap in a 180 base – basically what I'm saying is I'm giving you the thickest cap sheet and the thickest base sheet. A lot of guys will say two-ply system and mean they're using just a cap sheet and just a vapour barrier, but technically, it is two-ply, don't you know.

The devil's in the details. You want a quality cap sheet and a base sheet with a registered installer. Then you're going to get longevity on those roofs.

Bruce: Let's shift gears on this. A little earlier on you brought up a good point about the changing weather. This is an interesting angle. From your perspective, how is the weather changing roofing? You know, with the temperature, the winds and wind-driven rain, what's changed for you?

Phil: Well, of course there's the ice damming. Right? I mean, ice moves mountains. I've seen ice damming at eaves tug an eave detail away from a house. The whole wood socket heaved an inch-and-a-half to two inches. It's just a sheet of ice running down and that is a symptom of significant heat loss.

Typically in older homes, insulation gets depressed in some areas or sculpted because of the winds that get in there. Even in renovated homes, where for example, pot lights or exhaust vents were installed. If the vapour barrier is not completely sealed, heat just escapes into the attic. Then, of course, the -20 outside air and all this warm heat escaping – hot meets cold – and then condensates, hits the top of the plywood and starts to drip back down. This is a significant problem in Toronto.

The way you combat that on the roof side is you put ice and water shield and drip edge and metal flashings at the eave. That helps prevent rain, water, ice, snow from getting in. It doesn't guarantee it, but it helps.

Bruce: I know, that is why we have to pay so much attention to getting it right the first time. Coming back and correcting the problem is difficult and costly. Phil, can you talk about the steps we take to assure that this is not an issue?

Phil: Sure. To start off, you want a significant amount of insulation in the attic. Optimal is around an R-50. You make sure that insulation is raked off the soffits. Why? You don't want any restricted airflow from around the perimeter. So, when it's raked off the soffits, make sure that you have perforated soffits and that there is air flow. Then slide in baffles so the insulation doesn't blow back over the soffits that you just raked off. You then want to make sure that you've got the attic sealed off from the warm air inside the house. Next are the attic static vents on the roof to help draw that air and circulate. So, if you've got an attic with R-value and have the attic well sealed from the house, now you're significantly reducing the heat loss into the attic. Those in conjunctions with good airflow that circulates, you now know it's not going to collect condensation.

Of course, when you do a roof, you have your roofing contractor make sure that if there are any bathroom vents, the flex hoses are tied off and steeled around the exhaust vents. Hopefully, they're insulated flex hoses, because if they're metal they tend to condensate and they can cause problems. So all these things make a big difference.

Bruce: Yeah, that all makes sense. Hey Phil, earlier on you mentioned that on the flat roofs, water management is an issue. What about the crazy heavy rains? The volume of water and wind driven rains can be an issue. What can you say about handling the flash rains that we've seen the past few years?

Phil: With the high, driving rains and lower sloped roofs, we used to rely on ice and watershield protection on the entire deck. In some cases, you really sort of restrict the roof from breathing because you've just waterproofed the entire deck. So, you have to be careful not to suffocate the roof. So, while the ice and water shield is a big plus for the ice damming in the winter, we have to make sure that the roof is breathing properly.

Bruce: Are you saying that the balanced approach in an underlayment? What is the underlayment we are installing now?

Phil: It's a synthetic felt, which allows the roof to breathe, but sheds water, which is brilliant. It's also like a woven polymer or fabric. So while a driving rain or a tree limb hitting the roof can lift and rip your shingle off and Rocky Raccoon gets up there after he sees the shingles off and wants to go live in your attic, he can't tear the synthetic felt.

Bruce: It's brilliant. What about managing the volume of water?

Phil: When we typically go to a home, if I see the eavestroughs that are put in with nails, that's a red flag, because the nails don't have much integrity. They pull out and the eavestrough can slump quite easily and hold water and of course, when it holds water it pulls out even more. It compounds the problem. So, we just like to make sure you've got a proper eavestrough system. No less than five-inches wide; sometimes six-inches, if it's managing a long, sweeping roof slope. Then, of course, you have the large downpipes that are strategically placed to manage the water.

Bruce: How large? So, how large would those downpipes be?

Phil: At least 3-inch by 3-inch square. When we can, we'll require 3 by 4-inch and getting it away from the foundation wall. So, with these things you've got to know what you're looking for.

Bruce: On the eavestrough, when you said not to have them nailed, then what type of system is it that you recommend?

Phil: Well, it's a hidden bracket. So, it just grabs and gets nailed in or screwed in from there and it just gets a good bite versus those aluminum spike and furrow that they just don't hold up.

Bruce: Right.

Phil: They're a little cheaper, but they just don't hold up.

Bruce: Okay, all that sounds good. One last thing, with the gutter guards, that's something that you are using on many of our roofs.

Phil: Well, there's only one type that I'll use if I use them and that would be the Alu-Rex. Alu-Rex is good because it has round perforations, but very small. It allows dirt and sand and maybe very fine foliage that will easily get through the elbow assembly of the downpipe.

That's what you want. That doesn't plug up the elbow assembly, so it runs free and clear. The bigger stuff tends to lay on it and blow off with ease. With other products, stuff gets caught in it, so you're up on a ladder cleaning out the gutter guard. Then you say, "Well, why did I put it in in the first place?" So, I'm not big on gutter guards except for Alu-Rex, because that has the best track record with me. The downside with any gutter guard is that if there's snow and ice, it will get caught at the eave a little bit, because there's a gutter guard there now. It doesn't just fall into the trough, but if the roof's done right, with good underlayment, it's a non-issue.

Bruce: Phil that's all great information! Thanks for the talk.