There are two main categories of roofing. Low slope and steep slope. Low slope is anything that has a pitch of 4/12 or less and Steep slope is everything above that. Slope or pitch is usually noted in a fraction of twelve. Twelve is used as it relates to the old school measurement of 12 inches. Slope is based on rise compared to run. Rise is how vertical the roof climbs and run is the horizontal distance on an equal plane to the flat ground. So a slope of 4/12 means the roof rises 4” on a horizontal distance of 12 inches (or one foot). So if I may jump ahead and offer this example, a pitch of 12/12 would be a 45 degree angle. This is not a pitch someone can walk on without any form of help or fall protection gear.
The general rule of thumb for usage of roof systems is that everything above 4/12 gets shingled with asphalt, cedar, concrete, clay, metal or composite shingles. Anything below 4/12 should get roofed using a membrane system such as SBS torch on, EPDM rubber, TPO, PVC and the quickly disappearing tar and gravel roof systems. Shingles depend on positive watershed and are not entirely self-sealing. They rely on overlap and gravity. A membrane roof is equivalent to enveloping and is a continuous membrane system with no weaknesses. I have waterproofed planters in torch on membrane and they are continuously holding water.
Now shingle manufacturer’s want us to buy more of their product so they say we can put shingles on roofs that have as low a slope as 2 or 2.5/12. They have their low slope requirements and necessitate changes to the roof system other than what would be done on a 4/12 or greater roof. I have seen it done and it has worked. I exclusively use a GAF-Elk shingle because they are so well self-sealing and become almost membrane like once they have had the heat of the sun to activate the patented Dura-grip adhesive on the back of each shingle.
The steeper the slope/pitch of the roof the less water is going to adhere to the surface and erode the waterproofing material. Therefore it goes to say that a flat roof requires a qualitative waterproofing system as it may hold a large quantity of ponding water for extended durations, depending on the effectiveness of the drain placements. Flat roofs commonly settle over time also (or sag in the middle of the roof area where the structural support is at the least) and when the rigid drain, supported by cast iron plumbing pipes, doesn’t move it results in excessive standing water as the drain is now “high”.
The attached photo shows an interlock shingle application on a low slope roof in which the contractor should have chosen a membrane roofing system. Trust Crucial Roof Services to choose and install your low slope roof system. Roof system selection and integrity thereof is of primary regard in all our roofs installed resulting in absolute customer satisfaction and continued referral business.
A critical component of any roof system, whether low slope or steep slope and whether torch on, metal, concrete or clay tiles, tar and gravel or asphalt shingles is ventilation. Ventilation is made up of two components – Intake and exhaust. Intake is commonly achieved through soffit vents and exhaust is commonly achieved through roof vents near or along the ridge.
When discussing this pertinent issue now days, clarification is needed. With the development of the LEED program – Leadership in Energy and Environmental Design – and the energy efficiency certification available to specific roof systems, a Green Roof is no longer just one specific detail.
The most common asphalt shingle roof leak happening today has to do with T-lock or Interlocking shingles. These are also known as duroid shingles and should not be confused with a metal shingle made by Interlock. Back to the leak issue: Of course, as previously blogged, leaks commonly happen where there is a protrusion or change in direction of the roof plane. However with these T-lock shingles, leaks can show up anywhere. First let’s get a history of this shingle. No longer in production today, this organic shingle was originally sold promoting the feature of wind uplift resistance. It’s my understanding that these shingles were sold with a 20-25 year life expectancy but current evidence details that most roofs in Vancouver and the Lower Mainland area are not achieving that extent. Technically, the unique design of this shingle allows for water to get in behind a corner of the shingle but escape out from underneath, lower down on the shingle. This critical water-entry point is where the tabs or T-Locks interlock with the below fastened shingle. This is all fine and dandy while the shingle is lying flat but as this ORGANIC shingle ages, it becomes dry and curls. It is at that age point the shingle systemically fails. Because of the curling, water will begin to travel sideways…which is never a good idea on shingle roofs…and leaks become inevitable. Because this shingle is no longer in production and extremely rare in ability to acquire, (I found some guy in Richmond BC selling bundles that used to cost $20 for $100 on the internet) roofers are left to try and stop the leak with our magic muck called mastic. BUT, unless the repairing roofer is aware of the “unique” feature of water entry and exit on this shingle design, he could potentially cause more problems than benefit by applying the mastic at the exit point and actually trapping the water under the shingle. If you have a shingle roof leak and want the best in service, 
A tar and gravel roof leak, in the general field area of a roof, can be a tricky thing to find. For starters, the crack or split in the roofing felts or roofing plies is concealed by a flood coat of asphalt and the ensuing gravel broadcast into that. In addition, there will be general and common debris such as dust and dirt, rain pollutants, tree droppings and anything else you might find on a roof. (You’d be surprised at what we have found!). These “additives” are commonly sunken into the spaces and gaps between the rocks in the built up roof system making it even more difficult to see a deficiency. Whether it is a 3 ply tar and gravel or 4 ply T&G roof system, the point of entry or stain on your ceiling is not necessarily exactly below where the leak in the roof is. Water travels. It takes the path of least resistance and escapes the attic or ceiling space through the easiest port. In bedrooms, it’s usually through the light fixture and in bathrooms it’s the skylight or the fixture again. It occasionally even runs in-between walls and skips a floor!