Shingles On Low Slope Roof
I’ve been estimating roofs for twenty+ years in Tallahassee. I’ve probably walked on tens of thousands of roofs in my career. I often run across situations where architectural shingles are installed on low-sloped area’s of a roof. Some define “low-slope” as below a 2/12 pitch. However, for this topic I am referencing roof slopes of a 2/12 or a 3/12 (up to but not counting a 4/12. I’d like to explain why I never recommend architectural shingles on a 2/12 or even a 3/12 roof. These lower pitched area’s are often additions on homes (screened rooms/patios) and mobile or modular homes. Lets take a look at why I don’t recommend architectural shingles below a 4/12 roof slope.
What is considered a low slope?
A low slope for shingle applications is generally considered to be a 2/12 up to a 4/12 pitch. These are situations where the water doesn’t drain as quickly as higher sloped roof angles.
The Common Bond Line
Architectural shingles (laminates) are actually two pieces of shingles laminated together horizontally.
This laminated intersection is what we call the “common bond”. Architectural shingles are thicker than the conventional 3-tab shingles because of this common bond.
You can see from this illustration that the total thickness of the shingle could be 3-4 layers at the shingle overlap. Item #5 is already 2 layers thick. When the next shingle edge (item #3) lays down the total “layers” would be 3 to 4.
Shingle Overlap
Here’s a great picture I took showing the shingle overlap along a particular row (this roof was a 5/12).
Even with a higher slope of 5/12, you can clearly see the elevated layers at the common bond line area.
Roof Slope versus Affective Slope
Your overall roof slope could be a 3/12… or even an 8/12. However, the affective slope at the common bond line will be reduced due to the extra layers of overlapping.
We can see this roof deck is a 3.5/12 pitch. The affective slope due to layering could be as little as a 1/12.
Traversing Water
Most all shingles are approximately 36″ in width. So water will get between the shingles at the side laps. Generally this is not a concern and expected. That is the reason that nails shouldn’t be placed too closely to side laps and the shingle side laps should be staggered by minimum of 6″.
When the affective roof slope is reduced then water can traverse along the common bond (since its slightly higher then the top of the shingle).
Roof Leaks -Low Sloped Roofs
Roof leaks can easily occur from water traversing along the common bond. This is most common when architectural shingles are installed on low sloped roof structures. Water will eventually find the next side lap or even rust out the nails. This is why we do not recommend architectural shingles be installed to low sloped roofs.
