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Flat roofs are in style in the residential roofing world. You will find flat roofs on tiny homes built in urban centers and on rural off-grid homes. Flat roofs are also a popular option for home additions, new porches, garages and garden features such as pergolas and gazebos. Increasingly, homeowners are adding flat roofs to their existing pitched roof in order to accommodate these great features. However, when you add a flat roof and tie it into existing roof shingles, you will face some challenges.
Much like a roof valley, the spot where a the flat roof meets a pitched roof is more vulnerable than other parts of the roof. Roofers must take a lot of care during installation to prevent leaks in this area and to allow for proper water drainage. We’ll explain why the shingle to flat roof transition needs special attention, and how your roofer can connect a flat roof and shingle roof properly.
Essentially the problem stems from the change of pitch. Water moves faster down steeper surfaces than flatter surfaces. So, as fast-moving water runs from the shingle roof onto the flat roof, it slows down. This increases the potential for ponding water and leaks. If the flat roof covers an unheated area, it also adds additional risk for ice dams, which may also cause leaks.
While roofers successfully connect flat and pitched roofs all the time, they must be careful to create a smooth transition between the two surfaces. A very simple mistake a roofer could make is installing the flat roof materials on top of the shingles. While this might look waterproof to the untrained eye, it presents a serious problem. As water rolls down the shingle roof it will meet a lip where the flat roofing begins. The lip slows down water and holds some of it back, which can cause leaks. Further, if this spot never dries, the standing water will erode it over time, leading to early replacement of the roofing materials.
Further, flat roofs typically require replacement before shingle roofs. So, you will likely need to replace your flat roof at least once in the life of your shingle roof. Your roofer will have to remove some of the shingle roof to allow the new flat roof to be nested beneath it, or the likely result will be the problems listed above.
Why not just install shingles on the flat roof? It’s true that flat roofs aren’t exactly flat, they have a small pitch of a few degrees. Still, you cannot put shingles on a flat roof as water does not drain off them quickly enough. The absolute minimum slope for asphalt shingle installation is 2:12.
This job is best left to a professional roofer, of course. However, as the homeowner you may be curious about how your roofer should be tying in your two roof planes. Also, it is wise to be knowledgeable about how a flat roof and shingle roof should be attached, so that you can ask your roofer the right questions and get the best results for your roof.
So, here’s what you need to know about how to tie into existing roof shingles with your new flat roof.
Some professional roofers add a thin piece of metal flashing to the flat roof to pitched roof transition. In most cases, this is acceptable (although, usually not necessary.) Potentially, there is risk for ice to form on the flat roof and creep up beneath the metal flashing. Such ice penetration beneath the flashing may increase your odds of a leak.
While it is uncommon to have a flat roof home that transitions to a smaller pitched roof, they do exist. These roofs may look like Mansard roofs, but with a completely flat top.
The transition from a flat to a sloped roof is just as vulnerable as when they are reversed. Installing a new pitched roof to your flat roof will require a bridging membrane, just as we discussed above. Also, when installing this roof transition, it is more likely that your roofers will choose to add metal flashing to the joint between the roofs.
Flat roofs do not drain water as quickly as sloped roofs, so water will drain more slowly in this transition than the reversed roof planes. Metal flashing is a wise way to protect the rest of the roof from slow-draining water.
When your roofer is planning out how to tie in a flat roof to a shingle roof, they will also have to choose between an external and internal drainage system. Most pitched roofs have external drainage, or a gutter system. Flat roofs may have external or internal drainage.
So, when you are combining these two types of roofs, what kind of drainage should you choose? It will depend on your specific roof, and factors such as the size of the flat roof, whether it is above a heated or unheated space, and more. It’s best to work with a professional roofer to get the right solution for your roof.
That said, here are some things to consider when discussing drainage options with your roofer:
If the flat roof is sitting on top of an enclosed area, such as a home addition or garage, it will need insulation, just as with any roof above a residential home.
Homeowners are often concerned whether the color of their flat roof can match their shingles. Of course, you want your home to look good and retain its curb appeal and matching roofing materials is key to both of these goals. The good news is that IKO’s residential low slope Roof-Fast Cap membranes are available in colors that match select IKO asphalt shingles. So, your flat and pitched roofs can match each other.
When you choose a roofer to work on your flat roof to pitched roof transition, choose one that has experience with both types of roofing. With an understanding of both systems, they can do a better job of predicting and resolving any issues your specific roof may develop. You can find local roofing contractors through IKO’s Contractor Locator.
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