A well-constructed and maintained flat roof will protect your commercial or multi-dwelling business and provide many years of use. But a poorly-constructed flat roof can lead to a variety of issues that cause damage, leave the interior and exterior of your building unprotected, and decrease the roof’s lifespan.
We’ve identified 16 issues which leave your flat roof susceptible to damage and cover every aspect of its construction – from venting and flashing to membrane fasteners and other issues such as ponding water. We’ll briefly describe each issue and the correct way to prevent it from happening. Many components work together to ensure that a building’s flat roof is always doing the job it is supposed to do and not presenting frequent maintenance issues.
Flat Roof Junctions and Joints
1. Flat Tie-In Not Sealed Properly
The junction between your pitch roof and the flat roof is called the tie-in. This area is specifically important because there are two different roofing materials joining. It is very important that two roof systems have the correct flashing detail and extra sealant protection.
2. Offset Not Correct
The way membrane roofs work is by their overlapping design. As the previous layer is covered by the next it creates stair-step watershed design, disallowing seepage and water backslaps. This design also has a vertical component to it. If each row is not offset correctly, water may enter the system through T joints. It is standard practice to stagger each row of the membrane.
Venting and Flashing
3. Vent Not Lapped/Sealed Correctly
As with all rooftop objects, it is important for proper water flow and installation to be considered. Flashing which is not layered correctly with sealant applied in the right junction areas will allow water seepage. The vent flange should be installed correctly with the proper overlap procedure. When it isn’t, adding sealant may prevent seepage temporarily but is not a roof cycle solution. The vent should be replaced and installed correctly.
4. No Sealant At Flashings
Although flashings bridge transitions in the roof, when not sealed properly they are ineffective. Flashing is designed to turn corners and breaks in a roof, which tend to have more movement than other areas. If not sealed properly, these flashings do little to stop water from entering the roof system. Adding sealant can be done with minimal expense.
5. Ell Flashing Is Not Sealed Properly
Ell flashing bridges the transition from the roofs surface and a wall or upright surface such as a curb. When ell flashing has not been sealed properly, the shingles could peel back from the flashing allowing seepage. This can be corrected by removing the area and installing mastic sealant. New membrane may need to be installed when this is done. Also, if the ell flashing cannot be cleaned or has too many old fastener holes in it, the entire flashings detail area may need to be replaced.
6. Ell Flashing Behind Wall Surface
Ell flashing is designed to bridge the roof to wall transition and allow for protection against water intrusion. Water running down the roof and wall is caught by the ell flashing and channeled to the edge of the roof. It is important for the ell flashing’s end to be flared so it sticks out past the siding/stucco. When it terminates behind the siding/stucco, it provides a direct path for water to flow behind the wall surface and into the building. Often this type of leak takes time to show and may only be noticeable in heavy rains or after the wall has soaked enough to be evident. The only way to repair this issue is to add a layer of membrane over the existing flashing to direct water flow out and away from the wall. Some roofers may suggest cutting out the old flashings and installing new. We do not suggest this, as it is impossible to replace the wall surfacing material and regain integrity.
7. Gable End Flashing Not Correct
Steel metal flashings exposed to the elements will rust over time. Flashings are now made of galvanized metal, a process which protects the steel core from rusting. When rust begins to form, it cannot be simply sealed over, as it will continue to grow. The rust needs to be cut out and a new flashing installed, then sealed properly. Since the rust is open to the surface and flashings are vital to the integrity of your roof system, we suggest repairing the rusted flashing areas before seepage causes structural damage.
8. Improper Number of Modified Membrane Fasteners
Modified membranes are designed to be installed over an approved surface and fastened according to specific guidelines. If modified membranes are installed over a surface without the proper fastening, the roof assembly is out of code and the manufacturer warranty becomes void. This also leaves the roof susceptible to roof blow-off in the event of strong winds. As hurricanes have left hundreds of modified roofs sitting in the street as the entire system uplifted all in one layer, the current building codes have designated our region up as far as Orangeburg to meet the construction guidelines for 130 mph wind code. However, most of the good roofing contractors go above code and install more fasteners than required due to lessons learned after the storms.
9. Improper Fastener Type
When two different types of metals come in contact a chemical reaction occurs. Electrolysis uses moisture as catalysis at a molecular level to pass ions from one metal to the other. The process causes a deterioration of both metals similar to the effects of rust. Using the wrong type of fasteners causes a real concern due to the importance of the fastener itself. Electrolysis causes the fastener puncture to deteriorate and widen, allowing a water path and seepage.
10. Improper Fastener Installation
It is important for all fasteners to be installed perpendicular to the roof surface and driven in at a perfect 90-degree angle. On one roof we inspected, we found areas where the fasteners were not driven straight in and the fastener heads were lifted slightly. Over time this will damage the roof system and allow water to get into the building.
11. Fasteners Uplifting
During roof underlayment installation it is easy for installers to forget about proper pressure setting on their nail guns. In most cases, too many nailers are hooked up to a low volume compressor and the air supply is not sufficient to fully drive each fastener. At the time of installation only the severe, high nails are noticeable. When the modified membrane seals to itself a tight bond is formed and high nails begin to protrude through the top membrane surface, causing rust and seepage.
12. Valley Metal Buckling
Valley metal is installed by crimping the valley center, fastening the entire length of both edges and then applying mastic to cover the valley metal edge and fasteners. When valley metal is not properly pressed, fastened and/or sealed it causes buckling and waving. Since a valley is designed to channel water between two pitch areas a valley receives more water flow than any other part of the roof. Valley metal buckling disrupts the immediate flow of water and creates water vortices, a circular flow of water that leads to roof seepage.
Other Flat Roof Damage
13. Ponding Water
Ponding water is a code violation and warranty exclusion on all asphalt-based roof systems. It happens when water sits on a roof for more than 48 hours without evaporating or running off naturally. Water ponding breaks down oil-based asphalt. Imagine a pot of boiling water. When you add oil, it stays at top of the water. The same principle applies to a roof: the oil trapped under the water must come up. This pulling out of oil and waterproofing properties from roofing material is called oil migration and causes the roof system to break down.
14. Plastic Boots Used
Plastic typically dries out rather quickly in our environment. The sun’s UV rays cause oil migration much the same as it does to exposed asphalt. On one roof we inspected, a plastic stack flashing was used to seal the pipe. A gap between the pipe and surrounding flashing was visible and allowing seepage. We removed the boot and replaced it with a new lead boot to prevent dry out and further damage.
15. Delaminating and Sliding TDM Membrane
Delaminating and sliding of a Torch Down Modified roof membrane are caused by improper fastening. On roofs with a slight pitch, each row is required to be fastened. Without lifting up every row and breaking the sealing asphalt seams, it is impossible to tell if the problem is consistent throughout the entire roof or just the areas visible. Generally, if the same crew or person installed the roof, the problem is consistent. There is a good possibility that roofs with this issue will continue to experience this problem in the future, especially after high wind events.
16. SPF Foam Roof Damage
As a roof expands and contracts, the roof system moves. This is a daily occurrence and is associated with Thermal Shock. There are many ways the SPF foam can sustain damage.
- During movement when the SPF foam binds against itself.
- Sometimes a tree branch or flying debris may impact the roof.
- 3rd party damage is another issue, such as people walking across the roof.
- Birds are a particular problem with SPF roofs – they love to poke holes and nest in the surface.
When SPF Foam on the roof has been damaged the underlayment showing and it is our recommendation these areas are repaired.
You can avoid all of the issues listed above by working with Tri-County Roofing and Siding to construct and maintain the flat roof of your Charleston-area commercial building. We’re a multi-generational company which prides itself on customer service, our credentials, and expertise and it’s service we want to provide you, your children, and your grandchildren.
We also place a high value on continuing education while keeping on top of changes in the roofing industry. In turn, we can always provide superior value and service for our customers. You can count on us to always be there for you, and offer a 24-hour response guarantee for existing customers and a 48-hour response for new customers.