A well designed and functioning roof drainage system is essential to the long term integrity of your properties and their surroundings. Poorly maintained and malfunctioning roof drainage systems can cause water to be concentrated or directed to areas that you don't want, like a lower level unit or basement. Serious property damage can result from misdirected roof runoff. Your correctly installed and properly functioning roof drainage system protects your property in two critical ways.
The first important task of the system is to provide roof drainage, especially on a flat or low-slope roof system. Water that is allowed to pond on the roof will eventually find its way into the building envelope and cause problems for you and your residents. If enough water is allowed to accumulate, the weight of the water can exceed the limitations for your building’s structure. It is absolutely essential that your low-slope roof be equipped with an adequate system to get that water off the roof and prevent ponding from happening. Furthermore, standing water accelerates the collection of debris which, in a vicious circle, can worsen the problem and increase the volume of standing water. If, after heavy rain or melt, standing water remains on the roof for over 24 hours, you need to evaluate the functionality and effectiveness of your system.
The second important task of the drainage system is to keep water away from the foundation of the structure. If all the water run off from a large roof is allowed to simply collect under the eaves and saturate the ground around the building it could easily lead to soil erosion, wicking into building materials, leaky basements, damage to basement walls, uneven settling and the eventual deterioration of the building’s foundation. In northern climates this process can become accelerated as the moisture in the concrete goes through thaw and freeze cycles. A well designed and functioning drainage system should include components that collect the water being shed by the roof and transport it away from the base of the building.
The most basic building drainage systems collect the water at the eaveline. Components of this system usually would include eave troughs, downspouts and downspout extensions. Then there are systems which collect and direct water that is in the ground. These can include drain tile, foundation drains and French drains. Generally these types of systems involve elaborate underground structures that direct the run off water through storm drainpipes to catch basins which deposit the water into the city’s storm water system (or if this is not feasible, water can be run into Drywells installed for the specific purpose of harmlessly draining the water into the ground a safe distance from the building).
If you are convinced that the system you have at your property is incapable of performing its task, then you should consult with a contractor who specializes in this sort of work to engineer a new system or redesign the current system. There is no one system that will be the most cost-effective in every situation, so it will be necessary to evaluate your specific property to determine what components (or combination of components) will work best for your building. In addition, local weather patterns, surrounding landscape and how far the building extends below grade all need to be taken into consideration when devising a system.
For everyone else, who just needs to make sure their existing system is functioning correctly; obviously, space constraints prevent commenting on every possible aspect of every type of system that is out there, but there are three basic rules that are going to apply to almost any drainage system, of which everyone should be aware:
- The components must be of adequate type, size and number to handle the volume of water produced during the worst conditions. Your system may work great during normal rainfalls, but what will happen when the once-in-a-decade or even once-in-a-century storm hits. Are 5” eave troughs and 2” x 3” downspouts going to be sufficient, or will 6” troughs and 3” x 4” downs be required to handle the volume of water? Is one downspout on a run of eave trough enough, or should there be one at each end? Are the 10’ downspout extensions getting the run off far enough away from the building, or would it be advisable to route this water somewhere else and/or install an additional collection system like a French drain? These are the kinds of questions you need to be asking yourself or your contractor when evaluating your current system.
- The components must be kept clear of debris to ensure proper function. The problem with almost any drainage system is that foreign matter can enter the system, accumulate and cause a stoppage. Your system should be checked several times a year, especially in the fall when falling leaves can really exacerbate the situation. When clearing debris from around a drain or scupper, don’t just clear enough to get the water flowing. The entire roof should be cleared to prevent the drain from becoming clogged again.
- Any damaged components of the drainage system must be repaired quickly and correctly to ensure they continue to function as designed. For whatever reason, it seems like there is no other building element which suffers more neglect where the consequences can be so catastrophic. The misdirection of rainwater can cause rapid decay in almost every component of a building. Any leaking, staining, excessive wear, cracking or indications that water is collecting where it shouldn’t need to be investigated immediately and repairs should thoroughly address any issues that are found. This is not a situation where it makes sense to try and save a few dollars and just “put a band-aid on it”.
The bottom line to all of this is: the relatively small amount of money spent to install or improve a drainage system and the time it takes to maintain it could potentially save tens of thousands by avoiding costly and inconvenient repairs.