A large roof with long straight sides and no equipment or skylights is quite easy to install and to make waterproof. However, when walls turn and bend,or when holes are placed in the roofing for equipment supports, the chance of a leak increases dramatically. Every corner, every penetration is a potential leak. Knowing this, and working with this basic knowledge,
will guide you in achieving a roof that is truly waterproof, even though the building has great architectural forms and there are numerous roofing
penetrations.
Roofing penetrations are a fact of life. For example, much of the mechanical equipment used for heating, ventilating and air conditioning (HVAC)is located on the roof. Also, in the retail business, the roof may be
the best place to locate and to install a sign. To maintain all this equipment there is usually a roof access opening and hatch cover. There will also be fall protection devices. It is rare to find a roof with no equipment on it.
The first step to minimizing leak potential is to minimize the number of holes required in the roof. Consider building a penthouse and placing
your mechanical equipment with its ducts, pipes, and conduits under another roof. This has the additional advantage of reducing the cost of the
mechanical system because it no longer has to sit out in the weather. Next, consider moving equipment anchor locations to vertical surfaces such as penthouse walls and parapet walls. Here they have a far greater chance of remaining waterproof, because they will never be sitting in a puddle.
Sometimes penthouses and vertical surfaces do not exist, and the only option for equipment location and support is on the roof. Following is a
discussion about roof penetrations that may help you avoid some of the situations that result in roofing failures.
Roofing penetrations take many shapes. A large HVAC unit, for example, is usually placed on a continuous curb that forms a rectangular box on
the roof deck. The same is often true for rectangular duct penetrations and for roof hatches. Roofing at these curbs is installed much like it is at the perimeter walls except that the HVAC equipment itself (or roof hatch)
is often the counterflashing. This condition is rather basic and should be easy to install and maintain.
Some equipment, like pipes and ducts rest on curbs or stub walls that are built on the structural roof deck and have roofing covering all four sides. Unlike equipment curbs, these supports usually require additional counterflashing and often a cap flashing. These curbs and stub walls may be difficult to make waterproof because they will contain holes for the attachment of the ducts or pipes that they are supporting. As before, if the supporting device is anchored in the vertical surface of this stub wall there is a far greater chance of preventing a leak. In no case should there ever be a penetration of the cap flashing for an equipment support.
Equipment, like a retail sign, may need a structural support frame that is anchored directly to the roof deck. Roofing is installed around each leg of the support frame and closed with flashing and counterflashing. This kind of roof penetration makes a relatively small hole and is easy to make waterproof when the frame is round. But if the frame is any other structural shape this penetration has a far greater chance of leaking. Imagine trying to flash around a steel "L", or an "H" section. If the structural requirements for the equipment support demand these shapes consider placing a pipe sleeve around them at the roof so a round flashing can be made. The transition to the more structural shape should be welded solid so there are no leaks there either.
Each of these penetration devices (curbs, stub walls, and support frames)work to support something above the roofing while making the
penetration of the roofing waterproof. Following are some guidelines for their use, but first a few suggestions for basic roof design. A typical commercial roof is a lot like a swimming pool. The edges turn up at parapets and penthouses and this contains the rainwater that falls
on the roof. The bottom is sloped and this controls where the water can flow.
There are drains to carry away the water. Sometimes, because of building geometry, we add crickets and saddles to direct water to the drains. This may seem too obvious to state here, but adequate slope may be the single most
important element of a successful roof. Roofing should slope a minimum of ' inch in 12 inches. This slope should be developed at the valleys, and that means the main roof might slope ½ inch in 12 inches. Why so much you might ask? Roofing has thickness, and every lap in the roofing causes a small "dam" in the path of water flow. In the main field of the roof, this "dam" is
seldom a problem, but at flashings and crickets and in valleys there may be three or even four layers of roofing overlapping at the same spot, and then the thickness really adds up. If you have a ' inch slope at the field there may be very little or no slope left in the valleys. The result can be ponding, and standing water will often produce a leak.
Since it is so critical to get water off the roof, anything in the way of water flow should be considered as a potential leak waiting to happen. At large equipment curbs it is very important to maintain slope to drain
around the equipment curb. Try to place the least dimension of the curb against the slope of the roof. This will reduce the size of the cricket behind the equipment curb and, therefore, the potential for ponding and leaks. Whenever possible, place the large HVAC unit on a ridge so water flows away from it. At smaller curbs, like at roof hatches, make sure there is a cricket at the high side of the curb. Cant strips are not sufficient to maintain slope and drainage.
When stub walls are used they can also get in the way of proper roof drainage unless their location is given proper consideration. Try to locate
them so they are parallel to the flow of the roof, not perpendicular to it. There should always be a cricket at the high side of any stub wall that is blocking the flow of water. Never place a stub wall across a valley line.
The least offensive of all roofing penetrations and the easiest to build is the round pipe penetration. It doesn't block the flow of water, it is easy to flash and counterflash, and it is relatively inexpensive. So, as you plan your next roofing project remember the old adage,
"Location,Location, Location" as it relates to supports and roofing penetrations. Try to locate supports on vertical surfaces first. If that is not possible, make sure you don't block the flow of water.
