Careful Selection of Roofing Curbs can alleviate leaking problems.
You are confident that the roof you chose for your retail project will be dependable. It's a big expense. You chose roof-mounted equipment carefully. You can relax, right? Wrong. Improper roof curbs can be a disaster waiting to happen.
Curbs that cannot be flashed in correctly, such as curbs furnished by some skylight and equipment manufacturers, are the main cause of future roof leaks. The culprit is lack of coordinatoin between the architect, specification writer and mechanical engineer. Specifications should be listed in the 07720, roof accessories, section, for structural insulated, fully finished, pitched roof curbs.
When listed in other sections, such as mechanical, curbs have to be field assembled and ususally require the addition of reinforcing angles.
The equipment manufacturer which supplies the curbs concetrates on supporting its specific units and usually does not have the facts or knowledge to adhere to proper roofing practices that enable the curb to mate with the roog design and slope.
The specification manner recommended by the Construction Specifications Institute is for roof curbs to be included in Section 07720, roof accessories, This ensures that curbs will mate with all roof applications and will coordinate with all equipment - mechanically, architecturally and structurally. Responsibility then rests with one source, which allows curbs to be on-site at the proper time, when the steel is delivered, thereby, assuring rooftop integrity. It's also a cost conserving measure.
One money-saving feature is that structural roof curbs eliminate most supporting angles for roof-mounted equipment because of internal reinforcing and continuous welded corners. These fully-furnished, insulated curbs eliminate the costly method of using curbs that require field modifications on the job site. When a structural curb is installed and fastened to the top of bar joist, additional structure is not required to support roof-mounted equipment. Curbs can be mounted up to four feet on either side beyond bar joist or supporting members. For installations of smaller curbs between bar joist, two reinforcing angles would be required. Structural curbs may also be attached to the top of the steel deck. Holes for supply and return for rooftop units are cut into the deck as required.
Double specification is a costly mistake
A common mistake made in project design is writing roof curbs in two parts of a specification for the same project.Coordination should be observed by the architect, mechanical engineer, and specification writer. A footnote should be added in the HVAC and skylight sections that reads "all curbs are specified in section 07720." This way the curbs can be delivered and flashed in before the roofer is required, therefore becoming part of the roofing system instead of being installed on top of the roofing or insulation. When installed properly beneath the insulation, most roof leaks will be averted.
The right product for the project
The roof curb manufacturer should supply curbs for all penetrations on the roof. This includes expansion joints, control joints, pipe penetrations, HVAC, fans, vents and skylights, This will result in a single source, single responsibility and in maximum cost-effectiveness.
Best way to retrofit an existing project
When retrofit is required for rooftop equipment, there is no need to disturb the roof itself. The old rooftop unit is removed and a roof curb adapter is installed on top of the existing curb, Installing the new equipment onto the adapter is typically accomplished at approximately one-forth the cost of replacing structure curb and roofing. This application works particularly well to change equipment in many retail projects. Further, it allows tenants who hire vendors to change-out equipment to preform this task with-out disturbing the roof in process. Shopping centers, for instance, have always provided a unique set of circumstances. Each project is different and needs to be handled so that there is no downtime for tenants. Changing needs for HVAC systems are a frequent problem for owners. Adapters are the ideal solution.
New construction can benefit
On speculative retail space, it is not neccassary to design an HVAC system beofre the tenants' actual needs are determined. A pitched, structural base curb is installed and a tempory cover is placed on top after the curb is roofed in. Later, after the tenant's needs are known, an insulated, structural adapter to match the HVAC configurationis installed atop the base curb, This eliminates the costly process of distrubing the roof structure and prevents a frequent cause of roof leaks.
Make sure the adapter has a sloping top surface to provide positive water drainage.
There is also an answer for future bathroom vents and plumbing stack requirements, A curb is installed with a gravitity hood and pipe boot. This allows the mechanical contractor to insall a ceiling fan in the bathroom and duct up to the gravity hood. The plumbing contractor installs his plumbing stack through the pipe boot. There is no need to have a roofer on site to roof-in the penetrations.
Wheather your project is new construction or retrofit, conventional or prengineered metal building a major expense is the roof.
Be certain the intergity and dependability of your roofing systemis maintained. Don't take roof penetrations lightly.
Curb the problems.
Make sure the specs are done right. You won't go wrong.