Coy Fuller, Co-Founder of Fuller Miller Construction
Last week in part one, we discussed the special considerations that need to be applied to the floor when building 100,000 to 300,000 sq. ft. warehouse/manufacturing projects. This week, we are going to discuss special considerations needed for the roof.
While conventional steel roof framing will have column spacing of 30-50 feet in each direction, a custom metal building can provide a rigid frame structure with a clear span of 120 feet or more. However, a super large building of 300,000 sq. ft. usually goes well beyond the width of even 150 feet. For that reason, a building of this size will have some interior columns, even in a metal building.
At this size, a building is usually comprised of four or five clear span structures situated side by side with no separating walls, which creates one super large space. This can be done by making each structure a single slope so that the roof of each structure continues the slope to one single ridge at the center of the overall arrangement.
Depending on the use of the facility, the possible downside to this arrangement is that it creates a high ridge height that results in a large volume of space inside the building above the eave height of the building. This unused space must then be heated and or ventilated.
The other option for this roof configuration is to have a separate ridgeline within each of the structures that are side by side. Historically, this configuration has been avoided since it creates a valley in the roof where the adjoining roof slopes come together. This point of connection can be a source of water leaks. However, with proper design to contain and control this water, this valley connection can be completely controlled.
FMC, along with Star Building Systems, has completed several super large buildings comprising over one million sq. ft. and containing 10,000 lineal feet of valley gutter – with absolutely no leak problems. This was accomplished by implementing the following guidelines:
Drainage Design. The goal of proper drainage design of a roof with internal valley gutters is to get the water off the roof as quickly as possible. In doing so, both volume of water and rate must be addressed. To accomplish this, the water must be drained from the valley gutters through downspouts to under-floor storm drain lines. The size and spacing of these downspouts are determined by the area of roof to drain.
Valley Design. The method of attaching the downspouts to the valley gutter is critical to achieving the necessary flow rate. The width and depth of the valley gutter must be adequate to hold the volume of water long enough for the downspout flow rate to keep up with the rate of rainfall.
Roof Sheeting Installation. The roof sheeting manufacturers installation manual addresses all of the required procedures and conditions necessary to ensure a watertight roof. The provided sealant tape must be installed as shown in the manual at all side laps and end laps, as well as all connections with various flashing. The critical areas in terms of water penetration are the panel side laps and end laps. The provided sealant tape must be installed as shown in the manual, and at all locations shown in the manual.
Flashing of Penetrations. The most common source of roof leaks on all types of roofs is the flashing at roof penetrations, such as plumbing vents or electrical services. Metal roofs are no different. The use of the correct and proper flashing and the proper installation of the flashing can easily eliminate this problem.
Quality Control. Aside from the gutter and downspout design, the rest of the installation of a metal building roof is not rocket science. The most important ingredient in obtaining a watertight roof is ensuring that the various components are properly installed with the proper sealants. This can only be accomplished with a third party inspector/observer on the roof at all times when the installation is occurring.