In developing a BIM roof system, it is important to understand that the actual materials are what determine the performance and construction of the overall roof. The type of roof covering used is considered based on not just aesthetics, but roof slope and local weather conditions. The secondary roofing materials are determined based on their individual merits as well; substrates are based on the building’s overall design, structural properties and locale; a vapor barrier is selected when or if certain moisture vapor properties merit its need; insulations carry specific R-Values important to the energy performance and code compliance of a building; the roof system attachment method is typically based on requirements of local codes. It is not necessary to understand how and why to use the specific materials when developing a BIM roof object. Realize though that the roof system is not just a slab that stops water from coming in. The appearance of the materials used in the roof system will determine how the details turn out. Use an appropriate surface pattern to convey the basic appearance of the upper most layer of the roof. Layers embedded inside of the roof will never be visible, and have little or no value to the model’s success. Cut patterns assign a symbolic appearance to various types of materials. The Uniform Drawing Standard (UDS) and National CAD Standard (NCS) have uniform symbolic patterns assigned to various materials. Using industry accepted patterns allows existing technology to be implemented into BIM technology without the need for reinventing a perfectly useable process. Each material has the ability to carry its own set of performance values which can be leveraged for model analysis and quantity estimation. A model calculates the length, area, and volume of a roof or roof material, but does not consider the unit by which the material is sold (Roll, Pail, Gallon, Cartridge, etc.) Where unit coverage rates are specified for given materials, an accurate estimate of the number of necessary units can be derived from the model, considerably decreasing the amount of time required to estimate a project. Later in this chapter, we will discuss in more depth the types of calculations that are possible from roof materials which carry a substantial amount of information. Roof System Components There are five main components that make up the basis of design for a roof system. In some cases others may be added and some omitted, but the primary components that should be considered for roof systems will allow others to be built from them and easily reused. § Substrate - The roof substrate is the structural component associated with the roof system. Functionally, it is the deck on which the thermal and waterproofing products sit. There are several types of substrates which may be used, the most common being Wood, Concrete and Steel. Unless the framing members are placed individually as described in chapter 28, roof systems should contain a material layer for the structural framing in order to consider the overall thickness for potential clash detection analysis.
*While the roof deck is arguably not a part of the roof system, it is not appropriate or effective to create one roof for the structure and deck and another roof above for the waterproofing system. It creates a conflict when placing components on the roof that are designed to cut a hole in the roof on placement.
§ Air/Vapor Barrier - Vapor barriers are very thin membranes typically placed between the substrate and the insulation. Even though the thickness is negligible, it is important to represent them in the model with a thickness appropriate enough to graphically display the material in a section view. Without giving the air/vapor barrier a thickness, providing call-outs or annotations of the material in detail section views is made more difficult.
§ Thermal Barrier - Roof insulation provides the thermal barrier between the building and the outside world. Just as a hat protects your head, since hot air rises, insulating a roof keeps the building’s heat from escaping. In some cases the insulation may be placed below the roof deck, and in other cases, it may be placed above the deck. In either case, its thickness should be easily adjustable and carry its R-Value in order to determine qualify the appropriate thermal protection used in a project. In addition to thermal properties, roof insulation may also provide slope, so the ability to taper the insulation independent from the roof structure is important.
§ Waterproofing Material - Shingles, membranes, metal and sprayed elastomeric coatings are all types of waterproofing. In most cases, this is the uppermost layer of the roof, and should carry some type of surface pattern to differentiate it from other roof types within the model view. Often, the roof covering is selected based on its appearance, and since there are so many options for roof coverings, it is important to have a good sample image swatch available for rendering the roof’s final appearance.
§ Roof System Attachment - Regardless of the type of roof, it needs to be attached in one fashion or another. Because it can affect how the roof performs or whether it meets the local building code’s requirements, it is important to add it as a layer in the roof system. Having it in the roof system as a layer allows the attachment method to be quickly called-out or switched if necessary.