Exploring the Building Code - Back to Basics / Construction Types

Our office has been doing a lot of growing recently and as a result it’s easy to lose site that what maybe an established code concept for some is new or foreign to young professional. With that in mind our office has been focused on bringing some ideas back to the basics. This blog focuses on the 5 construction types and the differences between them. 

In general, there are 5 construction types with each type of construction becoming more stringent as the type descends from type V to type I, but as the construction type becomes more it also affords a greater allowable building area and height. Each of the five construction types except type IV also include a sub type designated as either “A” or “B”. “A” sub type is often referred to as protected while the “B” sub type is referred to as unprotected. This sub type does not indicate if the building is sprinklered or not but refers to if fire protection is applied to the buildings primary structure. The additional fire protection requirements can be found in table 601.

The Five Types can be summed up as

Type V – Non Combustible Construction – Typically wood frame construction, but actually refers to any material permitted by the building code. It is most often found in residential construction and small commercial structures.

Type IV – Typically referred to as Heavy Timber - As previously mentioned this is the only construction type without both an “A” and “B” sub type. The exterior walls are required to be built of non-combustible material but the interior building elements can be of solid or laminated wood without concealed spaces. Section 602.4 provides the specific details for the construction of walls, floors and roofs. This type exists because solid lumber (when of a certain size as established by the building code) provides natural fire protection. Even though timber is combustible the extra thickness takes longer to burn therefore providing extra time for the safe evacuation of life in the event of a fire. Type IV construction is an odd bird in that it does not follow the rule that as the construction type descends you are afforded greater allowable height and area as the allowable height and area for buildings built of heavy timber in most instances exceeds type IIIA and can even exceed type IIA.

Type III – Describes a hybrid where the exterior walls are built of non-combustible construction and the interior construction like type V is any material permitted by the building code. Like type V we typically use it in the construction of multifamily residential structures, but it can be used for almost every use group (The exception is type IIIB is not permitted for use group I2). The practicalities of construction often dictate going up to type V or down to type II construction.

Type I and II – Refers to buildings built of non-combustible construction. Typical examples of non-combustible materials include masonry (Including CMU), Steel frame (Beams, Columns, and Joists) and metal studs. The primary difference between type I and II is the level of fire protection given to the structural frame. IIB, Like IIIB and VB does not require any fire protection of the structural frame, but that is not the case for IB as it is required to be fire protected. In many ways type I construction is just an extension of type II, as the fire protection of the primary structural frame increases as you work your down from type IIB to IA.  (IIB – 0 hours, IIA – 1 Hour, IB – 2 Hours, and IA – 3 Hours). Not all parts of type I and type II construction are required to be non-combustible and section 603 outlines 26 locations where combustible materials can be used which addresses things like millwork including doors, windows and frames as well as interior wall, ceiling and floor finishes. (PS – It includes nailers and blocking as well)