What are the different construction types in the IBC?
Types of Construction in the IBC
The International Building Code (IBC) requires that all buildings be classified as one of five different construction types, which are described in Section 602. Types I, II, III and V all have subcategories A and B and starting in the 2021 edition of the IBC, type IV construction has types a A, B, C and heavy timber.
Each construction type has two primary criteria: allowable building materials and the required fire-resistance rating of building elements.
We’ll start with Type I and work our way through:
Type I construction requires that all building elements described in IBC Table 601, which includes the primary structural frame, interior and exterior walls, floors and roofs, consist of noncombustible materials. Conceptually, the easiest way to think about noncombustible materials is to ask “will this material burn?” Technically, noncombustible materials are those that have passed the test criteria of ASTM E136.
IBC Section 603 does list 26 applications where you are permitted to have combustible materials in Type I or II construction. Common examples from this list are fire-retardant-treated wood, combustible insulation, roof coverings and flooring.
So, in any Type I building, the materials must either be noncombustible or covered in the Section 603 list.
Type I construction is divided into two subcategories, Type I-A and I-B.
The only difference between these is the required fire resistance ratings, which are found in IBC Table 601.
Type I-A requires the primary structural frame and bearing walls to have a 3-hour fire resistance rating, whereas Type I-B only requires 2 hours. Type I-A construction also requires a 1.5-hour fire resistance rating for the roof, whereas Type I-B only requires 1 hour. There are a number of footnotes to IBC Table 601, which allow for a possible reduction in the fire resistance ratings in certain situations, so be sure to review these for your specific building.
Type II construction has the same material constraints as Type I construction – the building materials must either be noncombustible or covered by the list in IBC Section 603.
Similarly, Type II construction is also divided into two subcategories, with the only difference being the fire resistance ratings. Type II-A construction requires a 1-hour fire resistance rating for the primary structure, bearing walls, floors and roofs. Type II-B construction does have any required fire resistance ratings for structural elements.
Type III construction gets a little more nuanced – the exterior walls are required to be noncombustible, the same as Types I and II, except that fire retardant wood framing and sheathing is permitted when the wall has a 2-hour rating or less. The interior elements, however, can be any material permitted by the code. That means wood, even untreated wood, can be used in the interior of Type III buildings.
Type III construction is also divided into two subcategories, with the only difference again being the fire resistance ratings. Both types require exterior bearing walls to be 2-hour rated, but Type III-A construction requires a 1-hour fire resistance rating for the primary structural frame, interior bearing walls, floors and roofs. Type III-B construction does not have any required fire resistance ratings other than exterior bearing walls.
Type IV construction allows for mass timber or any noncombustible material. Mass timber includes solid, built-up, panelized or engineered wood products that meet certain minimum dimensional requirements found in IBC 602.4. Examples of mass timber include solid wood, Cross-Laminated Timber (CLT), Glue-Laminated Timber (GlueLam), Nail-Laminated Timber (NLT) or Structural Composite Lumber (SCL).
Starting in the 2021 IBC, Type IV construction was expanded to include Types IV-A, IV-B, and IV-C. These new subcategories have a range of required ratings.
In Type IV-HT, which is a carryover from previous code editions, the primary structural frame does not actually have a fire resistance rating requirement, rather there are specific dimensional requirements that must be satisfied in the timber members. The required fire resistance rating increase in Types IV-C and IV-B, all the way up to Type IV-A, which requires a 3-hour fire resistance rated for the primary structure and bearing walls.
Types IV-A through IV-C Construction have a unique requirement for noncombustible protection on some or all of the structural elements. This protection, typically Type X gypsum wall board, is required to cover a certain portion of the interior elements, exterior walls, floors and shafts. Again, the required protection is minimal in Type IV-C and fairly extensive in Type IV-A.
Finally, Type V construction is allowed to contain any material permitted in the IBC. This means 100% of a Type V building can be light frame wood construction. Similar to Types I-III, Type V construction is divided into two subcategories with different fire resistance ratings. In Type V-A construction, the primary structure, bearing walls, floors and roof all require a 1-hour fire resistance rating. In Type V-B construction, there is no required fire resistance rating.
In summary, the IBC requires that all building be classified as one of these construction types. Determining construction type is one of the key early decisions in the design of any building. The choice of construction type determines: what materials you can use in a building, the required fire resistance rating of building elements, and a host of other code requirements, such as allowable height, area and number of stories.
I'm Chris Campbell, this is MeyerFire University.
Sentry Page Protection