Among the changes approved in the current code change cycle was G109-18, which will, for the first time, allow concealed spaces in traditional Type IV heavy timber buildings. The concealed spaces must be protected with one or more of the following three alternatives:
- the building is sprinklered throughout and sprinkler protection is provided in the concealed space,
- surfaces in the concealed space are fully sheathed with 5/8-inch Type X gypsum board, or
- the concealed space is filled completely with noncombustible insulation.
There is an exception for stud spaces in framed one-hour interior walls and partitions, which are permitted in Type IV-HT construction.
These same provisions have often been accepted by code officials through alternate methods as necessary protection for concealed spaces in traditional Type IV construction. The 2021 edition of the IBC will have a code path for acceptance without resorting to the alternative methods and materials section of the code. This should greatly help in the rehabilitation of many existing heavy timber buildings that are being put back into use as new occupancies and will allow new construction of Type IV-HT with concealed spaces that are commonly found in residential occupancy groups where ceilings may conceal overhead plumbing and mechanicals.