March, 2008
Inspection News and Views from the American Society of Home Inspectors

Code Corner: Clearance to Combustible Materials


This installment of code corner will examine clearance to combustible materials reductions per Chapter 13 of the 2006 International Residential Code.

A common issue found in both new construction and existing homes is fuel-burning appliances, vents or chimneys that are too close to combustible materials or assemblies. The code defines combustible materials as “any material not defined as non-combustible.” Materials that are considered non-combustible have to meet certain criteria when tested in accordance with ASTM E 136. As a home inspector, the important thing to know is most typical wall and ceiling assemblies (including tile and stone finishes set on conventional wood-framed walls) are considered combustible. Drywall is also considered a combustible material because of the paper facing.

Most fuel-burning appliances come with listed clearances to combustible materials stamped onto the unit. In these instances, it is easy to determine the required clearance to combustible assemblies, then to measure the actual distance. There are many cases where a fuel-burning appliance, vent or chimney will be located too close to a combustible surface, including walls and ceilings. This is where a reduction in required clearance comes into play. A protective assembly, often referred to as a “heat shield,” may be installed between the combustible assembly and the appliance, vent or chimney in question in order to reduce the required clearance. This is often a practical solution. The code outlines rules for using a protective assembly to reduce required clearances.

In no case can the distance between a solid fuel- (i.e. wood or coal) burning appliance and a combustible wall surface be reduced to less than 12 inches. Solid fuel-burning appliances can burn very hot, and 12 inches is a minimum distance in the code to a combustible wall assembly. Additionally, solid fuel-burning appliances that are listed and labeled as having a clearance to combustible assemblies of less than 12 inches to begin with cannot be reduced in clearance at all.

When a space is required between the combustible material and the protective material, the same space is required at all edges of the protective material. Adequate air circulation behind the protective assembly is critical to proper performance, since this air circulation is being relied upon to dissipate heat that might otherwise cause a fire.

A common misconception of homeowners is that the stone or tile backer around their solid-fuel burning appliance is automatically a protective assembly that allows them to reduce the required clearance. However, more often than not these non-combustible finishes are set right against the combustible wall framing, rendering them ineffective as protective assemblies. To be considered a protective wall assembly, the materials used as the protective surface and the backer, if applicable, must be non-combustible. Also, there is typically a one-inch clearance from the protective assembly to the wall surface. Any spacers used to maintain the proper clearances must also be non-combustible and must be arraigned so as not to hinder air circulation behind the protective assembly. Additionally, any protective assembly cannot be positioned closer than one inch to the appliance or chimney itself.

Reductions in required clearances cannot be used for appliances, vent connectors and single-wall metal pipes, masonry fireplaces or kitchen exhaust ducts.

Refer to the 2006 International Residential Code, Table M1308.6, for specifics on clearance reduction methods and construction.