How to Design Exhaust Pipe Penetrations Through a Wall or Roof

Exhaust pipe thimbles are the cleanest way to handle exhaust pipe penetrations through walls or roofs. Thimbles are designed to isolate the hot exhaust pipe from the structure, and to allow for a better termination at these penetrations.

Important: Where the exhaust piping penetrates through a structure made of combustible materials, exhaust pipe thimbles are required by NFPA-37.

NFPA-37, Chapter 8

Exhaust pipe thimbles are available in carbon steel or stainless steel construction. The thimble consists of the main cylindrical body, and two flashings used to secure the thimble to the structure. The main body includes internal insulation and a ventilated air gap between the inner and outer shell. A rain guard is provided with roof thimbles to prevent water ingress. Rain guards may also be specified for use with wall thimbles.

The installation method should be as directed by the manufacturer, but generally consists of the following steps:

  • The opening in the wall or roof should be 1″ larger in diameter than the outside diameter of the thimble.
  • For combustible wall or roof installations, the thimble must extend at least 9” on each side of the wall or roof (per NFPA 37).
  • Roof thimbles are inserted through the roof, with the pre-assembled (pre-welded) metal flashing on the exterior, thereby using it to support the thimble’s weight. Install the second metal flashing on the interior of the roof (if needed).
  • Wall thimbles are generally furnished with both metal flashings shipped loose. Install the thimble through the wall, and support it so it does not rest on the structure. Once in place, attach the metal flashings to the wall’s interior and exterior surfaces, and caulk/seal around the flashings as needed.
  • The exhaust pipe should be supported so that it does not rest on the thimble’s interior surface. An equal gap should be kept between the exhaust pipe and the opening in the thimble.
  • If using a rain guard on the thimble’s exterior, slide it over the exhaust pipe and clamp it while leaving a 1/4″ gap between the rain guard and the thimble (do not obstruct the thimble’s ventilation holes).
  • The exhaust pipe should extend beyond the thimble assembly as shown in the picture above.

A final note… Even in seemingly “non-combustible” walls/roofs, it may be a good idea to provide a thimble to protect against burning of roof sealers, coatings, stucco, etc.

If your next project has special requirements pertaining to the engine exhaust system, whether due to strict noise regulations, long piping runs, or due to space constraints, I am ready to help. Contact me here to get your project moving forward!

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