Details for the Top of a Masonry Heater

Chimney Connection

These photos show anchor plates connecting the metal chimney system to the masonry core. There is one layer of 1/4″ ceramic paper serving as a gasket. Hole locations were marked and 3/16″ holes pre-drilled only to the depth necessary. Four 1/4″x1 1/4″ TapCons with washers are hand-tightened. In general, good to be as gentle on tiles as possible.

The anchor plates shown were custom made by Olympia Chimney to fit to their double wall connector. The Class-A anchor plate is more widely available and will accept Class-A.


The ASTM 1602 Standard requires 2″ of insulation on top of the core. This is a practical perscription which helps to keep heat down in the body of a heater.

We typically build our facing at least 2″ above the core, let that set over night, and then make a mix of perlite and lime the next day which we pack and level into the basin created by the facing. We dry-mix the perlite and lime at a 4:1 ratio and then sprinkle water carefully to achieve a mix which is just uniformly wet enough to form and maintain a ball when packed in your hands.

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Home Forums Chimney Connection

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    Here’s a good place for discussion about chimney connections for masonry heaters.

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Expansion Joint Between Masonry Heater Core & Facing

It is important to create a continuous expansion joint between the core and facing of a masonry heater.  It is inevitable that the core will undergo significant thermal expansion and you want to isolate the resulting cracks in the masonry to the inside core. 

The materials we use for such expansion joints are a combination of 1/4″ ceramic paper surrounding all openings and then single-corrugated cardboard everywhere else.  The cardboard both gives the bond break we need to provide a vertical slip joint for the core as well as sufficient “sponge” to allow for 1/8″ horizontal expansion on all sides, while at the same time not creating too much of a thermal break which would cut conduction from the core to the facing.  Using 1/4″ ceramic paper around the openings ensures that this potentially combustible expansion joint is sealed from access to oxygen by a similar, yet more technically robust (and more expensive) material. In general, I make the ceramic paper seal 2 3/8″ thick which is conveniently the width of most levels.  It is good practice to use additional ceramic paper above the firebox opening where things are hottest to ensure that the facing is additionally protected there.

I line the cardboard flush to the top of the heater.  Once I have built my masonry up past the core, I line that with 1/4″ ceramic paper and then fill with my insulative mix of perlite and clay/lime/cement.  

I am picky with my cardboard… specifically picking large sheets of single-corrugated cardboard and cutting and taping at the corners rather than bending around corners so that I don’t get any annoying bowing in the field that is pushing out against my facing. Avoid using double corrugated cardboard and cardboard with glossy inked surfaces. I use cardboard that hasn’t been walked on or otherwise had its corrugation compressed. 

This technique works well although it can be time-consuming, taking up to a half day, to detail correctly.

For context, here are some different strategies out there include:

  • dry-jointing, cardboarding only corners
    • this seems mostly only do-able if you are building the facing first
  • doing a full wrap of 1/4″ ceramic paper
    • more expensive / less conductive.
  • using other materials such as fiberglass cloth
    • probably has the most promise
It is a subject of some debate whether bench flue runs need to have gasketing as it adds work and materials and may not be necessary. I have poured benches solid to flues before and not had a problem but also tend not . Putting a joint on the top of a bench is a good idea so that the bench cap detail is supported on the masonry facing and not the core.
Our client did the cardboard part of the expansion joint. It is best to use an uncolored paper masking tape.
Part of this core juts out. In this case, you want to make sure you have enough expansion room above the shelf created for the core to expand upward slightly and not support the masonry above it directly on the core using a lintel or arch.

Home Forums Expansion Joint Discussion

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    Here’s a good place to post questions specifically about expansion joints on masonry heaters.

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Lost Creek Masonry Heater Completed!

This masonry heater features a “white” oven, ample wood-storage options, and a heated bench. The heater is faced with Idaho Bitter Root Ledge Stone, Sage Ledge Stone and the slab details are Pennsylvania Blue Stone. It serves as a visual focal point, as furniture, and as a resilient energy source for the home!

Heated Bench along the whole back of the heater.
Lost Creek Masonry Heater with White Oven, Wood Storage, Heated Bench, Ledge Stone
Our new “Hybrid Core” Approach! Very happy!
Heated Bench Flue on the “back” of the Masonry Heater.
Built by Max Edleson and Jonathan Mears, members of the Firespeaking Team!

Outline for a Resource on Masonry Heaters

Here is an outline for a project about masonry heaters. Ideally it would be a publicly available web resource which was developed dynamically. Items would become links as they were written and discussion generated would be incorporated into the text. The big question is how to fund the project. Ideally, people would be moved to donate based on the utility they got from it. Comments are open below to provide feedback on the project. Feedback will fuel its creation! Also, join our mailing list to receive updates.


  • Why Fire? Why Firewood? Why Masonry Heaters?
  • Gentle Giants of Healing Warmth
  • 2 Kinds of Efficiency – Combustion & Heat Exchange
  • The Challenges Presented by Masonry Heaters
  • “The Cathedral and the Bazaar” Approach


Masonry Heater Architectural Design

  • Heart of the Home
  • Inspirations
  • Placement
  • Clearances to Combustibles

Masonry Heater Technical Design

  • Design Methodology
    • Concept
    • Layout
    • Sections
    • “Brick-by-brick” Design
  • Comparison of Design Philosophies
    • Russian
    • Contraflow
    • Five-Run
    • Bell
    • Rocket

Masonry Heater Construction

  • Comparison of Basic Approaches
    • Pre-Fabricated / Manufactured
    • Core Kits
    • Custom
    • Hybrid Steel & Masonry
  • Foundations
  • “Double-Skinned” Construction : Core & Facing
  • Chimneys
  • Masonry Heaters & Construction Codes
  • Budget

Resources & Next Steps

  • Workshops
  • Organizations

Plans for a Masonry Heater Core

The world needs more masonry heaters and masonry heater builders! A shortage of viable plans as well as education opportunities means there are very few masonry heater builders in North America. This design work reflects an effort to simplify things in order to make the process easier for everybody involved.

New approach to masonry heater building!

For the masonry heater we are currently working on, we took a new approach to the design and construction of the core by casting the most complex parts. The goal of this is to simplify both the design and construction process so that we can streamline the process for our own projects as well as produce a viable strategy and parts for other masonry heater builders.

Masonry Heater Bypass Damper Update

We believe that bypass dampers are an important part of masonry heater function. They provide an easy way to heat the main chimney in order to then pull heated gases through a much longer heat exchange pathway.

We have been working on our design and currently produce and ship on a per order basis. We are getting close to making them more widely available. Contact us if you are interested.

Masonry Heater Clearance to Combustibles

I was invited to examine a masonry heater that was built by another mason in a town one hour north of us. One of the wooden studs immediately behind the heater, in the wall adjoining their bedroom, had been heated to the point that it had at least reached a smoldering point inside the wall….