What Is An Expansion Gap? - How To Measure Your Hearth

Once you've decided on the material you want your hearth to be made from, you need to measure up your hearth so we know what dimensions to cut your hearth to. 

This is very straightforward - it's just a case of getting your tape measure out and measuring your fireplace recess, or any other space the burner will be stood on. 

However, there is one thing to bare in mind when measuring and it is something a lot of people forget about - expansion gaps. Without these, your hearth is liable to crack when it heats up, so it's important to be aware of what they are, and how to account for them in your measurements.

What Is An Expansion Gap?

All materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated and contract when they are cooled. For this reason, when installing a new fire hearth you need to allow for expansion gaps. Installing with the correct expansion gaps will prevent your hearth from cracking due to the expansion and contraction of the stone.

Thermal expansion and contraction vary dependant on material type. For example Slate, Limestone, Sandstone or Granite will all have different tolerances. Other dependants are water content within the material and the temperature the stone is subjected. Of these factors, the material type has the greatest influence on the expansion and contraction.

  • Black Limestone Hearths will expand the most, so a larger expansion gap is needed.
  • Slate Hearths will a expand a little less.
  • Granite Hearths will expand the least.

Expansion gaps are the most effective way to protect your hearth from cracking by providing space for the stone to expand into. Cracking occurs when the stone expands and pushes against a solid area/surface. The pressure of this resistance is what cause the stone to crack.

One of the most economical methods for making a contraction joint is by simply having the hearth cut into 2 pieces - one piece cut for the fireplace recess and the 2nd piece for the front lip of the fireplace. Image a 'T' shape with the vertical stem of the 'T' being a single piece and the horizontal top being a second piece.

If, aesthetically, you would prefer your piece to be cut into a single T-piece shape, then you need to account for expansion gaps around any edge that the hearth meets a solid wall. A suggested expansion gap would be a minimum 5mm - a larger gap will do no harm but anything above 10mm can begin looking like a bit of an eye sore.

Examples of Accounting For Your Expansion Gaps

If the width of your recess is 1000mm then the width of the stone should be cut to 990mm. This will allow an expansion gap of 5mm for each side.

If the depth of your recess is 500mm then the depth of the stone should be cut to 495mm. This will allow an expansion gap of 5mm for the back wall.

Examples of a gap that is too small:

This hearth may look like like an ordinarily finishes piece, but upon closer inspection, you can see just how tight the hearth edges are up against the wall.

The right and left edges that are in the fireplace recess are right up against the wall as the follow image shows:

The hearth is within 1 or 2mm from the wall and will have no space to expand into, resulting in a cracked piece. No good! It's important that you follow the guidelines of a minimum of 5mm from all surfaces that are going to be in contact with a non-flexible surface, or risk losing your hearth when it gets its first use.

Filling The Expansion Gap

We appreciate that a large gap in your fireplace is going to be an eyesore. It may not offend some people, in which case you are free to leave the hearth as it is. However, it is a simple fix to hide the gap so it's worth considering.

Once your hearth has been cut to size, it is important that if you are going to fill the expansion gap that you use the correct substance. A gap that is filled with a hard setting substance i.e. mortar or tile grout will make the effort of creating an expansion gap completely worthless. To allow the expansion gap or joint to serve its purpose it must be filled with a flexible filler often called either mastic, silicon or caulk. Flexible fillers can be found in a multitude of different colours and we suggest where possible to try to match the filler as close as possible to your stone colour.

Once you've accounted for the expansion gaps in your measurements, you're ready to make some of the final decisions!

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