How to secure slate hearths

Written by kelly nuttall
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How to secure slate hearths
Secure slate hearths with latex modified thinset. (braises image by philippev from

When securing slate to hearths or hearth pads, you will need to use a product that is nonflammable and will not melt from the heat produced by the fireplace. The best product to use is called thinset---a common adhesive used to complete tiling projects. Different grades of thinset are available, along with different formulations that are meant for specific applications. As long as you use a latex modified thinset to secure your slate hearth, you will be using a product that will last for a long time.

Skill level:

Things you need

  • Level
  • Self-levelling compound
  • Floor patch and skim coat
  • Latex modified thinset
  • 25 litres (5 gallon) bucket
  • Thinset mixer
  • Electric drill
  • 6 by 9 by 6 mm (1/4 by 3/8 by 1/4 inch) thinset trowel
  • Tile spacers
  • Grout
  • Grout float

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  1. 1

    Place a level on the hearth area to be sure that the subsurface is level. If it is not level, use a cement-based, self-levelling compound to level it out. If the subsurface is level but there are holes and pits that need to be filled, use a floor patch and skim coat to fill the holes. Spread the floor patching compound with the straight edge of a thinset trowel. In either case, allow the compound to dry before proceeding.

  2. 2

    Fill a 25 litre (5 gallon) bucket with latex modified thinset. Add water and mix the thinset with a thinset beater and electric drill until it reaches the consistency of peanut butter. Let the thinset sit in the bucket for five to 10 minutes. Mix it again and it is ready to use.

  3. 3

    Use a margin trowel to take the mixed thinset out of the bucket and place it directly on the hearth area. Spread the thinset with the notched side of the thinset trowel. A 6 by 9 by 6 mm (1/4 by 3/8 by 1/4 inch) thinset trowel should be used when installing 30 by 30 cm (12 by 12 inch) tile. If you are installing a 40 by 40 cm (16 by 16 inch) tile, you have to use a 30 by 30 by 30 cm (1/2 by 1/2 by 1/2 inch) thinset trowel in order to apply enough thinset underneath a larger tile. Hold the trowel at a 45-degree angle to the floor when combing through the thinset.

  4. 4

    Set the tile directly into the wet thinset. Press the tile into the thinset to ensure the full back side of the tile is covered with thinset. Insert tile spacers in between the tiles to ensure that the grout line remains straight and consistent in width. Allow the thinset to dry and cure for at least 24 hours.

  5. 5

    Mix a batch of grout in a colour that matches, or compliments, the slate tile. Pour the grout into a plastic bucket, add water and mix the thinset, adding enough water to make the grout the consistency of peanut butter. Dump the grout directly onto the tile and spread the grout with a grout float. Force the grout into the grout lines to avoid leaving any holes or air bubbles in the grout lines. Allow the grout to sit for 15 to 20 minutes. Wipe the excess grout off of each tile with a damp sponge. Allow the grout to cure for 72 hours before building a fire in the hearth.

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