Butter mortar is mortar used between stones or bricks. To butter is to apply the mortar between stones or bricks. When putting down outside tiling or flagstones, a two step process is often recommended by contractors.
Butter Between the Stones Before Topping With Dry Mix
In setting stones such as flagstones or paving stones, dry brushing as a top coat is often employed. Dry brushing is done with a dry mix of concrete spread into the cracks between stones.
A butter joining is achieved if dry brushing is combined with butter mortar. The butter mortar is merely wet mortar. When you butter, the mortar is set in the spaces between the stones, at a level lower than the surface. The dry mix is then spread on top of the butter mortar.
There is enough water in the butter mortar to facilitate hardening of the dry mix above it. This method, dry mix and butter, forms the best seal. It is also good if the dry mix is pressed down onto the butter, with a trowel or other tool.
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