How to Grout Outdoor Flagstone

Grouting flagstone, those wide, flat, uneven stones used on patios, is the same general idea as grouting tile in your house. The major difference is that the lines between the flagstones are wider than they are on files, and the flagstones themselves are rough. This means you can't spread grout over the whole surface with a rubber trowel as you would on regular tile. A better method for grouting is to use a grout bag, which is a similar principle to a pastry bag, targeting the grout.

Powdered grout mix (modified with latex for outdoor use)
Bucket
Drill
Mixing bar for the drill
Wood spoon
Grout bag
Damp sponge

Instructions

- 1
  Combine powdered grout with water in a bucket, using the ratio listed on your grout packaging. Mix it well with a drill and mixing bar. The mixed grout should have the consistency of thick mud. Don’t make more than will fit in the grout bag (usually about half a gallon).
- 2
  Let the grout sit in the bucket for 10 minutes to “slake” (meld). Re-stir it with the drill and mixing bar.
- 3
  Put the grout into the grout bag, pouring it in the open back end of the bag, using the wood spoon to guide it in. Close the bag.
- 4
  Set the tip of the grout bag in the space between the flagstones, starting at one end of the surface. Squeeze the bag to push the grout into the line.
- 5
  Pull the bag backward along the line, slowly, while continuing to squeeze it, filling the space with grout. Fill all the lines, keeping the grout in the spaces and off the surface of the flagstone. If it gets on the flagstone, use a damp sponge to wipe it off.
- 6
  Fill all the lines you can with the grout in the bag. Press the flat back side of your wood spoon along the wet grout lines to get them flat and even with the flagstone.
- 7
  Mix more grout to refill the bag. Continue grouting and flattening the lines until they’re all done. Let it set for 24 hours before walking on it.

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