How to Install Rock Underpinning

Structure

• 1

  Using a plumb bob and chalk line, mark a line corresponding to the bottom of the home or mobile home wall on the ground. Place the plumb bob at each corner of the house or mobile home, and allow the bob to dangle 1/2-inch from the ground. Place a nail in the ground at the point where the bob stops rotating. Snap a chalk line onto the ground to connect the four corners, completing an outline of the structure.

• 2

  Install 2x4 posts 4 feet apart between the ground and the bottom of the house or mobile home. Cut the posts 6 inches longer than the measured height of the opening. Align the bottoms of the 2x4s to the inside of the line on the ground, insetting it another 3 inches to allow for plywood, mortar and rock. Tap the 2-by-4 into the ground and attach it to the structure above, insetting it 3 inches as well, using a 3-inch wood screws.

• 3

  Attach treated plywood to the 2x4 frame. Cut the plywood to fit, following the grade of the ground. Place the plywood horizontally along the 2x4s and attach it using 3-inch wood screws.

• 4

  Attach 30 lb. felt paper to the plywood. Attach the paper to the plywood by rolling it along the plywood and attaching it with a hammer stapler. Trim off the excess paper after it has been installed.

• 5

  Attach diamond mesh to the plywood. Cut the metal mesh with metal cutting scissors and attach it to the plywood, over the felt paper, using 1-inch wood screws. Space the screws 6 inches apart from bottom to top and left to right along the mesh. Overlap the mesh 6 inches at seams and at corners.

Rock

• 6

  Mix type S mortar and sand in a wheel barrow with a hoe. Add 1/2-bag mortar and 8 shovels of sand and combine. Add 3 gallons of water and mix until you have a pancake batter consistency.

• 7

  Place a 1/2-inch thick bed of mortar onto the wire mesh using a concrete hand trowel and mortar hawk. Load hawk with mortar from the wheelbarrow and place the board against the mesh beginning at the bottom. Push the mortar onto the wall with a hand trowel, distributing the mortar along the wire mesh in even strokes.

• 8

  Allow the mortar to dry for 24 hours. Attach the flagstone rock, which is found in 1-inch thick slabs, onto the dried mortar on the underpinning. Smear type S mortar onto the back of the rock and stick the rock to the wall as you would piece a puzzle together.

• 9

  Grout the rock joints with a grout bag. Place type S mortar into a grout bag, which resembles a cake decorating bag and squeeze the bag from the top until mortar is pushed out of the tip. Place the tip into the rock joints as the mortar squeezes out, filling the joints 1/8-inch past the top of the rock surface. Allow the mortar to set for 30 minutes, and cut the excess mortar smooth and even with the surface of the rock using a mortar trowel.