How to Anchor a Sill Plate to a Cinder Block Foundation

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A sill plate is nothing more than the bottom of the stud-frame wall that you will be erecting on your cinder-block foundation. You'll need to anchor the plate down to the top of your foundation in order to safely secure the wall in place. Sill plates, although they are the bottom of the wall, are put on separately from the wall itself. It is not a complicated process, but there are a few things you need to take into consideration when anchoring a sill plate to a cinder-block foundation so you do not run into difficulties later.

Things You'll Need

  • Tape measure
  • Lumber crayon
  • Fill mortar
  • J-style anchor bolts
  • 2x8 pressure treated boards (sill plate)
  • Drill
  • Lock nuts for anchor bolt
  • Level
  • Shims (wood or metal)
  • Starting from one corner on your cinder-block foundation, before you pour the fill mortar into the last course (the mortar that fills the hollow blocks to make it solid), measure and mark the top of the block with the location of the vertical studs in your upper wall. Must studs are placed 18 inches apart but make sure you are including the placement of the double studs at your corners and openings in the wall.

  • Measure and mark the location of your anchor bolts for your sill plate. You will want an anchor bolt placed every 24 inches along the length of your wall, with an additional anchor at each corner and at the ends of each planned door opening. When you are marking where your anchor bolts will be placed, make sure that none of the bolts will fall within 4 inches of either side of the marks you made indicating where your vertical studs will be. You want to keep the anchor bolts far enough away from the studs so the bolt and nut of the anchors won't interfere with your ability to nail the bottom of the studs when you install the wall. You also want to make sure that each anchor bolt is marked to be placed in one of the cells of the cinder block (the cell is the open space in the block; most blocks have two).

  • Pour your fill mortar into your cinder-block foundation so each cell is completely filled. Insert a J-style anchor bolt into the fill mortar at each mark you made for an anchor. The curved part of the bolt goes into the mortar and the threaded straight end will stick out. Keep the bolts as straight as possible and make sure there is at least 3 inches of thread exposed (2 inches to pass through your 2x8 and 1 inch to thread the lock-nut on). Let the mortar cure and set.

  • Drill holes in your sill plate to match the spacing of your anchor bolts. Place the sill plate onto the anchor bolts so each bolt passes through the board. Hand spin the lock nuts on each end but do not tighten the nuts completely.

  • Use a level to check that your sill plate is level horizontally on the top of the cinder-block foundation. If it is not, use wood or metal shims to adjust the level of the sill plate. An uneven sill plate will make your wall uneven and will cause problems when it comes time to install your roof. When you are satisfied, tighten the lock nuts on the anchor bolts and you are ready to begin standing the stud frames of your wall.

Tips & Warnings

  • Lay your sill plate on the cinder block next to your anchor bolts and mark the board where you will need to drill the holes through the plate. Do this for the entire foundation. Then drill out the sill plate and then install the plate to the foundation. Doing this in organized steps with each step preparing the sill plate for the whole foundation (rather than just a section) will save you time be making you more efficient in preparing the sill and attaching it.
  • Never attach a sill plate to a hollow cinder block wall; the weight of the upper building that will be carried on the sill plate will crack the wall if it is not filled and properly reinforced.

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