How to Install Stucco Trim

Stucco trim is a great low-maintenance product that is perfect for the do-it-yourselfer. It’s made from expanded polystyrene that is reinforced with fiberglass and coated with modified cement. It gives you the solid look of cast stone or concrete in a lightweight, easily tooled form. When you use this material, you will no longer have to worry about wood rot or pest damage. Once it is installed and painted, you can count on years of beauty and added value to your home.

This kind of trim works well with many types of cladding: stucco, EIFS, brick, concrete block, and fiber cement. This article will first discuss how to prepare the trim for installation around a window. Next we’ll discuss how to install stucco trim on solid surface cladding like brick, block, EIFS, fiber cement and stucco. Once you know how to install a window frame, you can follow the same process for any other stucco trim—quoins, bands, cornices; the sky’s the limit.

  • Materials:
  • Dust Mask
  • Eye protection
  • Tape measure
  • Carpenter’s pencil
  • Paper towels, shop towels, or rags
  • Circular saw or Miter saw fitted with masonry blade
  • Speed square (especially if you are not using a miter saw)
  • Putty knife
  • 3/8” notched trowel
  • Cement Adhesive (Some applications require a non-cement adhesive as well)
  • Low-modulous CaulkMeasuring and cutting the trim.


  1. Measure the bottom edge of the window, including an allowance for the width of the side trim on both sides as well as an additional inch or two on each side. For example, if your window is 72 inches wide and you are using 6 inch wide trim on each side, you would add 72+12+2 for a total of 86 inches. If you order your sill with mitered ends, this is the measurement you give to the manufacturer. Ordering pre-mitered sills is a real time saver, but if you prefer to finish the ends yourself, a miter saw will be your best friend.
    1. Mark the 8 foot length of sill to your measurement.
    2. Cut each end of the sill from front to back inward at a 45 degree angle. You always want the saw blade to go through the cement coating first, not the foam core.
    3. Using the remaining bit of sill, mark a section that matches the width of your sill and cut at a 45 degree angle on one side. When placed against the angled cut of the sill, you should create a perfectly matched 90 degree corner. Repeat the process for the other corner. Remember that the second angled cut is changing directions, so think it through, visualize the final result, and double check what you’re doing before cutting. “Measure twice, cut once” is a cliché because it saves you time and money!
    4. Using a clean putty knife, gently scrape any loose foam off of each piece, and rasp the cement coating slightly.
    5. Mix a small amount of cement adhesive. Coat the angled sides of both the sill and the return pieces with your cement adhesive, and press them together firmly, smoothing away any excess adhesive along the joint.
    6. Allow this to cure while you cut the remaining trim for your window.
  2. Measure the height of your window and add the width of the trim. If your window is 65 inches high, and you are using 6 inch trim, your measurement would be 71 inches.
  3. Find the length of the top trim piece by using your width measurement plus the width of both pieces of side trim. (This measurement should be shorter than the sill measurement if you cut your sill to stand out an extra inch from the side trim.)
  4. Measure and mark the trim pieces. Cut the top of each side piece at a 45 degree angle from outside to inside. Cut the top piece so that the 45 degree angles match the side cuts. If you are using a hand-held circular saw instead of a miter saw, you can mark these angles with a speed square and use the edge of the speed square to keep your saw straight while cutting.

Attaching the trim (Stucco, EIFS, cement block, fiber cement):

  1. Mix your cement adhesive according to the manufacturer’s instructions. Wear a dust mask and eye protection while you do this. The dry mix can irritate your lungs and eyes.
  2. While it slakes, mark the center point of the sill and the center point of your window.
  3. Using your notched trowel, apply adhesive to the back of the sill from bottom to top, leaving a slightly thicker layer at the top of the sill. The notched trowel creates a ridged application of adhesive which increases suction and gives a better hold.
  4. Press the sill into place, aligning your center marks. Check the level of the sill from side to side and adjust as needed. Check the level of the sill from front to back at several points. You want it to be slightly off of level so that water does not pool around the window.
  5. Tap a few nails under the sill to hold it in place while the adhesive cures. You’ll be removing these and caulking the holes later, so they only need to be tapped in far enough to hold.
  6. Wipe away and smooth out any excess adhesive.
  7. Using your notched trowel, apply adhesive to the back of a piece of side trim. Put adhesive on the bottom and top edges as well. Set it into place and level. If you need to adjust the position of the side trim, use a block of wood and mallet. Do not strike the trim directly.
  8. Once it is level, press it firmly into place. Wipe off and smooth any excess adhesive. You can use a nail or two at the outside edge while it cures if needed.
  9. Repeat steps 7 and 8 for the other piece of side trim.
  10. Using the notched trowel, apply adhesive to the back of the top trim piece, as well as to the cut ends. Align the top piece with the sides, pressing firmly. Wipe away any excess adhesive, smoothing it between the joints at the top and sides.
  11. Allow the adhesive to cure for 24 hours. Remove all nails. Gently sand any rough edges.
  12. Caulk all edges and fill any nail holes to prevent water intrusion.
  13. Prime and paint the trim with masonry paint.