Installing a window box on brick or stone is a little different than installing one on siding. Typically you do not have to worry about finding studs so that sometimes can make the job a little easier. The tricky part however is that you will need a drill with a hammer action on it, sometimes called a hammer drill. A hammer drill is a drill that not only spins, but pulses front to back to create a hammer action that helps break up the brick. Special drill bits called masonry drill bits are used since they have a chisel point at the end that assists in breaking up the brick.
If you are on brick the method is more straight forward than if you are on stone. Some stone is fragile and can't be drilled into and also a consideration about stone is that there may be some unevenness in the stone surface where some stone sticks out farther than others. If you are unsure if it is safe to drill into your stone I highly recommend consulting a local handyman first. Now let's begin.
Method 1: Installing a Window Box on Brick without the use of Support Brackets Underneath
For installing a window box on brick or stone, you will need a hammer drill, 5/8" masonry drill bit, a level, a ratchet wrench, some 3/8 lag bolts, some 3/8" lag shield anchors, 5/8" paddle bit for woodworking, and some silicone with a caulking gun.
If you are installing a window box made from wood or hollow plastic, then you will not want to use this method. This method is only for composite PVC window boxes that can be drilled directly through the back board. A direct mount is secure and does not require a support bracket underneath. Instead these types of boxes usually use a fake bracket underneath, sometimes called a faux bracket and they are usually decorative only. To mount a box like this it must be made of solid material that can be safely drilled into. Fiberglass usually does not fall into this category nor does hollow plastics, but solid PVCs do.
Figure out how many lag screws are needed to mount the box. A good rule of thumb is one lag screw every 24" length of box. In this example we will use a 6 foot long box. We recommend you use one lag screw on the ends then two more lag screws at the 24" and 48" mark for a rough total of 4 lag screws. Rather than putting the lag screws at the very end of the box they are usually installed 3-5 inches from the end of the box.
Begin by marking a horizontal line across the box that is halfway from the top and halfway from the bottom of the box. Mark a spot to drill at 4", 28" from the left end and 4" and 28" from the right end. Use a 5/8" drill bit to drill a hole at these 4 spots roughly the size of a nickel. Next, mark the center of the box on the back/top section and mark the center of the window. Hold the box up beneath the window sill so the centers align. Now, trace with a pencil 4 circles onto your brick through each of the 5/8" holes you drilled through the back of the box. You can now set the box down and pull out the hammer drill.
Using a 5/8" masonry drill bit drill a hole through the brick that is 3" deep in each of the 4 marked spots. Place a 5/8" lag anchor that is 2" long in the hole and tap it in with a hammer so it is snug, but not too tight. Make sure the opening where the lag screw enters is correctly facing outward. Once all four lag anchors are tapped into the holes in the brick you have your 4 anchor points. Now hold the box up and pass the 4 lag bolts through the holes in the back of the box and into the lag shield anchors. Tighten each one 90% with a ratchet wrench. Once all four are tightened, then place a level on the box and finish tightening 100%. Since the hole in the back of the boxes was 5/8" it will be slightly larger than the lag bolts passing through which are 3/8". This gives you slight wiggle room to level the box.
Once the box is fully leveled, you can now silicone around the edges with clear silicone to make it water tight. You are now finished installing the box, happy planting!
Method 2: Using a Support Bracket to Install a Window Box on Brick or Stone
If you are using a support bracket then many of the principles from method 1 will be reused. I highly recommend reading method 1 first, then coming to method 2. Now let's begin.
Determine how many support brackets will be needed. For this you will need to purchase support brackets and get a load bearing capacity for the weight from the manufacturer. Usually a support bracket is either wood or a metal bracket bent 90 degrees. For a 60" long box weighing under 100 pounds when filled with dirt usually 3 support brackets is common. First measure the height of the box. You will want to make sure the top of the bracket is at least this distance lower than the bottom of the window sill so that you have adequate space. Adding an extra inch of cushion is common to make sure you have clearance.
Next, mark the center of the window sill with a pencil and also mark the center of the window box. For a 60" box measure and make additional marks on the window sill approximately 24" to the left and the right of the center mark. This will be where your brackets need to be. Depending on your bracket and who manufactured it you should follow their guidelines for installing them. However, since this is on brick you will likely be using some form of a hammer drill and anchor system so that you can fix the bracket to the brick really strong.
Most support brackets mount to the brick or stone and the box is simply placed on top of that and can be removed for the winter. It would be advised in this case to mount the box on top of the brackets at least 1" away from the home. When you are directly mounting it you can silicone around the box to make it water tight but when you are using a support bracket with intent to take the box off and on, then leaving it 1" away from the house will help with drainage and keeping water from getting trapped behind the box.
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