DIY: Installing a Window Box on Vinyl or Wood Siding

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 To install a window box on siding is a moderate task that many DIY enthusiasts can often accomplish. The hardest part is usually finding the studs to anchor the box or brackets, but other than that it can be done with a fairly basic set of tools. There are a few ways to attach a window box to siding. Some involve screwing brackets into the siding and setting the box atop of the brackets. Another way that is common is to anchor or screw the box directly to the house for a more permanent solution.  Let’s first start with installing the boxes using a bracket system. If you are using a window box made out of wood, fiberglass, or hollow plastic then you will likely need to take your window box down for the winter so using a bracket system may come in handy. 

If you are using a window box made out of composite or solid architectural PVC then you can likely leave your window boxes hung year round and decorate them in the winter.  This means you can mount them with a variety of methods without fear that the winter freeze will crack or destroy the boxes. A common technique for no rot composites is to simply bolt the box directly to the house since it can become a part of the house and be properly sealed to be water tight. 

Installing Window Boxes Using Brackets 

This technique is one of the older techniques and when the flower box is installed during home construction often works well. The main consideration with using brackets to support the window box is making sure that you have symmetrically placed studs underneath the window, otherwise you can end up with the brackets underneath not looking correct or not being attached securely. If the home builder took this into consideration then your studs may already be strategically placed, otherwise the studs could be 16” off center and that may or may not line up symmetrically with your window box and where you are wanting to put the brackets. One good tip to combat this is to utilize the studs that frame the window out. In almost all situations a window is framed out with studs on either end that run all the way to the floor. 

This means you typically have two known studs which are symmetrically placed and are there 99.9% of the time. For boxes that only require two brackets this technique can often work without too many issues. You will, however, want to make sure your window box is longer than the window is wide by about 12”. This allows the box to overhang the window a good 6” on either side so that the brackets will fit comfortably underneath. Just remember, if the window is longer and the box requires a third or even fourth bracket then there is a chance that there may not be a third or fourth stud directly in the middle, or in the case of four brackets placed evenly.  

To mount the brackets you will want to screw them to the house making sure that you drill through the brackets and into the studs with sufficient sized screws or lag screws considering the weight. It’s important to put a level across both brackets to make sure the box sits properly and doesn’t have drainage issues. In some instances the box can merely sit atop the brackets, but for liability reasons screwing through the bottom of the box into the top of the bracket is advisable because there are circumstances where heavy winds can knock the box off the brackets causing it to fall and break or even hurt someone. If the box is planned to be taken down during the winter then it is recommended that it sit away from the wall by about 1”. This will keep water from getting behind the box and getting trapped between the siding where it can potentially be an issue. If the boxes are going to be left up permanently then they can sit flush to the siding and you can silicone all the way around the box to make it water tight. 

Installing the Window Boxes without Using Brackets 

So in many instances there may not be enough room under the window for brackets. For example, sometimes a window rides 8-10” above a roof line leaving very little space to put a window box. There is a good solution to this which is to omit using a bracket underneath altogether. For this, you will simply bolt from the inside and out the back of the box and into the studs behind the siding. By directly bolting the box itself it does make it a little more permanent, so I recommend getting a high quality window box made from architectural grade cellular PVC. 

It won’t rot, it’s waterproof, and it can be left out in the winter so it’s freeze resistant. Another advantage to this installation method is that you don’t have to worry about stud placement at all as the fasteners are hidden inside the box and buried in soil. Many companies offer “faux” (as in decorative only) brackets which merely glue to the bottom of the box and give the illusion of support. Since they don’t have to screw into the house, but can sit flush to the house, it is ok to space them out however you want since they don’t have to align with the studs. This gives you greater flexibility in customizing your look.  

To install a box using this method you will first want to find the studs which is often easier to do from inside the house using a stud finder over the drywall. Next, transfer the marks to the window sill and go outside. Mark the center of the window and the window box and line the two up. Now you can transfer the marks from the window sill to the back of the box. From here this tells you where to bolt through the box so that you are hitting the studs. I recommend drilling a hole out of the back of the box slightly larger than the lag screws you use for attachment because this gives you wiggle room to level the box. I recommend quantity 4 of 3/8” diameter, 3.5” long lag screws for boxes weighing around 125 pounds. Use an oversized fender washer larger than the hole you drilled in the back of the box with your lag screws to help suck the box up to the house tight. Also predrill with a ¼”drill bit into the siding where the lags will go to make tightening the screws easier.  

If in doubt always contact a professional. A handyman or contractor is usually knowledgeable and can assess your individual situation for any necessary modifications.  

About the Author

Matthew Buquoi is the owner of Flower Window Boxes, an online window box company that makes PVC window boxes and ships them all over the country