How To Build A Window AC Support Frame (DIY)

Are you sure your window AC box unit is secure in its place? What if it’s heavy, clunky, or old?

As the hot days of summer approach, you could wake one day to a new breeze coming from the hole of your window – where that AC box unit once was.

But don’t worry, this article will teach you how to quickly set up an air conditioner brace for even the clunkiest of old units.

How To Build A Window AC Support Frame (DIY Installation Guide)

How to Build Window AC Support Frame

What You’ll Need

Here is your go-to list of items you need before starting. Write these down or take a photo if you find a trip to your local hardware store is needed:

  1. Adjustable wrench (for secure tightening)
  2. Level
  3. Pencil
  4. Power drill
  5. Measuring tape
  6. Air conditioner brace, or “bracket kit”

Larger stores also tend to have air conditioner braces on hand, making all this a straightforward Saturday morning errand.

Step By Step Guide

Once you have all your tools, follow these steps:

  1. Test the weight of the AC box unit by lifting one side at a time. Is it centered, or does it feel heavier on the right or left side?
  2. Measure the width of the exterior window sill. This is usually the slightly raised, jutting part of the window frame on the outside of your window.
  3. Find the exact halfway point and mark it with a pencil.
    • If the AC had a balanced weight, use the middle pencil mark.
    • If the AC felt heavier on one side, make a second pencil mark three inches to the left if it was heavier there or to the right if it was that side.
  1. Find the concave mounting block which came with the air conditioner brace kit. Then, assuming the AC box unit had a balanced weight, line up the inner circle with the first and only pencil mark.
  2. Following the mounting block, predrill a hole into the exterior window sill.
  3. Now use the correct screw (usually provided in the kit) to attach the concave mounting block.
  4. Take your second curved block from the bracket kit and place it on the concave block. You want to line the top of the blocks to the height of the interior window sill or the horizontal section receiving the window when you close it.
  5. Bracket kits come with a bracket bar – the piece of hardware which holds the AC box unit. At the window end of this piece, notice the holes lining up with the support blocks.
    • Attach this upper bracket bar to the mounting blocks, but do not tighten the screws completely.
    • Take the lower mounting bracket and attach it to the top bracket bar with the provided bolts.
      • The lower mounting bracket is the part with the “feet” designed to rest against the exterior portion of the wall beneath the window.
      • Align these feet with the wall such that the upper bracket bar forms an approximately 90-degree angle.
  1. Finally, adjust both brackets using a level (sometimes provided in the kit) on the top bracket. Find the exact middle point for the bubble and tilt the AC support brace slightly down to allow the condensation to trickle outside.
  2. Tighten all your screws and bolts, and then mount your AC box unit according to its manufacturer’s instructions.

One important safety measure to remember: always opt for safety glasses when handling a power drill. Specks of wood or cement can fly rapidly towards you. Not to worry, however, this is an easy job even for a total DIY beginner!

Steps to Build Window AC Support Fraome

What Happens If The Window AC Has No Support Bracket?

There are three critical matters to keep in mind with your AC box:

  1. The risks.
    A poorly installed AC unit without an air conditioner brace can fall at any time, particularly during a rainstorm. Potential damage includes your: window, wall, and another person passing by.
  2. The AC window unit itself.
    An unsafe or misaligned unit will not function optimally. If the downward tilt to the outside isn’t ensured, you could experience some leaking from the unit.
  3. Yourself, the homeowner.
    Why expose yourself to risks of damage or legal liability when an inexpensive air conditioner brace gets the job done?

Related Article Should A Window AC Be Tilted?

Is A Support Frame For A Window AC Absolutely Necessary?

It isn’t necessary for all cases, but it is for most. We’ll explain with two key points:

  • The chances of a falling AC box unit are small but can lead to significant liability. If you have a strong window frame and live on the ground floor, your need is much smaller than the fourth floor of a New York high rise.
  • In addition to removing virtually all risks, an air conditioner brace ensures longevity for both the AC box and your window.

Some types of AC, like over the sill window air conditioners, do not require brackets. Instead, these units will use the window and wall frame as their support.

Related Article Can Rain Damage A Window AC?

People Also Ask (FAQ)

Here are some basic window AC support brace questions.

How much does an air conditioner brace cost?

Solid and reliable units usually cost from $20 to $60 at any major department or hardware store.

How long does it take to install?

Not long at all. We estimate a total beginner could install it in 30 minutes or less.

Can you install one without drilling into the window sill?

Yes, there are units for sale that use the support of the vertical pillars of the window frame itself. However, these tend not to be as sturdy as screw-based AC braces.

How much weight can an AC brace hold?

This depends on the brace, but the production description often mentions the maximum bearable weight.


We always recommend an air conditioner brace for window AC boxes, without exception. It’s easy to install even for a total beginner and could even be your gateway into DIY!

Sturdier window frames and private homes do have less need for a brace, but if you’ve got your hands full, no problem–many reputable handypersons accept private jobs for a modest fee. We’d rather you skip on the DIY than the air conditioner brace.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.