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Window Rough Openings: A Complete Guide

August 23, 2024

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It's a great feeling to install a window. If you're a DIYer, performing your own window installation can help you save money while at the same time giving you the satisfaction of a job well-done.

Before you can install a window for your home improvement project, the first step is to measure the window rough openings. You'll need this information when filling out the order form for your window purchase.

When you work with Woodland Windows and Doors to perform your window installation, we'll measure the openings during your in-home consultation - no worries about measuring the rough openings for yourself. However, if you plan to install the windows yourself, this guide can help you measure the window rough openings and place your order.

What is a Window Rough Opening?

A rough window opening is the opening where a window frame is installed. The rough opening size information you need in order to buy a window replacement or a window for a new construction home.

When you're ordering a window, the window manufacturer or dealer will probably ask for two measurements: the rough opening size, and the net frame size. The net frame size is the size of the window to be ordered. The net frame size will be just slightly smaller than the size of the rough opening.

Understanding Rough Opening vs Actual Size

Compared to the actual size of the window, the rough opening will be approximately a half inch larger on the vertical and horizontal measurement.

For example, if you're ordering a double hung window for a rough opening that's 50 inches by 40 inches, then the actual window (net frame size) will be approximately 49-1/2 inches by 39-1/2 inches.

This leaves 1/4" of space on each side of the window when the window is installed in the center of the rough opening. Some manufacturers may recommend a different rough opening size depending on the window, so it's important to pay attention to rough opening recomendations every time you order a window, even if you've ordered windows before.

How to Measure a Window Rough Opening

Measuring a rough opening is easy. Use your measuring tape to measure the opening where the window will be installed from jamb to jamb, first measuring left to right (horizontal), and then measuring top to bottom (vertical). Measure the opening only - do not measure the trim around the rough opening.

Horizontal measurement: Take the measurement in three places along the horizontal axis - near the top, in the middle, then at the bottom. The smallest measurement recorded is the measurement you'll record on the spec sheet you'll fill out when ordering the window.

Vertical measurement: Take the measurement in three places along the vertical axis - first near the left-hand side of the window, then in the center, then on the right-hand side of the window. The smallest measurement recorded is the measurement you'll record on the spec sheet when ordering your window.

Window Rough Openings Measurement Tips

When you're measuring rough openings, accuracy matters. Mistakes can lead to problems during window installation or after. A poor fitting window can leak or experience wear and tear that leads to a shortened service life.

Gather your tools before beginning. You'll need a step stool, measuring tape, level, and you may need a partner.
Measure twice. Take every measurement twice to ensure you're correct.
Watch for measurements that seem incorrect. If a measurement stands out as incorrect, verify it.

Are Rough Openings Different for New Construction & Replacement Windows?

Measuring the rough opening for new construction window installation is not much different than measuring the rough opening in a pre-existing home. However, if you're planning to install a rough opening in a location where no window currently exists, then it's better to buy a window in your preferred size, then follow the manufacturer's instructions to create a rough opening in the proper size.

If the manufacturer recommends a rough opening that's 1/2" larger than the window itself, cut the rough opening to the size recommended by the manufacturer.

How to Prepare Rough Openings Before Window Installation

A close up of a window with a view of trees.

Preparing your rough opening before installation helps ensure a good fit when the window is finally in place.

Make Sure the Wall is Plumb & Level

Use a level, plumb level, or laser line to check the wall is plumb (straight up and down vertically). Use a level to check that your window is level (straight across horizontally). Make repairs if the window is not plumb or level.

Angle the Window Sill

Angle the bottom of the rough opening slightly downward, so that the window sill too will be angled slightly downward. This angle prevents water from collecting in the window when it rains. This protects your window from water damage and can prolong the life of your window.

Install a Back Dam

Install a back dam on the window sill to prevent water from entering the interior. The back dam is a strip of wood that's attached to the interior edge of the sill and covered with flashing tape. Alternatively, if the windows you're installing have jambs attached, you can create a back dam with a generous bead of sealant after attaching the windows.

Experience the Beauty of New Windows – Call Us Now!

If you're not familiar with the parts of a window and construction methods, installing a window can be intimidating. You don't have to install your own windows. When you buy windows from Woodland Windows and Doors, we'll install the windows for you. We install single and double hung windows, bay and bow windows, and a range of other window styles. We perform window replacement and window installation on new construction homes.

Have questions about window installation in the Chicago area? Contact Woodland Windows and Doors to learn more about window installation in your pre-existing or new construction home. Call today.