Measuring for a garage door is not as simple as measuring the rough opening width and height. Other measurements need to be accounted for to ensure the springs, track, hardware and garage door opener fit properly. Key spacing for headroom clearance, sideroom and backroom must also be factored into new garage door installations.

Failing to account for  headroom, sideroom or backroom requirements will cause you huge time and financial burdens. In the following article I will explain how to measure and plan for garage door installation, as well as measuring existing doors. This step by step guide will help you measure your garage door opening correctly the first time.


What size should my rough opening be?

As a general rule the rough opening should be the exact same size as the door you intend to install. A common single car door size of 9ft wide by 7ft tall should also have a rough opening 9ft wide by 7ft tall. When measuring garage doors the width is always given first followed by the height. So 9×7 implies a door that is 9ft wide and 7ft tall.

Some builders frame the garage door rough opening width down to 8’10” or 8’11” for a 9ft wide door so that the metal door end stiles rest up against the door jambs. This isn’t necessary, but some builders claim this has a few important benefits – reducing the amount of rattling in windy environments and providing a better seal in climates receiving heavy snowfall. It also hides the non-painted metallic colored end stiles of some doors from exposure in the absence of perimeter seal.

While door widths can be cheated short on rough openings the door height is almost always framed to match the exact rough opening height.  The door will naturally sit about 1/2″ off the floor due to the bottom aluminum retainer and seal. Installing a door taller than the rough opening height can also cause cause problems with headroom clearance requirements.

Chances are whoever constructed your rough opening did a decent job at keeping everything level and square, but in the event you are re-purposing an opening its important to take several width and height measurements.

Instead of measuring from the center of the header and jambs you might consider taking two width and height measurements from offset positions. This ensures that there aren’t any usual dips in the concrete or slopes in the header. This becomes increasingly important on taller doors on older buildings.

If you are measuring for an existing garage door to be replaced you should be measuring both the rough opening and the existing garage door panel configuration.  While a 7ft tall garage door has (4) 21″ sections, an 8ft garage door is made up of (2) 21″ sections and (3) 18″ sections. Some carriage house doors and commercial doors are (4) 24″ sections.

It is important to match the existing configuration, especially if multiple doors are installed on the same garage side by side. You want them to have identical panel configuration and layout.


What if some measurements aren’t level or square?

Its pretty common to have an unlevel floor with dips, crowns or slope. Garage doors are never installed to match the slope of the floor. Instead the bottom retainer and seal can be adjusted. Instead of a standard 2″ flat retainer and 3 3/4″ wide bottom seal adjustable retainers and wider seals are used.

Adjustable aluminum extrusion have 3″ side walls in the shape of the letter “U”. The go over the bottom section of the door to match the slope of the floor. Then a slightly wider bottom weatherstrip is installed in the retainer to help provide better seal.

All garage doors should have perimeter weather seal called vinyl door stop. This type of seal is nailed to both door jambs and header to seal the door and cover up any gaps. Matching color trim nails are installed 18 inches apart to secure the VDS in place.


How much space should be left for an opener?

Garage door openers with drawbar operation have rails that are about 2 1/2″ tall. If space is tight above the door this added space requirement will need to be accounted for. If you want to install an opener on an existing door you will need to about 3″ of additional headroom to fit the rail and mounting bracket above the torsion spring shaft.

Space for sidemount jackshaft openers is similar to standard openers. The casing for the opener sits above the torsion spring line. Because of this 4 inches of space above the torsion shaft is required.


1. Measuring The Width

Measure the width of the finished opening at the widest point. Take measurements from both ground level and eye level – in case the frame isn’t square. You will be measuring how wide the door is from one side of the opening to the other. To prevent splitting and unsafe conditions residential doors are best framed with 2″ x 6″ wood. Look for openings that are out of square and weak, rotting boards. Never install a garage door on an unsafe door frame.


2. Measuring The Height

Measure the height of the finished opening at its highest point. Be sure to look for irregularities in both the floor and header. An unlevel floor may require wider bottom seal on the door or the use of adjustable bottom retainer. If the concrete floor hasn’t been poured yet you will need to consult the builder to determine the thickness of the finished floor – subtracting that from your measurement.


3. Measuring Headroom

As a general rule a minimum of 18″ of headroom is recommended for all residential garage doors – if you have an extremely large commercial size door 24″ of headroom is best. This allows for pretty much any type opener, hardware, track combination to be installed. Sure you can get by with less headroom clearance, but this severely restricts you to certain door configurations.

Most people forget that headroom isn’t just a header to ceiling measurement – its a header to nearest obstruction measurement. While you might have 3 feet of available space above the door, if you have a pipe or duct work 10″ above the header in the travel of the door that presents a problem. You will either have to go over the obstruction with high lift track or under the obstruction by framing down the opening.

The easiest way of measuring headroom on residential doors is measuring from the header to the nearest obstruction. Given residential doors are under 9 feet in height you can easily use a standard tape measure and see obvious obstructions.

The second method is measuring from the floor to nearest obstruction and subtracting the door height. This method could be used on taller doors if you don’t have a ladder available. A laser tape comes in handy for commercial sizes openings that lack proper lighting as well.


4. Measure The Sideroom

The vertical tracks in the opening require space for the “L” shape brackets to fasten properly to the door jambs. Standard residential tracks are installed “leg out” where the jamb brackets extend outward toward the exterior wall and secured with lag bolts. On each side of the garage door a minimum of 4 inches of side room is required for residential 2″ vertical track. For commercial doors with standard lift 6 inches of space is required.

Sideroom is measured from the door jamb to the nearest obstruction, usually being a wall. In addition to the track brackets needing space to be fastened, the tube or shaft for the torsion springs also needs space. The typical shaft or tube on a 16ft wide garage door actually measures 16’10”. Of course the tube can be cut down on each side, but the minimum tube side is about 16’8″.

Standard drawbar openers with a rail and J arm system are almost always installed within the width of the door, but sidemounted openers have special sideroom requirements. The motor and casing for the Liftmaster 8500 jackshaft opener requires 8 inches of space to the nearest obstruction (sidewall).


5. Measure The Backroom

Also called “depth”,backroom is a measurement from the header to the nearest obstruction behind the travel of the door while overhead. Backroom is important as it ensures the door, opener and horizontal tracks will have enough space to be installed.

Backroom for Openers

  • On my standard 9×7 garage door the horizontal tracks measure about 18 inches longer than the door height of my door (8ft 6 inches)
  • My standard chain drive opener rail and motor extend 36″ past the door in the open position (130″ total from the header)
Measurement DescriptionDoor opening header to nearest obstruction (ceiling)Door jambs to nearest obstruction (side wall)Interior face of header to nearest obstruction (back wall)Rough opening inside of both door jambs.Floor to rough opening header.
Recommended Space (Door & Opener)24 inches12 inchesDoor Height + 48 inchesMinimum 7ft width
Maximum 30ft width
Minimum 6ft height
Maximum 18ft height
Minimum Space (Door & Opener)7 inches4 inchesDoor Height + 12 inches


Rollup Door Measurements

Unlike sectional garage doors that link together with hinges, rollup doors roll into a barrel and mount over the header.  The diameter of the barrel increases proportionately to the height, gauge and insulation inside the curtain. Measuring the height and width of a rollup door is much the same as the sectional door, but the headroom and sideroom requirements differ greatly.


Rollup Door Rough Opening

Measure the height and width of the opening at its highest and widest point. Look for irregularities like jambs that aren’t square.  Sectional doors can be 30 feet wide where as rollup (sheet) doors usually stop around 20 foot widths. To get a wider barrel style door you will need to go with a rolling steel door vs a sheet (rollup) door. Rolling steel doors are strong because they are composed of interlocking slats as opposed to one continuous sheet.


Rollup Door Headroom

If you intend to install a rollup door, but don’t know what model or features you want its best to leave yourself 24 inches of headroom. Minimum headroom space for barrel style doors is dependent on the thickness of the curtain and the height of the door. Taller openings require more headroom since as the door rolls up the barrel increases in diameter.

The same can be said for insulated rollup doors and doors with heavier gauge steel. Entry level mini storage models like the m650 only require 18″ of headroom while the model 2500 26 gauge steel door requires 23″.

If you plan on adding a hood cover or insulation you will need to add an additional 2 inches of space.


Rollup Door Sideroom

Sideroom requirements are higher for rollup doors vs sectional doors.  You will need at least 9 inches of sideroom space on each side of the rollup door. If you will be adding a side mounted opener you will need 24 inches of room on at least one side. Actual sideroom needed for an opener will depend on the model being installed.

Measuring Tips

Things to keep in mind when measuring.

  • If you are measuring a job with an unfinished floor be sure to consult the contractor or homeowner to determine how high poured concrete will be. If you have to install the garage door before the flooring is finished you will have to cheat your vertical track to compensate. Always compensate for un-level flooring low spots in the concrete where water may leak into the garage.
  • Always ensure you have an electric outlet within reach of an automatic opener. Most electric openers come with a 4-foot power cord. Running an extension cord is not recommended.
    Check all of the wood work in the garage. Ensure that the structure is sufficiently secure – taking note of split, warped or rotten boards. Again all wood work should be done with 2 x 6 boards.
  • Make sure vertical bucking extends 24 inches beyond the height of the door. End bearing plates must be fastened at the level of the spring line.
  • Make sure you have a spring pad for the center bearing plate and spring line to anchor into. This will be mounted in the headroom space in the center of the garage.
  • Take note of any impeding structure like duct work, pipes and electrical cords that may obstruct your garage door installation.
  • The most common garage door sizes are 8×7, 9×7 and 16×7. Never assume that a door is a common size – measure all doors yourself.