Remove the screws holding the existing doorknob in your door. The screws usually are located on the cover plate behind the handle. Some doorknobs have hidden screws. To access hidden screws, insert a narrow-blade flathead screwdriver into the small slot located on the doorknob shaft between the handle and the plate.
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Next, determine which type of door knobs, handlesets or levers you'll need throughout your home. Privacy lock: Push-button locks, ideal for bedroom and bathroom doors. Non-locking: For use in hall and closet doors. Non-turning: For use as pull handles, typically matching the trim on french or pantry doors.
http://www.schlage.com/en/home/support/understand-product-options/knobs-handletsets-levers.html
Stand outside the room for your door. For example, if you are installing a bath or bedroom door lever, stand outside the room with the door shut. Look for the door hinges, and select a lever in which the handle points toward the hinges. If the hinges are on your right, you need a right-handed lever.
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Lever Handles. Lever handles for doors are available in many different styles and finishes. To find the perfect lever for your home you can filter your results by brand, finish or style on the left. Because of varying design styles, door levers are often handed, meaning we need to know which way our door swings in order for the lever to curve up or down in the right direction on the door.
http://www.directdoorhardware.com/Lever_Handles.htm
Some door hardware may require handing to fit your installation needs. You may see right, left or reversible in the handing field. To determine what handing you need begin by standing at the exterior of the door (for example: if this is a bathroom door, stand outside the bathroom). Locate your hinges. If your hinges are on the right, you need right handed hardware. If your hinges are on the left, you need left handed hardware.
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Next, determine which type of door knobs, handlesets or levers you'll need throughout your home. Here are simple definitions to get you on the right track: Privacy lock: Push-button locks, ideal for bedroom and bathroom doors. Non-locking: For use in hall and closet doors.
http://www.schlage.com/en/home/support/understand-product-options/knobs-handletsets-levers.html
Or you can use the standard height for a handle, which is 1050mm off the floor. After deciding the height of your door handle, measure and mark it on the side of the door. Before you start installing the door handle, place a door wedge under the door to stop it from moving while you’re working. Move the wedge as you change the side of the door you’re working on.
https://www.bunnings.com.au/diy-advice/home-improvement/doors/how-to-install-a-door-handle
The advantage of a door lever over a knob is due to its ease of use. Children, disabled and elderly find levers much easier to open a door. Door knobs can take a firm grip to open a door, while a door lever opens with a simple pull down. Many door levers are ADA compliant and the grip can easily be used by children.
https://www.handlesets.com/door-levers/c3860
Concealed Screws. On many knobs and Levers, the screws that mount the hardware on the door are visible on the interior. Concealed Screws means that these screws are hidden behind the rosette, providing a cleaner more decorative look to your door hardware.
https://www.handlesets.com/door-levers/c3860
Step 1. Measure the height for the door handle There are a number of ways to work out the height for your door handle. You can measure the height of the handles on the other doors in the house and use that. Alternatively, you can read the instructions that come with the door handle kit and use that height.
https://www.bunnings.com.au/diy-advice/home-improvement/doors/how-to-install-a-door-handle
correct way to install lever door handles
The correct way to install a lever door handle is stand outside the room with the door shut, look for the door hinges, and select a lever in which the handle points toward the hinges, and if the hinges are on your right, you need a right handed lever.