A raised threshold at a side door is a trip line that only shows up when someone is carrying something or walking in the dark. The install is simple when you run it in order. Skip a pass and you end up re-setting the ramp in the doorway with the door open and the cold coming in.
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Pass one: survey the sill
Measure the height of the step or threshold at the door, from the walking surface outside to the top of the sill or lip. Measure the width of the opening and the depth of the landing on each side. Note the door swing. An outward-swinging door can strike a ramp that sits too far out, and an inward door can catch on a ramp that sits too close to the sill. Check whether the surface is level and sound, and whether it is concrete, wood, or a vinyl or tile floor, because the mounting method depends on it.
Pass two: set the ramp
Choose a threshold ramp or bevel whose rated height covers the step you measured. Read the product's stated height range and the mounting method the maker specifies, whether adhesive, screws, or anchors. Clean and dry the surface. Mark the position with painter's tape, check it with the door closed, and only then fix it in place. If the step is taller than the product's rating, or the sill is out of level enough that the ramp rocks, stop. That is a contractor's job, and you should get the scope in writing before anyone starts.
Pass three: clear the door
With the ramp set, open and close the door slowly several times. The door should clear the ramp at every point of its swing and should not drag on the top edge. If the door rubs, the fix is the door or the hinges, not more ramp. Check the bottom gap with the door closed. A gap that catches a shoe toe or a wheel is a fail.
Pass four: test the step
Test in the dim light Dad actually uses. Walk the route slowly with a cane or rolling walker, then again carrying the bag or box you normally bring in. Press on the ramp edges with your foot to see if it lifts. Look for water pooling on the ramp and for edges that could catch a trip. Let Dad set the pace and let him try the step first. He knows how the door feels to him, and the test is wasted if he is only watching.
Stop conditions
Stop and reset if the ramp moves under weight, if the door strikes it at any point, if an edge catches a foot, or if water sits on the surface after rain. A full flight of stairs at the door is not a ramp job. It belongs to a contractor who can say what the structure needs.
Next move
Tonight, measure the step height at the outside of the side door and the door clearance on both sides of the swing. Write those three numbers down, then match the ramp to the height before you order anything. If the numbers fit no product you can find, the next move is a contractor quote, not a stack of shims.