I'm replacing the access door to my garage (not the garage door) with a pre-framed metal door from Home Depot. The new door comes complete with a threshold. My existing garage door frame has no threshold and there is no sill plate under the door; the door sweeps just above the concrete pad. Temperatures here range from a low of -40 degrees to a high of about 95 degrees Fahrenheit. There's not a lot of snow, but the drifts sometimes gather in front of the garage (2 feet max). The slope is OK for rain. I can either remove the pre-built threshold from my new door and install like the existing door, or add a sill plate for the threshold to rest on. If I remove the threshold, will I weaken the structure? If I add a sill plate, how do I fasten it to the cement pad and how do I keep moisture (particularly ice) from infiltrating? Thanks.