If you're going to use a free-standing ladder to get on the roof, it needs to extend beyond the edge of it. Since your rig is approximately 12 feet high, you'll need a ladder at least 17 feet long (working length) to safely get up there and have it set at a comfortable climbing angle (about 75 degrees). You need to have beam and rungs above the edge so you can safely get on and off of the roof from it. You also need to have it footed at the base (secured), so it doesn't slip out from under you. To use a ladder that just reaches the edge of the roof is courting disaster.
If buying an extension ladder, be sure to consider how much you weigh when selecting it. A Type 1A ladder, rated for 300 lbs., will handle most people that have any business on a ladder. I carry an Xtend & Climb, 12.5 foot, Type 1A ladder for working on the cap and sides, but still use the rear, fixed ladder for accessing the roof. Fortunately, my weight is only slightly more than half of the load rating.
