I have a walkway to my door that is cedar and hemlock mulch, and a downspout from the roof where water runs through it. I'd like to keep the area low maintenance and good looking and just bought 900 lbs of red and green grey flagstone to lay over it. Because some of the area in that part of the house without any sun will remain in mulch, I am wondering if I need to prep the area to be laid with gravel and sand, or if I can just put it right over the mulch. Thanks,