I need to rebuild porch stairs onto a cement patio. The height from patio to porch entry is 23". I would like them to run the width of the porch (15 ft.) and the tread to be 18". I have 4 questions for my fellow diys - 1. should I use stringers or a box? 2. How do I calculate stringers for an 18" tread (the patio has a slight pitch for water run-off)? 3. I'd like to use TimberTech - wd you recommend? This is my first time building stairs, so go easy on me. I've got small kids and need stairs now. Carpenter can't come until end of July. Help!