Laying Down Natural Flagstone

Q. Building patio 16 feet x 20 feet with covered roof and need a procedure for laying down the natural flagstone. What is the best procedure?

A.There are a few options for flag patios. You can either set it on sand; limestone or trap rock screenings and leveling layer or set it in mortar. A lot will depend on if you expect the ground to heave during freeze or thaw and the look that you want.
You will also have to decide if you want to use rectangular, square flag pieces or random and which type of flagstone you want to use. The softer sandstone flag is the easiest to cut with a hammer and chisel, but be prepared for at least 20 percent will be wasted if you have not done this before. You could also use a wet saw or worm driven saw to cut the pieces to the appropriate size and shape. If you are going to be using granite or shale, you should consider using a saw for your cuts. They are too brittle to get an accurate cut with chisels unless very experienced.
First step in laying your patio is to excavate at least six inches and then compact the subsoil. Put in a leveling layer of screenings or sharp sand about three inch deep and compact again. Depending on the thickness of the flag, you should add another layer of sand or screenings to allow for the stones to be leveled if using the dry joint method or to create a base of concrete if mortar joints to be used.
For sand, dry joint, use a straight board or line from one edge of the patio to the opposite edge. This will be the level of the top of the stones. Remember to add a slight slope away from your house for drainage if the patio is butting against it. Put each piece of stone in place and with a rubber mallet or the handle of a hammer, push sand under each edge of the stone until it is level or even with the line. Continue laying stones, matching contours or edges to allow a bit of space ¼-inch - ½-inch, between each stone. If using random, you can place large pieces first and then look thru the rest of the pieces for something that matches the spaces or cut to fit. Rectangular, matching pieces to fit is usually a bit easier. In either case, try not to have joints creating an "X" pattern where four stones match up in the same joint. The pieces tend not to lock together when this type of junction occurs.
For mortar joints, you should pour a concrete pad that will allow at least ¼-inch of mortar plus the depth of the paving pieces to the final grade. After this pad has cured, you can mix small batches of mortar and using a straight board or string as your guide, set each piece of your patio in place. After the patio has set up, you can use mortar as grout between the stones, sponging off excess before it sets up to keep things neater.
Seems like a long, drawn out process, but if you take things a step at a time, you should be able to create something that you will be able to proudly say you did yourself.