Flagstone patios and walkways add an elegant touch to any landscape. Whether they are natural stone or made from concrete, the colours, patterns and sizes are limitless. No one builds a patio intending for it to last only a few years. As a part of the home, it should last for years. The foundation determines how well the patio will hold up. This is where the most material and effort will be spent, especially on an uneven grade.
Build a 5 cm x 10 cm (2 x 4 inch) wood frame around the perimeter of the new patio's location. If the flagstone patio will be square, the frame will resemble a box. The box, of course, will represent the outside edge of the new patio and provide a frame to fill and level with material. Keep in mind, to prevent water from standing on the new patio, it should have a gradual fall of 5 cm (2 inches) for every 3.66 m (12 feet_) of the patio's length.
Level the 5 cm x 10 cm (2 x 4 inch) wood frame by digging a trench under the part of the frame that is too high. For the other half that may be too low, drive wooden stakes in the ground next to the frame about 60 cm (2 feet) apart. Fasten the frame to the stakes with nails or wood screws. The result is now a level frame whose top edge represents the level to which the ground will be built up.
Fill the frame with foundation or fill dirt. Spread the dirt evenly throughout the patio area. Leave about 1.3 cm (1/2 inch) space between the level foundation dirt and the top edge of the wood frame. This will allow enough space for Portland cement. Once the dirt is level, pack it down with a tamper until firm. The fill dirt doesn't have to be perfectly level, because the cement mix is self-levelling.
Mix the Portland cement and mortar sand for setting the flagstone. Mix two parts mortar sand with one part Portland and mix with a garden hoe or similar tool. Spread the cement mixture about 1.3 cm (1/2 inch) thick as the flagstone is laid. Work from one corner of the patio to the other, spreading the cement just ahead of the area where flagstone will be placed. Use a wet sponge and clean off any cement from the tops of the flagstone before it dries. Check the stones with a level to make sure the final surface will be flat. Follow this procedure until the patio is complete.
Allow the patio to set for a day or two while drying, depending on the humidity and outside temperature. Once the patio has dried, fill the flagstone gaps with mortar sand and Portland cement mixture and pack it down with a trowel. Go over the entire patio with a wet sponge and remove any cement or sand from the top of the flagstone.
Level the grade with good quality foundation dirt for future stability. Don't allow cement to dry on top of flagstone, because it will be much harder to remove once it has dried.