Flagstone Patio/Walkway Installation on Concrete Slab

Quick Reference:

Determine what the maximum thickness is allowed for cabinet elevations and door sills.
Figure area and amount of stone. (Length X width = square footage), 1 ton of flagstone at an 1″ thick covers 100 ft².
Choose material and color.
Prepare sub-floor with wonder board or concrete board.
Lay stone out dry in desired pattern.
Make cuts with a diamond blade in a circular saw or shape pieces by breaking them with a hammer and stone chisel to fill in gaps.
Set in thin set mortar mix with acrylic bonding agent. Fill joints with consistent mortar mix or joint compound all at one time.
Let set for a minimum of one day.
Apply final sealer and color enhancers.

Before Beginning:

Determine design of patio and/or walkway so that total square feet (area) is known. Working from a drawing or plan is usually the easiest way to do this.
Break your project’s areas into squares and/or rectangles making square footage measurements easier. Remember 1 ton of flagstone at an 1″ thick will cover a 10′ x 10′ area (100 ft².). Therefore, divide your total combined square footage calculations by 100 to determine amount of flagstone. For instance, a 15′ x 10′ patio and a 3′ x 75′ walkway measure 375 ft². require 3¾ ton of 1″ flagstone. Coverage for 1½” flagstone is 75 ft². per ton.

Choosing Material/Color:
Take color samples of flagstone to find the proper match, compliment, or contrast the colors of house siding or trim, patio, furnishing, plantings etc..

Remember if walls and/or boulders will be incorporated later, make sure all stones and textures will work well together.

When choosing the flagstone make sure to select the size and thickness of pieces you can work with and manage. Flagstone comes in natural random shaped pieces or dimensioned (cut square or rectilinear). Cut flagstone is easier to work with, but is more expensive. This page will focus on the installation of the random flagstone.

Pouring/Preparing Concrete Slab:

Fluctuating soils, soil erosion, and maturing root systems should be the only concerns one has, when deciding to to pour a concrete slab. If existing soil is known to be a problem, it is best to replace the soil. Pouring a concrete slab on unstable soil will create cracking and breakage of slabs in the future.

A 4″ slab with reinforced steel meshing elevated up from the ground 1½” to 2″, framed out sub-grade on compacted soil is the standard for the average residential patio or walkway. (see fig. 1) Typical concrete mix (1/3 sand, 1/3 cement, 1/3 rock (½” – ¾” crushed)) is best. Mix with water so that the mixture is thick to work with, but is liquefied enough to work entirely into corners.

Work concrete with skreet boards until level and eliminate air pockets. (see fig.2) Before concrete sets up, use an old broom to finish the surface with enough texture to prevent slips and to give your stone mortar something to adhere to. Let concrete solidify for at least three days before capping with stone. Clean concrete well with water if dealing with an existing slab before starting to cap with stone.

Laying Flagstone in Mortar:

Dry lay your pieces directly on the concrete, before mixing any mortar. This will allow you to cut and shape your stone pieces together without having to hurry before your mortar base sets up.

For patios, begin by situating larger, heavier pieces in areas outside doors, beneath bottom steps of decks, where patio furniture will be placed, etc..

Next work along the edges, placing natural straight edges end to end around the perimeter of the patio. (see fig 2) Try to avoid too many narrow corners coming to the edge of your patio. This will help edge deterioration, make it safer and look more stable. Figure that the thickest stone, plus at least a ½” of mortar will determine the height of the patio.

Set up the guide boards with wood stakes and firing strips along the perimeter of the patio. Adjust these guides so they slope slightly away from the house to allow for positive drainage.

Your mortar mix should be approximately one part cement, one part sand, and enough water to mix a consistent thick paste. Use at least a 4 foot level to work off guide boards and other laid stone which insures the flatness of the patio or walkway.

Stay true to the guides to guarantee that water will drain away from the house or the desired area. Set stone securely so that mortar squeezes out from every edge of the stone. Leave joints open. Use a trowel to keep them clean if necessary; you will be mortaring or grouting them after all the stone are laid in place. This will allow you to mix up one consistent batch of mortar or grout to fill all the joints at one time.

Cutting/Shaping/Fitting Stone:

Paving stone can be worked with a hammer and cold chisel and patience. For a rough break, first draw a line where you want to break the stone. Elevate the stone edge so a gap will allow a fault to occur. (see fig.3)

Cutting stone with a circular saw and masonry blade is the best method of shaping paving stone. Score/cut ¼ the depth of the stone, then either cut on other side or tap other side on top of your cut with a hammer and chisel.

Renting a wet tile/brick saw is the best way to cut natural paving stone. Demo-saws or even circular saws with a diamond blade will allow you to cut entirely through the stone. Using clamps and a straight edged guide will help keep your cuts look good. (see fig. 4)

Filling Joints:

Mix joint compound of the same mortar mix used to set stone on concrete slab or use grout mix you prefer. Colors that match or contrast stone colors can be added in dry mix.

Make enough joint filler to cover entire area so color and consistency will be uniform over the entire patio. Use a trowel to work mortar in joints. Masons actually use industrial culinary bakery frosting bags with ½”-¾” application tips to squeeze mortar into joints.

Keep stone clean from mortar. Clean any mortar off flagstones while still wet, with water and a wire brush. Dried mortar can be removed with muratic acid.

Finish mortar or grouted joints with a slight concave appearance creates a distinction between stone and mortar. (see fig. 5) this can provide your patio with a distinguished look as opposed to a perfectly flat surface such as concrete.

Sealing Stone:
Sealing your flagstone patio is not always necessary. It is recommended to use a sealer when you-re using less dense stone in higher altitudes with temperature fluctuations. Most informal outdoor patios do not use sealers, however sealers can keep the appearance cleaner and easier to maintain.

Patio/Walkway Maintenance:
The reason natural stone is the best choice for paving material is its durability and few maintenance requirements. Colorado’s extreme weather conditions break down sealers effectiveness and will have to be reapplied every 1 to 5 years depending on sealer type.