20+ Tips to Remember When You're Installing Flagstone
Conquer all kinds of DIY patio and stair projects with these tricks of the trade for laying natural flagstone.
Four flagstone patios, repaired flagstone stairs, and one repaired flagstone sidewalk later — to say we've learned a lot about installing natural stone is an understatement. There's a lot to consider, from choosing the right product to knowing how to properly install for longevity and higher home value. Below, I share a list of things we’ve learned along the way. Whether you’re adding an outdoor flagstone patio, repairing damaged stones, or resurfacing a concrete slab (such as a foundation or concrete steps), this advice will help you plan and execute the job with confidence.
Things You’ll Need:
- knee pads
- dolly for moving larger stones
- rubber mallet
- sturdy work gloves
- long level (4’ or longer)
- cut-off saw with a diamond blade
Choosing and Buying Flagstone:
- Natural stone colors can vary by region and source. One supplier may have a completely different quarry source than another, so it's worth shopping around if you’re looking for a particular color or size.
- If you’re buying to match existing stone in your home, befriend an expert at the stone yard. Using photos or making an at-home visit, they can help you find a stone that's a great match. The nuances between colors in flagstone can make a big difference if you’re replacing a broken stone or extending an existing flagstone area.
- Know the square footage of material you need when you’re planning a patio, and add 10 percent to account for irregularities and extra cuts.
- Be aware of the stone's thickness. Smaller dimensions are easier for a DIY install than larger stones, which can quickly weigh upwards of 250+ pounds. Natural stone veneers are preferable for patio areas that are built upon an existing solid base, such as a foundation or concrete stairs.
- It's easy to lay flagstone if you choose a wide assortment of sizes, and you cut as needed as you work. Plan to have an extra set of hands on site to help with the big stones.
Consider a Roadmap Before You Start
If you're using irregularly-shaped flagstones, you'll have to wing the design. For flagstone patios with square or rectangular stones, make a to-scale replica of each stone (convert feet to inches), and organize how the stones will fit like a puzzle. If you've ever designed a picture frame gallery wall, follow that organizational concept for success. You can do this digitally, but trimming pieces of paper into "puzzle pieces" works well.
Doing this exercise early in the process isn't guaranteed to make your install perfect (After all, mortar lines that are a bit off can throw the whole game off), but it helps make it possible for visual people to understand how the stones will be organized:
- you won’t accidentally put all of your biggest stones down first
- you’ll be able to plan how to stagger joints from stone-to-stone
- you’ll get a better sense of how many stones will need to be cut along the way
- you can map and plan any obstacles or existing structures to work around (like trees and landscaping beds)
Things to Remember During Installation
Installing a flagstone patio is an easy way to expand an outdoor living area or to create a new space that increases your home's value. With any number of materials, it can be easy to DIY a patio, but natural flagstone is a favorite of ours.
- Creating a solid base on an existing soil surface is critical to the long-term success of your patio. You’ll need to dig out an area that’s universally 10" deep to start. Digging by hand is exhausting — consider hiring someone with a small bulldozer or excavator to do the digging, and save your energy for the actual installation. Freecycle the dug-up dirt by offering to people in the community.
- Use crusher stone as your initial base layer atop the dirt. It needs to be 5-6” thick and compacted very well. Tamping by hand is possible for smaller patios, but rent a vibrating plate compactor for large spaces to ensure an evenly-packed base beneath the flagstones.
- Use the base layer of crusher stone to establish your grade. If your stone patio is near your home, be sure it slopes away from it at a rate of 1/2-inch over 4-feet. If your patio is freestanding, make certain there isn’t going to be a low point in the center of your patio that’ll collect water.
- You won’t need more than 1-2” of sand between the flagstone and the compacted crusher stone. Irregularities in the depth of the natural stones may influence how much sand you lay for each individual piece of this puzzle.
- It’ll take some eyeballing, pounding with a rubber mallet, and triple-checking the level and grade to embed the stone into the sand (Use the longest level you can find). If a stone feels wobbly when you stand on it, lift it back up, add more sand, and reinstall until it is embedded very well.
- Polymeric sand can be great and can prevent weeds and grass from planting between stones, but be sure the spaces you’re filling between your stones are no wider than 1-1/2” and ideally as deep as the height of the stone to be sure the sand sets. Wide and shallow spacing can cause sand to crack and loosen. You’ll also need to be extremely careful not to have any sand atop the stones when they get wet. The material acts like concrete and will be hard or even impossible to remove.
Things to Remember When Refinishing a Concrete Surface
Our covered sunroom consisted of outdoor carpet glued to a concrete slab foundation. We removed the carpet and made a plan to replace it with 1/2” flagstone veneers to make the area cohesive with the rest of the flagstone accents in our home. Here are some tips:
- Thinner cuts of flagstone are easier to work with, and because they’re going atop an already solid base layer, there’s less concern about them cracking due to being too thin.
- To even cracks or irregularities on an existing slab, it’s beneficial to rent a concrete floor grinder (also known as a scarifier) to level the topmost surface of the concrete before adding stones. If there were grease drips, outdoor carpet adhesive, existing mortar, or any other materials on the surface, make all efforts to remove it so you’re affixing the stone to a fresh slab.
- Unlike flagstone set in the ground, you’ll only need masonry mortar between the slab and your stones. You might be conditioned to think you should use a notched trowel to spread the mortar, but take it from me, a solid layer is best because the increased contact holds the stone in place better, and it will sound less hollow underfoot.
- Consider mixing only a 1/2 bag of mortar at a time, so it doesn’t dry out quickly as you work.
- Invest in a flexible additive to help prevent underlying mortar from cracking with time. This is sold as a solution that gets added when you’re mixing the mortar.
- Don’t be afraid of using a little more water in the mortar than the instructions indicate. It needs to be spreadable and malleable (but not watery). The concrete slab is guaranteed to absorb some of the extra moisture.
- Have a few buckets of clean water available when you’re working with concrete and mortar — one bucket is for sponging the concrete slab to lift any loose dirt granules before you spread any mortar because it may affect how the mortar bonds to the concrete. Use another bucket and a separate sponge to clear off any excess mortar that pushes up through the cracks before it dries to the surface of the stone.
- Check the level of the stones constantly as you go, and check level left-to-right, right-to-left, and even on diagonals. Use a rubber hammer to make sure the stone is completely set in mortar when placing flagstones into the sand.
- You can use piping bags to help fill mortar into the cracks, but it is easier and more controlled to point and smooth the mortar by hand. The mortar between the stones can be the same as the mortar you used to set the stones, and pointing the stones as you go can be an easy way to use leftover batches of mortar.