Repairing and Replacing our Historic Flagstone Sidewalks

On a directive from city council, the City of Denver will be inspecting and possibly replacing poorly maintained sidewalks. Be prepared!


City of Denver Website

City of Denver Flagstone Information

List of Local Contractors

Dealing with the City

City Inspection

The City of Denver is sending out inspectors to review the condition of the sidewalks in central Denver.  These inspectors will check the condition of the public sidewalk in front of your home.   They said that they will not check the walk leading up to your home or the paving from the sidewalk to the curb.  They are only interested in the public sidewalk.  They are looking for sloped, broken, uneven or miss-aligned slabs of sidewalk.

If any portion of your sidewalk is lifted more than ¾” from one slab to the adjacent slab, this is deemed to be a “tripping hazard” and the stones need to be re-leveled.  If a slab of stone is shattered into several pieces, it is deemed to be beyond repair and the slab will need to be replaced.  Go to to find sources of new sandstone slabs.

Tree Issues

Invasive tree roots are the number one reason that sidewalk panels have been lifted out of alignment.  While it might be tempting to cut a root or remove a tree, do not do this without advice from the City Forester.  If you remove or damage a tree without permission from the Forester, the city can fine you for the value of the tree you took out.  (The Forester estimated the value of my large Silver Maple tree at $15,000).  The city has an online inventory of every tree in the tree lawns of the city at   Look under “Do I Have an Ash Tree” and “Interactive Maps”.

According to the Forester, trimming a root can actually be counter-productive.  The tree will naturally sprout multiple new roots at the site of the amputation, causing even more aggressive root activity.

Permits and Licenses

If you are blocking the public sidewalk or if you are using machinery, you will need a permit.  Homeowners can obtain a permit at no cost.  Contractors are charged $50.  You can file online at  You should file for your permit early.  It takes a minimum of five working days for the city to process your permit application.

The Department of Public Works will deliver 4 saw-horse-style signs to your house the day before your construction starts.  Put two of these signs on the sidewalk at the ends of the block.  Put the other two signs on the sidewalk just beyond your construction zone.

Repairing the Sidewalk

Mud-jacking or Foam-jacking

Mud-jacking or Foam-jacking is a method of lifting sunken slabs without actually picking them up.  The foam-jacking team will drill small holes (5/8”) through the stone slabs and will inject a liquid 2-part polyurethane foam underneath the slab.  This product is similar to the expanding foam in shaving cream.  As it expands, the foam will gently lift the slab.

If you hire a mud-jacker to align your slabs instead of a foam-jacker, he will drill a larger hole (1-1/2”) and will inject liquid concrete instead of foam underneath the sidewalk.

You can walk on a mud-jacked or foam-jacked sidewalk immediately after the job is complete.

Lifting and Re-leveling Slabs of Stone

Sometimes the best remedy for sloped or sunken slabs is to lift them out of place and adjust the ground underneath them.  If the slabs are small (3’ x 4’) you can do this work manually.  If they are larger slabs of stone, you would be wise to use mechanical assistance from a fork-lift machine.

Once the slab is removed, dig a few inches of dirt out from the exposed pit.  Use a tamping device to compact the soil.  Add a couple inches of crushed gravel and tamp that down, too.  Cover the gravel with a layer of rot-resistant geotextile fabric (landscape fabric).  This mesh keeps the sand from settling into the cracks between the gravel.

Add an inch or two of sand.  This sand will settle to adjust to the uneven surface of the bottom of your stone slab.  Measure carefully to make sure you have the right amount of sand.  Measure your slab at all four corners.  Then measure your pit at all four corners.  Your goal is to reset the stone and have it sit exactly right.  It is no fun to repeatedly lift the stone out to add or subtract sand.

Grinding the Edge of Flagstone

While grinding down the tripping hazard of a raised edge of pavement works well for homogenous materials like concrete and granite, it is not recommended for sedimentary stones like sandstone.  Sedimentary stone was formed at the bottom of an ancient lake bed.  Silt was deposited in successive horizontal layers.  Some layers are stronger than others.  Sedimentary stone tends to delaminate in sheets when a weak layer of the stone is exposed to freezing moisture.   Grinding the edge of the flagstone exposes these weak planes to moisture leading to premature failure.  Not a good idea.

If You Need to Replace Some Flagstone

If a Large Portion of Your Sidewalk is in Bad Shape

If one of your slabs is shattered into multiple pieces, it cannot be repaired or re-leveled.  It must be replaced.  If 30% of your walk needs to be replaced, it is probably most economical to remove the whole walk and replace it with concrete.   There are two masonry salvage companies in Denver (Hillen Corporation and Mendoza Used Brick and Demolition) that will remove the stone for you.  They will then re-sell the large pieces of stone to another homeowner.  This would be a sad outcome but it is better than having the stone go to the landfill.

If You Only Need to Replace a Slab or Two

The vast majority of the flagstones used in the sidewalks of Denver are sandstone slabs quarried from deposits near Rocky Mountain National Park.  Generically this stone is called Lyons Sandstone.  It is sold by a number of quarries (see the full list of suppliers at ).  It varies in color from grey to rose.  Lyons Sandstone is unusually hard, dense and water-resistant.  Many of the flagstone sidewalks in Denver are over 125 years old.

A large portion of the cost of the stone is the delivery fee (typically $400).  It costs the same to deliver one slab or a dozen of them.  You can save a boatload of money by banding together with neighbors who also need a slab or two.

While the city requires a 4” thick concrete sidewalk, I (Diane Travis) feel that it would be adequate to use 3” thick stone for sidewalks and 4” thick stone when the walkway crosses a driveway.  (You need stronger slabs because the cars weigh a lot).  Although the city officials said that they would consider changing their requirements, at present they still require 4” thick stone for sidewalks and 6” stone where the sidewalk crosses a driveway.  Check with Public Works to find their latest ruling on this.