Tuesday, September 6, 2011

Cracking from Tree Roots

I was on vacation this past weekend and stumbled upon this patch of road and being the nerd that I am I had to take a photo and share it with you all.

A few weeks ago someone asked me if alligator cracking can be caused by tree roots.  The photo above is an example of cracking caused by tree roots.  As you can see, the cracks are actually raised up from the ground instead of depressed like you will commonly see with alligator cracking.  Maintenance for roads with cracking due to tree roots is a little complicated.  In 99% of cases you will need to remove the tree to fix this on-going problem because even if you dig out the roots that are currently under the pavement, the tree will grow new ones and you will be right back where you started. 

The tree shown in the picture above is in a state park so the solution to remove it will not work.  If you need to keep the tree in place you'll need to find a way to reduce the roots below the pavement.  Typically a tree will have 6 to 10 large roots that grow outwards from the tree trunk which provide stability for the tree.  It is not advisable to cut these roots because the tree may loose its stability and become a safety hazard.   Cutting these major roots will also leave a large portion of the root exposed to the elements which can cause rotting and decay under the soil's surface and may ultimately result in the tree dying.  If you find that these roots are the ones that are causing your cracking, you are better off removing the tree.  If not, you will dig out the area below your pavement, saw off the roots that are under the pavement area and install a root barrier at the edges of your pavement.  The barrier should be placed to a depth of 18-24 inches.  If you have good soil depth, the roots will grow down and continue to grow laterally below the barrier but will not have as large of an impact on the pavement you install. 

A common mistake that contractors make is by only trenching along the edge of the pavement to cut the roots and install the root barrier.  By doing this they leave dead roots under the pavement which will rot and decompose.  Once this happens, the pavement can shift and create more cracking.  It is better to dig out the entire root system under the pavement. 

Another option is to pave with rubberized asphalt.  Although the rubberized pavement will still upheave from the roots, it will not crack into sharp cracks as you see above and form rounded mounds instead.  This can help aesthetically as well as reduce the tripping hazards from pedestrians and bicyclists.        

If you are planning on planting new trees near pavement keep some things in mind.  Most tree roots are 2-3 feet below the surface where the good soil is and can be as much as three times the length of the longest branch on the tree.  It is recommended that your pavements are 5 feet from any trees to eliminate the chances of the roots damaging it.  There are also certain species of trees that will be better suited for planting neared paved areas.  Avoid trees with roots that swell and raise near the surface.  Also avoid trees with dense canopies that will reduce the amount of sunlight available to the pavement.  The lack of sunlight will make it harder for moisture to evaporate which can encourage water damage to your pavement.  Some good choices for trees to plant near pavements or sidewalks are hedge maple, cornelian cherry, green ash, chinese juniper, crabapple, scarlet oak, and lacebark elm.  A few of the trees that you want to avoid when planting near a paved surface due to large surface roots are red maple, beeches, live oak, pin oak, willow oak, weeping willow, and american elm. 

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