Robert W. "Bob" Jensen, P.E.
bjensen@jensenengineers.com

Michael S. Jensen, P.E.
mjensen@jensenengineers.com

972-994-9872
972-994-0747
972-692-5713 fax

TREES - LETHAL WEAPONS

See also: Drainage Problems, Plumbing Problems, and Foundation Movement

The presence of large caliper trees and other forms of vegetation can have deleterious effects on a foundation.  Trees inherently grow their roots towards reliable sources of moisture such as the soils under your slab.   These root systems can extract hundreds of gallons of water from soils on a daily basis.  Studies indicate trees can influence moisture levels in soils over horizontal distances of up to 1.5 times their height.  These drying effects are maximized during the hot, dry summer months and exacerbated when the homeowner fails to adequately water his yard.  As a result, portions of the foundation can move lower as moistures are reduced.   

Conversely, during the wetter winter months, moisture is returned under the slab and the foundation experiences lifting.  Unfortunately, this lifting doesn’t always return the foundation to original elevations due to loadings imposed by the weight of the structure.  Consequently, after several seasons of moisture gain/loss, elevations across the foundation slab become irregular, resulting in distress to the structure.  This distress can result in permanent structural damage, reducing property value.   

While some people do not believe tree roots actually extend under a foundation slab, the following two pictures document the extent of root growth which can develop.  Note the relatively small size of the subject tree which has extended roots under this slab, visible in the plumbing repair trench approximately 30 ft. away.  Most roots were found growing along the top of soils just under the slab while some roots extend below 3 ft. depths.


Medium-sized tree near foundation


Roots exposed in plumbing trench

Of even more concern is the situation following foundation repair.  As a foundation is underpinned, perimeter elevations can be improved.  However, if trees are allowed to grow nearby and extract moisture from under the slab, volumetric soil shrinkage can occur, lowering the interior slab while piers continue to support the perimeter.  This typically results in reverse sloping of floors, drywall cracking in opposite directions and reverse door fitments.    Also see Soil Moisture Maintenance Tips


Soil Desiccation from Tree Roots

 

Many engineers and contractors see this as a problem to be solved by mudjacking (pressure grouting) the interior slab.  While this approach may initially address symptoms, it doesn’t solve the primary cause of uneven soil moisture content.  Further, this approach often leads to a subsequent need to mudjack again and again, never really solving the problem.  Each time this procedure is performed negatively impairs positioning of the slab.   

The older your home and the more developed your trees, the greater is the potential for problems.  Sadly, many homeowners learn about the devastating effects of tree roots the hard way - after foundation problems developFortunately, these problems can be addressed using either of two methods. 

• Trees can be removed and the stump ground below soil level. This approach should be strongly considered with less valuable, short-lived fast-growing trees such as Cottonwoods, Fruitless Mulberries, Willows, Maples, etc.

• With more valuable trees, a root barrier can be installed between the tree and foundation. Studies have shown that roots typically grow within the top 28 in. below ground, with only minimal roots extending below that level. It is important to design not only the appropriate location but also proper depth of a root barrier.

Through careful historical analysis, Jensen Engineers has developed procedures which properly addresses both cause and solutions to these problems.  Please call for an appointment to find out more on this subject.