Tree Roots, Foundations, Pools, and Sewer Lines in Arizona: What the Research Actually Says

Written by Justin Tornatore, ISA Certified Arborist | TIMBERx

Abstract

Few tree worries keep homeowners up at night like roots. Will that mesquite crack the slab? Is the palo verde going after the pool? Are roots in the sewer line? Peer-reviewed research gives a more nuanced answer than most people expect. A 2014 review in Arboriculture & Urban Forestry concluded that tree roots "rarely cause direct damage" to building foundations and rarely damage pipes, though roots readily exploit existing breaks and loose joints in sewer lines [1]. The bigger foundation risk documented in the literature comes from roots drying out shrink-swell clay soils, a mechanism studied mostly in the United Kingdom [1][2]. This article reviews how roots actually grow in Arizona soils, what the research says about foundations, pipes, pavement, and pools, and how to plant and water so trees and structures can coexist.

Background: How Roots Really Grow

Roots aren't drills. They're opportunists. A root grows where it finds water, oxygen, and soft soil, and it slows or stops where it doesn't. I explain it to homeowners the way water moves across a yard: it doesn't bore through obstacles, it follows the path of least resistance and pools where conditions are best.

In Arizona, that path is mostly outward and fairly shallow. UA Cooperative Extension notes that the root zone of trees like mesquites extends horizontally two to three times the width of the canopy and vertically two to three feet or deeper, if there are no barriers or compacted soil [3]. UA's watering guidance puts typical mature tree root depth at 18 to 36 inches [4]. Just as important, we steer roots with irrigation. Emitters placed near the trunk keep roots concentrated there, while emitters at the dripline and beyond build a wider, more stable root system [5].

Palms are a different animal. Their roots are fibrous and emerge at their final diameter, and UA notes that this growth pattern prevents the root-pavement conflicts caused by many broadleaf trees [6].

What the Research Shows

Foundations

Watson and colleagues, reviewing the research literature, state that tree roots "have been associated with interference with building foundations but rarely cause direct damage." The force a root generates as it grows in diameter is small, and damage occurs only to lightly loaded structures [1]. The documented mechanism is indirect. On soils with high shrink-swell capacity, roots near shallow foundations can deplete soil moisture during drought. The clay shrinks, and the foundation settles and cracks [1].

Lawson and O'Callaghan's earlier analysis in the Journal of Arboriculture examined the large volume of tree-related subsidence insurance claims in the United Kingdom. They found the commonly cited tables of tree "water demand" by species to be "clearly in error and misleading," based on limited data, and they called for closer collaboration between consulting arborists and structural professionals [2].

Two cautions apply to Arizona. First, much of this research comes from UK clay soils, so how well it applies to your lot depends on your soil. Second, the review found root barriers are "not considered an acceptable solution" for foundations, because roots can grow under, over, or through them. Crown thinning didn't reduce the tree's water use either [1]. The approaches supported in the review are planting trees well away from structures or using deepened footings [1].

Sewer and Water Lines

The same review found that roots rarely damage pipes outright, but they can enter through breaks and loose joints and then "proliferate rapidly once inside the moist, nutrient-rich environment" [1]. Older clay and concrete pipes without rubber gaskets resist intrusion the least. Willows, poplars, and eucalyptus are among the genera more likely to cause intrusion, and roots are less likely to grow into sewer pipes when trees are planted 6 meters (about 20 feet) or more from existing pipes [1].

Sidewalks and Pavement

Research reviewed by Watson et al. challenges the assumption that cracks near trees are always caused by roots. Older sidewalks failed more often, sidewalks did not fail at higher rates where trees were present, and 61% of pavement expansion joints were cracked even with no roots present. Cracks may actually invite roots by letting more oxygen into the soil beneath [1].

Pools

I could not find peer-reviewed research specific to tree roots and residential pools, so I'll be careful here. The same principles apply, though. A pool shell is a heavily built structure, and its plumbing behaves like any other pipe: roots exploit leaks and loose joints rather than creating them [1]. In practice, the more common pool-and-tree conflict is litter and debris. AMWUA's plant database lets you check whether a tree is light on litter and "pool friendly" before you plant [7].

Cutting Roots Has Costs Too

Before anyone grabs a trenching shovel, consider the tree's side. Reduced root systems from root loss can reduce stability and increase stress [1]. In Moore's study of 80 wind-thrown trees, trenching near the trunk appeared in 58.8% of failures and damaged lateral roots in 87.5% [8].

Field Observations

When a homeowner suspects a tree is damaging a structure, the evidence is often circumstantial. Roots get found near a crack or a leak, but that doesn't prove they caused it. The research backs up that caution [1][2]. I'll also say what I don't know: I'm an arborist, not a structural engineer or a plumber. A cracked slab or a backed-up line deserves a qualified engineer or plumber alongside the arborist, which is exactly the collaboration Lawson and O'Callaghan recommended [2].

Where trees genuinely cause trouble, the same setup tends to repeat. A fast-growing, high-water-use tree is planted close to a structure, and irrigation, a leaking line, or a downspout keeps the soil next to the house the wettest spot in the yard. Roots go where the water is.

Practical Recommendations

1. Plant for the mature size. Space trees from foundations, pools, and sewer lines based on their mature canopy and root spread, not their nursery-pot size. Twenty feet or more from sewer lines lowers intrusion risk [1].
2. Pull water away from structures. Place drip emitters at the dripline and beyond, on the side away from the house, and move them outward as the tree grows [5].
3. Fix leaks first. Leaking irrigation, pool plumbing, or sewer joints attract roots. Repairing the leak removes the attraction [1].
4. Choose species thoughtfully near pipes. Avoid high-water-use trees known for intrusion, such as willows, poplars, and eucalyptus, near older lines [1]. Check litter and pool-friendliness for poolside plantings [7].
5. Don't rely on root barriers or heavy pruning to solve foundation concerns. The research doesn't support either [1].
6. Get the right professionals. Pair an arborist with an engineer or plumber before removing roots or trees over a suspected structural problem [2].
7. Protect roots you keep. Avoid trenching and compaction near the trunk. Root loss can destabilize a tree [1][8].

Key Takeaways

• Roots rarely damage foundations or pipes directly. They exploit moisture, cracks, and leaks that already exist [1].
• The best-documented foundation risk is roots drying shrink-swell clay, mostly studied in UK soils [1][2].
• Sewer intrusion risk drops when trees are planted 6 m (about 20 ft) or more from pipes [1].
• Irrigation placement is your most powerful tool for steering roots where you want them [5].

Talk to an Arborist

Planning a new tree near the house or pool, or worried about an existing one? Text Justin at (520) 451-8790 (texting is preferred over calling) or email Justin@TimberxTrees.com. TIMBERx is a veteran-owned tree care company led by Justin Tornatore, ISA Certified Arborist, serving Tucson, Oro Valley, Marana, Vail, Sahuarita, Green Valley, and Benson.

References

1. Watson, G. W., Hewitt, A. M., Custic, M., & Lo, M. (2014). The Management of Tree Root Systems in Urban and Suburban Settings II: A Review of Strategies to Mitigate Human Impacts. Arboriculture & Urban Forestry, 40(5), 249–271. https://auf.isa-arbor.com/content/40/5/249
2. Lawson, M., & O'Callaghan, D. (1995). A Critical Analysis of the Role of Trees in Damage to Low Rise Buildings. Journal of Arboriculture, 21(2), 90–97. https://auf.isa-arbor.com/content/isa/21/2/90.full.pdf
3. Schuch, U. (2021). Drought and Extreme Heat: Plant Responses and Landscape Maintenance Practices (AZ1876). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/drought-and-extreme-heat-plant-responses-and-landscape-maintenance-practices
4. Call, R. E., & Daily, C. (2017). Watering Trees and Shrubs (AZ1298). University of Arizona Cooperative Extension. https://www.extension.arizona.edu/publication/watering-trees-and-shrubs
5. Schuch, U., & Kelly, J. (2012). Mesquite and Palo Verde Trees for the Urban Landscape (AZ1429). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/mesquite-and-palo-verde-trees-urban-landscape
6. Schuch, U., & Quist, T. (2023). Arizona Landscape Palms and their Management (AZ2021). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/arizona-landscape-palms-and-their-management
7. Tenney, W. (2017). 5 Trees To Shade Your Desert. Arizona Municipal Water Users Association (AMWUA). https://www.amwua.org/blog/5-trees-to-shade-your-desert
8. Moore, G. M. (2014). Wind-Thrown Trees: Storms or Management? Arboriculture & Urban Forestry, 40(2), 53–69. https://auf.isa-arbor.com/content/40/2/53