I often get questions about the trees that grow right next to a house. Popular opinion has it that this can cause cracks in the foundation and that the roots of the tree will actually penetrate the concrete, then expand and cause it to crack. Obviously this would be of concern to any homeowner. But it is really true?
The answer is almost always no. Here’s why.
It’s important to understand that tree roots actively seek moisture in the soil. They will grow towards moisture, but stop growing entirely if the soil is completely dry. And the soil immediately next to a foundation is usually very dry, even more so if there is a roof overhang, as that means little rain reaches it. Also, a foundation generates heat, even in summer, drying out the soil next to it. The result is that, when a tree starts to grow near a foundation, whether it was planted there or sprouted there on its own, the majority of its roots will grow away from the foundation, toward the moister soil nearby. Also, roots have no affinity for concrete which is naturally dry and also very alkaline and will instead, again, head away from it. So, no, roots will not penetrate the cracks a foundation and are even less likely to create any.
However, a problem can occur when the soil used to backfill the foundation walls is clay. During periods of deep drought, the roots of a tree planted near the foundation wall will tend to dry out the soil as they look for the last drops they need for their survival. This causes the soil mass to shrink and pull away from the foundation. If that occurs, the wall is no longer held in place by a mass of soil, can expand outwards and may then crack. Note that this will happen even if there are no trees nearby, but that the presence of a tree can exacerbate the situation.
Note too that no self-respecting contractor would use clay to backfill a foundation. Sand or gravel are far more commonly used. But this problem has turned up in some developments where norms were not followed.
This kind of cracking can largely be avoided by letting some rainwater seep into the soil near the foundation rather than draining it all away. And if there is a severe drought and you know your foundation is surrounded by clay, it would be wise to water the soil near the foundation (a soaker hose would be appropriate) whether a tree grows there or not. If drought is an annual occurrence, it would probably be worthwhile biting the bullet and replacing the clay backfill with sand or gravel… and maybe taking the contractor to court if he can be found!
Other Tree Effects
Under normal circumstances, therefore, the presence of a tree near a house doesn’t harm the foundation. But there are other factors to consider when a tree grows near a house.
The branches of the tree can rub against the building’s walls or roof, especially on a windy day, and that is not good for the tree or the house. It is best to prune back or entirely remove the branches that cause the problem… and that can detract from the tree’s overall appearance.
Also, depending on the species and also the dimensions of the house, a tree growing next to the foundation can eventually exceed the height of the house and its branches will therefore likely extend over the roof. This in itself is not harmful and will even help cool the house in summer. (Studies show that shading by trees reduces air-conditioning costs by up to 30%.) In addition, a shaded roof will last longer than one constantly exposed to the sun (it’s the sun’s ultraviolet rays that damage roof coverings, resulting in the need to replace them periodically). However, leaves or needles of an overhanging tree will tend fall on the roof, as will small branches, and that can clog gutters. It’s always wise to clean your home’s gutters in late fall whether you have an overhanging tree or not, but it’s even more important when tree branches stretch over the roof, as there will be more litter to remove. Furthermore, some trees also drop gutter-clogging flowers, fruits or seeds in other seasons. In that case, it may be necessary to clean the gutters more than once a year.
But what about the poor tree that grows next to a house? In general we prefer trees that are symmetrical, with a straight trunk and branches fairly equal in length spreading out in all directions, but the walls of the house create shade. A tree growing near the foundation will therefore tend to grow at an angle, leaning away from the house and toward the source of sunlight. The branches on the shady side will be fewer and shorter and die younger than the branches most exposed to the sun. So the perfect symmetry you’re looking for is compromised.
Ideally, therefore, you should plant a tree at a distance from the house at least equal to half its future spread. That will allow it to really flourish. That could be 10 feet (3 m) for a small tree, but up to 20 feet (6 m) for a tall one. However, if the tree is already there and you don’t mind its somewhat lopsided effect, you don’t have to be in a hurry to remove it. Many trees have grown near the foundations of old homes for decades or even more than a century, do no harm… and may even appear quite attractive.
But What About Underground Pipes?
That’s another question entirely.
Most trees like even moisture. They prefer soil that has some moisture, but also a good supply of oxygen. Constantly wet soil becomes stagnant and lacking in oxygen: their roots will not be able to grow under such circumstances. So the average tree has no affinity for leaky pipes and are unlikely to cause any damage to them.
However, some trees – mostly those that naturally grow in swampy soils and are therefore adapted to low soil oxygen – are so thirsty their roots will naturally tend to move towards sources of water, which can be leaking water, sewer, or drainage pipes. Even they will not grow towards intact pipes: to attract roots, the pipe has to be are cracked or broken, or have joints that are improperly sealed.
People seem to imagine that the roots wrap themselves around the pipe and crush them, as if they were strangling them, but in fact, the main damage actually comes from very fine fibrous roots that penetrate into small holes already present. Once in the pipe, they start to branch out, forming a dense mass of fine roots that slow the flow of water and may even end up clogging the pipes. Also, the roots that penetrate the pipe thicken over time and that can create pressure that may eventually cause the pipe to crack further or even break up entirely.
The basic rule is to not plant trees of any sort within 10 feet (3 m) of underground pipes. And some trees are more avid water-lovers than others – poplars, willows and silver maples are the worst culprits – and are more likely to cause problems. Better away keep them at least 30 feet (10 m) from any pipes. Some cities have bylaws preventing you from planting poplars, willows and silver maples at all, or else require planting them a great distance (65 feet/20 m or even 80 feet/25 m in some cases!) from any city pipes. Oddly, local garden centers generally still sell “illegal” trees in the very municipalities that forbid them.
Do Plant Trees… But Think Ahead
Remember that the presence of trees is beneficial to your home and increases its value. Do not hesitate to plant them… but do select suitable varieties and plant them under conditions that suit them.