Home & Garden Columns

About the House: How Trees Do and Do Not Impact Structures

By Matt Cantor
Friday July 14, 2006

Your Honor, does this lovely Liquid Amber appear capable of doing harm to anything, let alone Mr. Filbert’s 1926 Craftsman bungalow? No, I tell you, it’s a lie, a myth, a hit and a myth! 

My friends the trees have been sorely abused. And it’s all based on false information and a general lack of understanding about how they grow and what they do to the foundations of houses. Although I’ve read tracts by engineers and other experts on the dangers of trees, I’m here to tell you that it just ain’t so. 

You know how the nurse often has better info than the doctor, especially the one in the clinic that sees the same rash a thousand times and the doctor has to check the Merck manual to confirm her findings? Well, in some respects, I’m that nurse. 

I’ve been in the field, seen a lot of houses and have never, ever seen a tree that was overturning a foundation. I expect to see it one day but it won’t be for the reasons that are commonly attributed to trees and I’ll explain why. 

First let’s start with the things that trees do destroy and then we’ll move on to houses and we’ll see a very startling difference that only took me about 1,000 houses to figure out. 

Trees really wreck havoc with sidewalks. This the place we see the most mayhem wrought by our leafy friends. Every year, thousands of linear feet of city sidewalk get slated for replacement due to the formidable tripping hazard that trees create. 

They seem to be completely unaware of the presence of sidewalks, driveways (less than sidewalks), patios and concrete pathways as they toss them, albeit glacially, this way and that. 

That’s the key, they’re unaware and the reason they are is that concrete (I only learned this a few years ago and was shocked to say the least) weighs about the same as soil (given a specific volume, of course). 

So what this means is that a tree will treat a sidewalk the same as any patch of ground, although it may be somewhat more cohesive and a bit rocky. Since the breakages we see happen over long periods of time and as a function of the slow growth of a root, it’s really no great task to break up a three-inch thick skin of low strength concrete that has also been provided with some deep control grooves designed to ease breakage.  

Now again, in the examination of thousands of homes over the last 20 years, why haven’t I seen this happening to houses? Tree roots are certainly capable of doing enormous damage to house foundations and, over time, to entire structures. 

Trees are located right near houses in a large percentage of the homes I see. Well here’s the difference. 

As noted above, concrete doesn’t weigh more than earth and so a skin of concrete over a small area isn’t going to pose any significant resistance to the advancement of a tree root. But a house is a different kettle of bricks, right? A house has real and substantial weight pressing down upon the footing of the house and the earth below 24/7. 

In other words, the tree can actually feel the weight compressing the soils below the footing and can choose a less resistant path. This isn’t to say that there will be no growth below a foundation but I believe that trees will choose, like all of us, the path of least resistance as they grow and that means staying out from under the 80 tons of a house. 

Again, a sidewalk is just too light to feel, weighing roughly the same as soil. Now, I’ll make one amendment. 

I do believe that trees are less likely to break up large heavy and well built pads of concrete because they have some cohesive strength that soil will not exhibit and that slows the advancement of large roots growing near the surface. However, this is, perhaps, putting too fine a point on my argument.  

There is one type of structure that I do see trees destroy and that’s retaining walls. These are also lacking in the load provided by the weight of a house but seem so much more sturdy than sidewalks. Well, they are, but they’re still vulnerable, especially when they lack some sort of foot that resists overturning. 

When they’re just a wall coming up out of the soil and holding back a body of soil on one side, there isn’t that much resistance to leaning. 

If a tree is planted nearby on the uphill side, the roots have little choice but to crowd the space behind the wall and eventually push the thing over. 

Since there can be a moderate amount of designed resistance in better retaining walls, I have no doubt that trees try to send their roots away to easier digs (get it, digs, I sure crack myself up) but the unstoppable growth of the root system of a tree isn’t something you can send just anywhere. Like a woman who’s just broken water, it’s just going to happen … right here and right now.  

I want to wrap up a line that I opened up near the start of this column regarding the damage to a foundation I expected to someday see caused by a tree. Trees aren’t perfect things, sometimes they die and fall over, sometimes they grow in response to the light and become imbalanced and more than a few are gradually being tipped over by landsliding (you can see whole groves of them in the East Bay hills if you look for them). 

So I expect one day to see a tree root ball levering up the corner of a foundation as it falls the opposite way due to one of the above conditions, most likely earth-movement. 

I’m sure that the Animists and the Cartesians are going to line up on either side of this argument but I don’t think I’m imbuing our leafy friends with too much intellect in this theorem. I’m sure you could get a Planarian to do the same thing.  

Nonetheless, I choose to see this as a kind and stewardly act on the part of the arboreal world. But that’s just me. 


Got a question about home repairs and inspections? Send them to Matt Cantor, in care of East Bay Real Estate, at realestate@berkeleydailyplanet.com.