Do large roofed structures negatively effect trees?

I'm curious if a large covered structure built next to a tree (so over a large portion of its root system) would have a negative effect on the tree's health. It seems like suddenly denying a large amount of rainwater from the tree could be a problem, but structures are built around trees all the time, so maybe it's not a big deal at all.

I'm planning a covered work area next to a mature douglas fir tree. It won't have a foundation, so I'll still be able to water under the structure, if needed. Am I overthinking this? It's just a nice tree and I don't want to hurt it.

Just wondering if anyone has experience or knowledge about this. All I'm finding from google is how trees affect structures, not the other way around. I did find some stuff about how building around trees can hurt them, but just from compaction of soil, nothing about water.

Thank you for any discussion.

David

If you have to have the building in that particular spot, do it. If you don't, don't. When you have a building and a tree grows up next to it, the tree makes accommodations to the building's environment. It sends roots out to where the water is. Its branches avoid the roof heat if necessary. But where you put a building next to a mature tree, that tree has grown in an environment the tree is used to. When you add the building, you change all that. Wind patterns change. Moisture coming off the roof waters the ground much differently. Sun reflected off the roof is much different. You are putting the tree into a situation much changed from what it is used to.

You can try to water the tree extra, but the reality of life is that you will water religiously for a time and then you won't. You'll forget occasionally, then your habits will change, and you'll decide it no longer matters. But trees live according to decades, human people live mostly to the moment. So, if you change the environment too much, the tree will suffer and maybe/probably die. ~~~But you can always make firewood and plant another tree.

The Fairies won't like that. The other trees won't like that. You have been impolite to the life that existed long before you came along, But if you must, you must. That's what humans do.

Two considerations come to my mind when I think about this:
1) As you mentioned, lots of structures are built near trees. If it were so bad that doing so would cause them to die, then very few folks would be doing it. It wouldn't take a lot of trees suddenly toppling onto rooves for folks to stop building near trees. But this obviously isn't the case.
2) Something that people do often consider when building a structure is the fact that you are converting a certain amount of earth into a rooved structure that doesn't absorb rainwater. Instead of naturally percolating into the ground, and then running off when there is excess, you now just have all the rainwater running off immediately. This of course affects the earth around it, specifically where the tree is. Depending on your topology, you could have excessive water pooling around the tree trunk, or perhaps draining away in a ditch. That water being water the tree was relying on being there. Whether or not this will affect the tree badly enough or whether the tree will adapt and send roots elsewhere, I don't know.

I think if I were in your position, I would figure based on the first point that it shouldn't be an issue under normal circumstances. If you wanted to really take precautions, perhaps put gutters and some barrels to collect the rainwater off your new structure. You can then make sure the rainwater still benefits the tree.

I would build some compost bins, filled with carbonous material, inside the drip line.
I would then direct the water that rains on the roof directly into those piles.
This would create  reservoirs of moisture and nutrients for the tree to tap into, without creating another task for you.
These compost piles would also be a good place to pee.

Depending on what type of roof is built that might actually help the tree with runoff from the roof.

Like the runoff from a hip roof which might give added water that the tree might not get.

Also, the shade might help the root system in the summer heat.

Depends on how the roof water is managed.   Most houses/buildings are designed to get the water away from the building as quickly as possible which can impact trees.  Local advice is to have a rain garden or rain collection system to slow the waters progress to the ocean.

That said, I would worry more about the building.   Roots don't mind digging up houses.  They block drains and it's fun to come home to 2 feet of sewage flooding the basement.    But not as big an issue for you unless there is water moving in or out of the structure.

Where I live has strong wildfire seasons so having trees overhang the building pretty much invalidates insurance for the whole property and can make a property owner liable for other damages if a simple structure fire spreads.

Wind and widowmakers are a big issue with Douglas fir.   Something to plan for.

And worst of all, it makes cleaning gutters more frequent and difficult.

But yeh, if you can get the moisture away from the building but still in the soil within the dripline of the tree like with a rain garden,  the tree should be fine.

The local recommendation for making rain gardens.  It's good to adjust it for your local conditions.

https://www.crd.bc.ca/education/stormwater-wastewater-septic/green-stormwater-infrastructure/rain-gardens

Thank you all for the input and ideas. And many great points to consider. Love the ideas of the compost piles and rain gardens. I had also considered having the gutters and directing the water right back under the structure...maybe with something like french drains (but percolating, not draining) , or even just trenches filled with gravel. You're absolutely right, Jim. I would be forgetting to water that tree in no time. Redirecting the rain that's falling anyway seems more realistic.

Fortunately I dont feel worried about the tree affecting the integrity of the structure. Very basic pole-barn style building with simple concrete footings and no plumbing. Widowmakers are a risk that I can't do much about. If one happens to fall on it, I guess I'll just repair it.

Thanks again.

David