Why do unoccupied buildings deteriorate so much more quickly?

Factual Questions
1

“Because they are unoccupied, duh!”

Well, yes, but why exactly? I was thinking of this when passing some office buildings which have been empty for a few years. There has been a little bit of vandalism, but not much. The main observation is how “weathered” they appear to have become in a short space of time. They seem to attract moss, lichen and general grime, in places that occupied buildings don’t even though I can’t imagine they would be physically cleaned in those places. (Surely nobody washes the exterior brick walls of office blocks?)

I see the same thing with houses - the brickwork of empty houses seems to deteriorate, whereas the walls of my own house don’t, even though I have never cleaned or otherwise maintained the walls themselves in the 10 years I have lived there.

2

Up here, after ten years the house might be completely covered with blackberry bushes.

3

Just a WAG, but an unoccupied building may suck up more humidity and that prompts more growths and weathering of the structure.

4

Well, my house would probably disappear under creeper within a year if I didn’t cut it back. But even empty houses with no greenery on them seem to crumble away.

5

I would guess less heat and air conditioning. Maybe none. That would increase the moisture in both winter and summer.

6

I can only imagine it by comparing what would happen if I did no maintenance on my house for 5 years. The shingles that didn’t get replaced by a common wind storm would quickly deteriorate into an actual hole in the roof that let floods of water inside. Uncontrolled weed and foliage growth would damage my sidewalks and foundation. The rear deck would rot away and become unstable after three years of doing nothing. Mold and mildew growth caused by the extreme swings in temperature would make the place unlivable.

In fact I have an excellent example of this just down the street. When I was looking at house in the neighborhood I am in now I it came down to the house I choose that had been maintained and the house that was bank owned and abandoned for three years; pretty much the exact same house. Much of this stuff had already happened, the people that ended up buying for $80K less than I paid for mine had to sink more than that to get it back up to decent shape.

7

My WAG is that it has a lot to do with constant interior temperature. If a house ls left so that the inside freezes or bakes, according to the season of the year, there will be a great deal more expansion and contraction and moisture absorbance and desiccating in the structural materials, than if there was somebody in there keeping it 72 all the time. There might also be some value in having somebody in there walking around and flexing the floorboards and the joints and the trusses.

8

There’s a documentary called “Life after people” that goes into this stuff in great detail.

9

The average building requires moderate to major restoration work every 25 years, and periodic repairs as needed. Roofs last 10 to 25 years. Painted wood exteriors last about 5 years; vinyl siding lasts a few decades. Aluminum and masonry can last indefinitely, assuming minor damage is repaired so as to not accumulate.

Most office buildings have firms that regularly clean the exteriors and windows (and incidentally inspect for damage), because otherwise city grime from exhaust and other sources would accumulate. In Hartford, a major office tower was at risk of being mothballed for want of a tenant, and the city would have required it to be wrapped in plastic, lest minor damage not be promptly spotted. The plastic would help prevent loose masonry or broken glass from falling on the public below. In an occupied building, a broken window would likely be promptly spotted and replaced. In economically depressed areas of that same city, the lack of this regular maintenance is very apparent.

Heating and plumbing are the other major points of wear. Most building materials will last indefinitely if dry. Once water is gets inside, mold and mildew can cause materials to deteriorate, and the freeze/thaw cycle can cause cracking. If the heat is turned off, but the pipes are not drained, the building will only last a few years.

About a decade ago, Connecticut vacated a public psychiatric hospital, and turned off the steam lines providing heat, but left the water system to the campus turned on so the fire system would work. The never cut off water to individual buildings, and the plumbing would freeze, burst and leak; they now have to demolish everything, even the stately main building facing a major road that the locals wanted to use as a town hall.

In the Deep South, uncontrolled humidity will eat away plaster/drywall and other inappropriate materials. In tropical countries, the building material of choice is concrete inside and out to resist moisture (this is in areas where electricity is too spotty to allow air conditioning). In the Northern US, humidity is less of an issue, causing mostly cosmetic issues addressed by repainting.

Mostly, occupied structures have people who can catch minor damage before it spreads. A little bit of patching and cleaning goes a long way. Occupied structures are generally heated in the winter as well, heading off the major causes of deterioration.

10

The roof leaks. Seriously, before it was abandoned it was very likely in an already rundown and dilapidated condition. Chances are the roof had a couple of issues already.

A leak in the roof is all it takes to destroy a structure, and quickly too!