Why do roofs leak so easily?

Today we had a thunderstorm. The agency that I work at is made up of nine buildings and we had a few leaks. Even though it has been “fixed” a few times, my parent’s roof leaked around the fireplace as usual. My girlfriend is into real estate management and helps take care of different kinds of buildings and she says that roof leaks are a fairly common problem in all of her buildings, regardless of thier age or building type (Her old metal warehouses leak, her newer class A office buildings get leaks). When I bought my first house and was trying to figure out the whole home insurance thing- I asked homeowners if they have ever used their home owner’s insurance and what they used it for. It seemed as if a lot of people who had to use their insurance reported that thier roofs leaked.

I figure that we humans have been making roofs for quite a while now. We have even been making relatively leak free boats for some time now as well.

With all this time to try different things, why do roof leaks seem to be such a common problem? I realize that for every roof that leaks, there are many more that do not. I also realize that heat, wind, and rain take a toll on materials over time, but what is it about building roofs that is so difficult? Is it just a cost thing?

Because water molecules are very small?

Flat roofs are notorious for leaking. A roof on a pitch is much better, but you still have to pay attention to transitional areas such as chimneys, and ensure the caulk around the flashing is in good shape.

Great question, addressing a pet peeve of mine.

I think there’s no good reason, except perhaps the evolution of roofs has been slow and people expect them to leak. In fact I doubt the great majority don’t leak.

They mass produce light bulbs with vacuums in them, and not a thimblefull of air leaks into them throughout their life unless you actually shatter the whole thing. And for what, a buck?

I think roofs should be made with, say, 0.020" aluminum sheet 6’ wide rolled out in single, long horizontal paths and sealed with epoxy at the overlapped seams. You’d never hear of them leaking unless somebody drilled a hole - and you’d never expect to repair them unless a tree dropped something heavy on them.

Turtles grow waterproof roofs that never need replacement in 100 years or more. It can’t be that hard to figure out.

You might say most leaking roofs leak because most roofs are made of paper, felt, sand, and oil mashed together. Jeez, who didn’t see that coming?

Old joke, from an RCA Victor 78 record, circa 1915, titled The Arkansas Traveler:

Visitor: I noticed a hole in the roof of your house. Why don’t you fix it?

Farmer: ‘Cause it’s been a-rainin’ lately.

Visitor: Why don’t you fix it when it’s not raining?

Farmer: 'Cause when it don’t rain, it don’t leak! Haw, haw, haw…

They just don’t do comedy like that anymore!

Few roofs or houses are currently build according to time-tested methods. Quite often, they’re built to be cheap and nice-looking. So, experience accumulated over the centuries is essentially irrelevant to this kind of issue.
Also, though I couldn’t say if it’s true, I’ve been told that the average american house is of poor quality (low quality materials, no durability, and more specifically crappy foundations) in order to build houses which are at the same time large and affordable (but won’t last long). A stone house with good foundations will provide excellent insulation, is unlikely to have cracks in the walls, or to be damaged by dampness. But not many houses are build this way, nowadays (and not many people could afford them). The issue with roofs might be similar.
Now, that’s purely a guess (except for the part about accumulated experience being mostly irrelevant regarding modern houses).

Roofs that leak weren’t done right in the first place. It’s too bad the usual fix is just a patch job, not to do them right from scratch. So a roof that leaks today is more likely to leak next year.

In the mountains we use a material called bitithane as an underlayment. Unlike tar paper, it is about as completely waterproof as you can get. We have a problem with ice dams on our roofs. This can create situations where we get standing water even on sloped roofs.

Do not ask anyone but a very good friend to help you work with the stuff. It comes in a roll with a paper backing. Once the paper is off, it sticks to anything and everything, including you.

It’s simple: weather, penetrations and lack of maintenance. You don’t see it, so you don’t think about it. Water will enter through the smallest of openings and continue to wick until it finds an entry.

Keep your gutters clean, your roof de-mossed, check all flashing periodically, ensure trim pieces are tight, and that no shingles are missing. For built-up roofs, stay off of them unless you absolutely must walk on them. No do-it-yourself penetrations.

Then there’s glaciation (ice damming), which is a whole 'nuther topic.

The conditions roofs have to endure are more extreme than most people understand. In particular, they have to handle large temperature variations. A roof might start off in the morning with a layer of frost on it and could end up in the afternoon sun at over 150F. This produces huge stresses on materials that need to expand and contract reasonably well, day after day for years. Not a lot of materials can handle that. And that’s ignoring wind and such.

Just take the aforementioned example of large sheets of Al sealed with epoxy. The individual sheets are going to expand and contract at varying rates, the epoxy will soon fail. And how are these sheets held down and still able to expand and contract? Throw in the places where the sheeting meets up with different materials, e.g., chimneys, vent pipes, etc. and there is a huge number of ways for additional failures to occur.

It just isn’t anywhere as easy as it sounds.

Yes, cheap building practices are a common problem. Chip/flake board has no business being used in exterior siding let alone on roofs. Nail guns do horrible damage to plywood. Etc. Roofs and foundations are the two areas of houses where no cost cutting should be allowed.

Gee, wish I’d said that. :smiley:

It will if the walls are quite thick. But if the walls are of a thickness comparable to those of a reasonably well built wood house, the insulation they provide will not be nearly as good.

There is a corrugated, galvanized, steel sheet roofing, and more recently colored metal roofing in manageable panels on the market. 20 mil aluminum would be hard to lay on a pitched roof, still has to be fastened to the framing, etc.

Best bet is to get a reputable roofer that does the job right the first time with whatever type of roofing suits your fancy. Don’t under any circumstances settle for a 'bargain" job.

As has been mentioned, thermal expansion will crack the joints. And as the building settles, and they do to some extent, the expoxy will also crack.

The roofing needs to be resilient. There is a material called Torchdown (which might be a trademark) which is close to ideal, except - there’s always an except - its quality depends on the skill and dilligence of the person who lays it.

Could someone explain this comment to an Aussie? What the hell is something made out of such materials doing as a primary component in waterproofing a roof?

He’s referring to asphalt shingles, far and away the most common residential roofing material in the U.S. They’re relatively cheap, installing them doesn’t require a brain surgeon, they look good, and they usually give you about 20 years of waterproof life. Then, it’s time to strip them off and start over.

Well, the comment was a little exagerated. Best quality composition shingles these days are made with a fiberglass base impregnated with asphalt or other bituminous material and coated on the outside with sand. Roll roofing is also available with the same composition. There are also tiles both real and plastic and metal sheets.

The underlay is asphalt impregnated roll-roofing paper except on roofs that have less than a certain pitch depending upon the local building code. Those must have the Torchdown which is a plastic-like stuff that is melted together with heat, or hot tar mopped under and over the roofing paper.

The best guarantee of a roof that won’t leak is to have no holes in it and stay the hell off it. I cringe when I see people up stomping around on their roof.

There are also metal shingles, which are expensive and a bitch to install, but which will look as good 30 years from now as they do today.

A number of reasons for leaks exist. From a number of articles I’ve read in trade journals, 15# felt isn’t what it was in my Dad’s day. I spec nothing less than 30# to counteract the cheapening of materials, and insist on Grace Ice and Water Shield® at rakes, eaves, valleys, and adjacent to other details such as chimneys. This self-adhering bituminous rubberized underlayment requires no heat for application. Shingle installation can be faulty, too. Some guys still use narrow crown staples, and/or don’t regulate pneumatic guns to preclude overdriving.

Leakproof roofs are easily created (even with asphalt shingles) by skilled professionals who take the time to install proper flashings, penetrations, etc. Bump up to a standing seam metal roof done by a professional, and you have a roof that is leak free for 50 to 100 years with proper maintenance. You also have a roof that costs 5 to 10 times as much as the low bid fly-by-night contractor throwing up asphalt shingle roofs.

Flat roofs on commercial buildings are a different story. For a long long time, flat roofs were made by hot mopping tar onto paper or fabric in multiple layers. This was a bad system, but no one had come up with anything better. There are still millions of these roofs out there, and more being put up. However, now you also have the choice of a solid rubber membrane roof, which (properly maintained) should have a long leak free life.

It is just a cost thing.