Why do roofs NOT leak?

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Ultimate Geek
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#3384788 16-Jun-2025 16:21

>Happily my house is already over one hundred years old. If looked after well it'll last another hundred years.

Exactly - but when talking to suppliers now, questions of durability are often answered with the supposedly reassuring statement that it will, of course, last for 50 years.

When I was looking for a more durable roof than long-run, a supplier was able to show me a track record of over 50 years (his warranty) with an expectation of 200+ years. Unfortunately, between scoping it out and giving him the contract, our dollar plunged enough that I ended up with long-run. I think his 200+ years was conservative - but in line with your house's performance.

Sure, lots of stuff needs & deserves renewal - but not everything should be made so that it's required.

Handle9

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#3384789 16-Jun-2025 16:21

Comparing construction to motor vehicles is fools gold. Motor vehicle manufacturers make a very small range of products in an integrated supply chain. There is significant opportunity to optimise the design and manufacturing as the entire process is driven from the vehicle manufacturers

Construction isn’t like that. It’s a massively diverse range of components designed and assembled in a largely bespoke fashion by SMEs. You don’t get the opportunity to optimise the process in the same way that you do in manufacturing. This leads to a hierarchy of controls being necessary

It’s not a problem unique to New Zealand, construction innovation and productivity is a massive global issue. Productivity has dropped in construction over the last 30 years, which is in direct contrast to pretty much every other industry.

pdh

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#3384796 16-Jun-2025 16:44

>Comparing construction to motor vehicles is fools gold.

If you believe that making the modern range of Teslas is as simple as churning out Morrie Minors - or as simple as building a sub-division of houses in Hobsonville... you're not paying attention.

I drew the comparison to the advances in our philosophy of building (other) things - to design out problems rather than design stuff with an expectation that it will fail - such as self-draining leaky building envelopes & leaky attics being adequately ventilated to dry out.

One of the major causes of our building productivity decline is micro-managing by local bureaucrats.

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#3384799 16-Jun-2025 16:58

pdh:

>Comparing construction to motor vehicles is fools gold.

If you believe that making the modern range of Teslas is as simple as churning out Morrie Minors - or as simple as building a sub-division of houses in Hobsonville... you're not paying attention.

I drew the comparison to the advances in our philosophy of building (other) things - to design out problems rather than design stuff with an expectation that it will fail - such as self-draining leaky building envelopes & leaky attics being adequately ventilated to dry out.

One of the major causes of our building productivity decline is micro-managing by local bureaucrats.

If it's as simple as you claim why does every revolutionary new building company go bust? It's a complex, extremely capital intensive industry.

The reason why you design with a hierachy of controls, and why a leak test is almost useless for waterproofing, is that building last for decades and a certain number of components will fail, either through normal product defects or damage. If you have unseen damage to the building fabric it shouldn't result in complete failure of the framing.

A leak test tells you that the interior is sealed, it doesn't tell you whether the waterproofing is intact. You can have a fully sealed interior and still have leaks.

What you deem "micro managing" is driven by risk. Local authorities don't want to wear the cost of building failures so take a risk adverse approach. Management of liability and risk drives a lot of the behaviour in construction. Once again it's easy to hand wave away but in reality very very difficult to change and not unique to New Zealand.

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#3384801 16-Jun-2025 17:03

wellygary:

cddt:

Happily my house is already over one hundred years old. If looked after well it'll last another hundred years.

Betcha it has at least 2 or 3 roofs over that time though... Maybe more

It's also probably been painted 10 times, had 5 kitchen and bathrooms, the carpet replaced 3 times, and half the window panes replaced. It's been rewired, had insulation added, a minor extension and some walls moved.

Houses are like jacks axe that is 50 years old and has had 4 new handles and 2 new heads.

tweake

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#3384808 16-Jun-2025 17:55

heavenlywild:

This is a left field question on GZ of all places. I have been pondering how roofs don't leak.

Roofs with solar panels where you have to drill holes into your roof. How does it not leak?

Even without solar panels, the construction of a roof, where you nail in / staple shingles in, how does it not leak?

I am genuinely curious. We (speaking generally) are nailing stuff all over the roof, and yet somehow they don't leak.

Can someone explain this to a noob?

gravity.

primary mode for keeping stuff dry is to drain it away using gravity. the 2nd thing is understanding pressures.

solar panel or tv aerial doesn't leak because its on the top and covers over the hole.

the tricky one is wind, aka air pressure, blowing water up hill. which is why roofing has back dams under the top flashing to stop water being blown up and over into the ridge vent. this also applies to windows/doors where its open at the bottom to allow water to drain out but its sealed on the inside so wind can't blow back into the house and push water into the house. why there is little flaps on the window drains.

then there is the back up which is the roofing layment, or building wrap etc. so when water gets past the primary defense it has something to catch it and drain it away. (notice the gutter heights are designed to stop wind blowing water back up which is why the gutter front is higher than the edge of the roof) in modern residential housing its a multi layered system.

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#3384834 16-Jun-2025 18:49

pdh:

>Roof spaces also tend to have a lot of air flow to dry out. It is a system that works with NZ's climate.

That's classic defeatist thinking...

My Dad (born in London, in Quebec from the age of 17) asked his English brother why all the plumbing pipes were still put on the outside of English houses... The answer was 'so that they could get at them when they froze'. My Dad told that story for years.

If you want to see a wet & humid climate - go and look at the Pacific NW in North America.
Coastal British Columbia & Washington State winters make Auckland look dry (and I've lived here for 43 years).
Our tradition of mould doesn't encourage a belief that what we're doing 'works with NZ's climate'.
We chose tiles / longrun and roofing paper as the best option in 1920 - and haven't stepped up our game since.

Sometimes when NZ tries to do something different from what we know works, we end up with problems like the leaky building crisis

tweake

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#3384852 16-Jun-2025 19:21

mattwnz:

Sometimes when NZ tries to do something different from what we know works, we end up with problems like the leaky building crisis

yes, tho leaky homes was really a case of ignoring the science and the issues to make money. direct fix EIFS cladding caused leaky homes well before it was ever used in nz. but they ignored that and all the building science.

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#3384866 16-Jun-2025 20:26

tweake:

yes, tho leaky homes was really a case of ignoring the science and the issues to make money.

Not really. It was an outcome of making huge changes to the building code, building industry qualifications as well as some supply chain changes all happening at once. There were absolutely some systems used which were not fit for purpose but it's much more complex than what you were claiming.

Prior to 1991 there was no national building code, it was a hodgepodge of 60 different acts and local bylaws. The basic intent of the new act was good, to unify the building code through the country, and to introduce a performance based system to allow more modern building methods. It was loosely based on the Norwegian model but there weren't a lot of performance based systems around.

The difficulty was the industry wasn't set up or trained for a performance based system and there were big changes to the inspection and training regimes at the same time. Essentially the checks and balances in the system were insufficient and systems which weren't fit for purpose were allowed to be used. This was the point I made earlier about the level of complexity when making changes to the materials and systems being used.

You need the people and supply chain setup to support new products and systems as well as the regulatory authorities being setup to monitor and control them. It really was a perfect storm.

gzt

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#3384871 16-Jun-2025 20:39

pdh: How did we get the expectation that a house (or component) would only last 50 years ?

That's a good question. I don't know the answer. It's in the building code as more than 50 years or not less than 50 years. I don't know which.

gzt

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#3384876 16-Jun-2025 21:03

wellygary: Betcha it has at least 2 or 3 roofs over that time though... Maybe more

Roofing steel production thickness has reduced over 100 years. The original roof if well maintained may have lasted longer than it's replacement. Maybe there was a sweet spot when galvanizing came in.

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johno1234

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#3384882 16-Jun-2025 21:25

heavenlywild:

This is a left field question on GZ of all places. I have been pondering how roofs don't leak.

Roofs with solar panels where you have to drill holes into your roof. How does it not leak?

Even without solar panels, the construction of a roof, where you nail in / staple shingles in, how does it not leak?

I am genuinely curious. We (speaking generally) are nailing stuff all over the roof, and yet somehow they don't leak.

Can someone explain this to a noob?

Water will always follow gravity via the path of least resistance. Roofs are designed so the path of least resistance leads to the guttering.

mattwnz

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#3385076 17-Jun-2025 15:16

johno1234:

heavenlywild:

This is a left field question on GZ of all places. I have been pondering how roofs don't leak.

Roofs with solar panels where you have to drill holes into your roof. How does it not leak?

Even without solar panels, the construction of a roof, where you nail in / staple shingles in, how does it not leak?

I am genuinely curious. We (speaking generally) are nailing stuff all over the roof, and yet somehow they don't leak.

Can someone explain this to a noob?

Water will always follow gravity via the path of least resistance. Roofs are designed so the path of least resistance leads to the guttering.

Not when it follows capillary action. Then it can travel upwards or sideways and sometimes good distances.

heavenlywild

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#3385102 17-Jun-2025 16:48

Thanks for your comments all.

I just can't understand why we think putting nails into our roofs is a good idea? You can put in plugs and what not but under the hot sun it won't last 50 years, let alone 5!

richms

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#3385105 17-Jun-2025 16:59

Well the only other options are the awful crimped seam systems that make removal and reinstatement near impossible, so if you need to lift it to redo a flashing around a chimney or access wiring, you are basically up for a reroof as its damaged so much when they undo it to remove it.

Seals on screws and nails have worked well for 100s of years, its usually where its walked on and bent down or some ham-fisted sky installer has been working on it that leaks happen. Left alone the older nails work fine till disturbed, and newer screws with seals also work well.

Richard rich.ms

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