Why do roofs NOT leak?

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heavenlywild

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#319943 16-Jun-2025 09:32

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?

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#3384623 16-Jun-2025 09:46

On long run they have gaskets around the nails, now screws. Alternatively you can put silicon, and other UV stabilised material to prevent rain getting in.

You also have large overlaps between sheets so rain is not driven up/under sheets.

Tile roofs are shaped so they cover the tile below, again preventing rain being driven up.

No idea on other systems.

Behodar

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#3384627 16-Jun-2025 09:49

I don't know how common this is, but at least under my roof there's plastic sheeting, so even if water gets through the tiles themselves, it'll then just run down the sheets and back outside.

wellygary

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#3384632 16-Jun-2025 10:07

Behodar:

I don't know how common this is, but at least under my roof there's plastic sheeting, so even if water gets through the tiles themselves, it'll then just run down the sheets and back outside.

Roofs tend to be lined with moisture barrier (back in the day it was bitumous black building paper), now its likely some breathable teflon,

As well as stopping any water that gets past the metal/shingles/tiles, its also stops condensation from entering the house envelope...

gzt

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#3384635 16-Jun-2025 10:24

Why do roofs NOT leak?

Because they are installed by competent tradespeople. No other reason.

Batman

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#3384640 16-Jun-2025 10:39

My roof leaks. I've been wondering how to make roofs not leak.

heavenlywild

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#3384641 16-Jun-2025 10:40

Batman:

My roof leaks. I've been wondering how to make roofs not leak.

I wonder if it is how we build them? Of course nothing lasts but putting screws in don't make sense from a water proofing perspective.

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MadEngineer

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#3384650 16-Jun-2025 11:24

Regarding solar panels there a few things to note. The rails that the panels sit on are in turn sitting on a mount that has a large rubber pad that seals the hole. They’re mounted on the upper ridges of the corrugating so have limited water running over them.

Something to watch however is if debris builds up behind such mounting gear, especially with tiles as this can cause pooling or other water damage which would then allow water to go through. The sheet liner under this would then hide the issue for a length of time.

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Handsomedan

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#3384654 16-Jun-2025 11:48

Gutters and roofs leak.

Ask me how I know...

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Kookoo

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#3384664 16-Jun-2025 12:35

They do, we just prefer not to notice until it's too late.

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gabba

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#3384665 16-Jun-2025 12:37

heavenlywild:

Batman:

My roof leaks. I've been wondering how to make roofs not leak.

I wonder if it is how we build them? Of course nothing lasts but putting screws in don't make sense from a water proofing perspective.

Yes wrong thread for me today as well. Currently waiting for someone to come fix a leak in our corrugated iron roof!

Suspect it's one of the seals around the screws that has perished or silicon needs reapplying

Neighbour has had alot of trouble with their flat (un)sealed roof.

Maybe I just live in a bad roof neighborhood!

richms

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#3384666 16-Jun-2025 12:38

I can assure you that the do leak, and that the smallest apparent leak in a roof can let a huge amount of water thru when the wind and rain is in the right direction.

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#3384675 16-Jun-2025 13:01

I have a concrete tile roof - it leaks - in fact they knew it was going to leak - so they put in builders paper to act as an additional protective layer.

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heavenlywild

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#3384686 16-Jun-2025 13:48

So this poses the question - why do we design roofs this way knowing eventually the water will come through?

I was thinking why don't we put two layers of roofing timber on - sandwiched in the middle a sheet of builder's paper? Wouldn't that make it more waterproof?

I mean nothing is going to be bullet proof but surely there is a better way than today's methods? Wouldn't cost that much more?

Handsomedan

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#3384691 16-Jun-2025 14:16

heavenlywild:

So this poses the question - why do we design roofs this way knowing eventually the water will come through?

I was thinking why don't we put two layers of roofing timber on - sandwiched in the middle a sheet of builder's paper? Wouldn't that make it more waterproof?

I mean nothing is going to be bullet proof but surely there is a better way than today's methods? Wouldn't cost that much more?

Because Thatch and Tar is really expensive.

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mattwnz

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#3384693 16-Jun-2025 14:17

Roofs can leak even new ones. Some fixing may only leak a small amount and that water may only leak down to the underlay which can act like a second barrier and either evaporate or drain to the bottom of the underlay. You may need a lot of consistent rain over many days to notice some.

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