Thread: A quiz about old roofs

it only rains on every 2nd sheet

good pick up
i want to see the end of this
glen

I know because I used to work for Lysaght/BHP but I wont tell but if you do a search i'm sure I am have posted the answer here before.

Originally Posted by mic-d

Here's an interesting question I heard the other day. If you look at old unpainted corro iron roofs pre 1960's I think, you will frequently see sheets alternating between rusty and not(so rusty), ie one is rusty, the next less so, the next rusty, the next less so... etc. Why do you think that is (I know the answer BTW)?
If you know too, don't post the answer straight away

Cheers
Michael

Unders and Overs

as in the lap

I dont know so i will have a guess - if its right i'm sorry for spoiling others attempts.

How did i go? yep got it right so i deleted it for others to guess

Cheers

Originally Posted by Calm

I dont know so i will have a guess - if its right i'm sorry for spoiling others attempts.

I think when the roof was laid to get the overlap right every second sheet is turned upside down and the galvanising was not the same on both sides.

How did i go?

Cheers

In other words .....

Unders and Overs

as in the lap



I seem to remember something about an oil coating sprayed on one side , to stop the sheets sticking together .

But I could be wrong eh

That's one of two reasons covered
What's the other reason David?

Originally Posted by echnidna

That's one of two reasons covered
What's the other reason David?

Bob that was a guess - got no idea and i deleted mine so others can guess

Cheers

Originally Posted by echnidna

That's one of two reasons covered
What's the other reason David?

Reason for the alternating under sheets and over sheets , was so that the whole iron roof did not peel off in high winds .

Originally Posted by Barry_White

I know because I used to work for Lysaght/BHP but I wont tell but if you do a search i'm sure I am have posted the answer here before.

You better post the definitive answer Bazza, just in case I'm wrong

Cheers
Michael

Originally Posted by Manuka Jock

Reason for the alternating under sheets and over sheets , was so that the whole iron roof did not peel off in high winds .

Well that's not really the reason for overs and unders. When corrugated roofing was first manufactured it was made on a barrel corrugator (see picture) and the maximum sheet length was 3.000m or 3.600m.That is why all the old roofs on long runs had sheets end lapped. When the sheets were put through the corrugator the sheets had a overlap or an underlap on both sides depending on which way the sheets were laid.

Attachment 104809

The under sheet was always laid with laps turned up and then the next sheet was laid on top with the laps turned down. Hence the term overs and unders.

All corrugated roofing is now made on a rollformer (See Picture) and with sheets when they come out of the rollformer one side of the sheet has an upturn and the other side has a downturn which always goes on the top of the upturn so you can then start at one end of the roof and keep laying the next sheet on the top of the previous laid sheet.

Attachment 104810

As far as the wind is concerned you always lay the sheets away from the prevailing winds and if a roof is properly laid with the correct number and type of screws it is unlikely a roof would blow off anyway. In cyclone areas all roofs should be fixed with cyclone assemblys.

Originally Posted by Barry_White

Well that's not really the reason for overs and unders. When corrugated roofing was first manufactured it was made on a barrel corrugator (see picture) and the maximum sheet length was 3.000m or 3.600m.That is why all the old roofs on long runs had sheets end lapped. When the sheets were put through the corrugator the sheets had a overlap or an underlap on both sides depending on which way the sheets were laid.

Attachment 104809

The under sheet was always laid with laps turned up and then the next sheet was laid on top with the laps turned down. Hence the term overs and unders.

All corrugated roofing is now made on a rollformer (See Picture) and with sheets when they come out of the rollformer one side of the sheet has an upturn and the other side has a downturn which always goes on the top of the upturn so you can then start at one end of the roof and keep laying the next sheet on the top of the previous laid sheet.

Attachment 104810

As far as the wind is concerned you always lay the sheets away from the prevailing winds and if a roof is properly laid with the correct number and type of screws it is unlikely a roof would blow off anyway. In cyclone areas all roofs should be fixed with cyclone assemblys.

Interesting info and photos Barry , not overly relevant to the question.

So how many roofs , in New Zealand , were screwed down , rather than nailed down , before 1960 ?

And when did cyclone assemblies first get used in New Zealand ?

Note that the opening question refers to "old unpainted corro iron roofs pre 1960's"
And does not specify any particular place on earth.

cheers,
Jock

Originally Posted by mic-d

You better post the definitive answer Bazza, just in case I'm wrong

Cheers
Michael

This is my take on it but as I think Calm said it was in the process of dipping the sheets in the galvanising one side of the sheet received a heavier coating of gal than the other.

With the process of galvanising the heavier the coat of gal there is a direct relation to the time that the gal sacrifices itself to protect the black streel.

With Zincalume coating the relationship of time to the deterioration of the coating is enhanced by up to eight times. So with gal lasting up to 80 to 90 years zincalume could last up to 700 to 800 years.

As I heard it and it fits with what's been written. The older corro was 9 corrugations wide, that is, ends with 2 upturns or 2 down turns, to get the correct overlap, adjacent sheets were laid 'flipped'. Because the sheets have two different faces, they will experience slightly different environments in the manufacture or handling process if those processes require a particular orientation of the sheet to feed through machinery etc. Manufacturing tolerances mean that one side may get less gal, or some property that makes it rust faster. Modern sheets are 11 1/2 corrugations wide so could be laid either face up, they might therefore rust randomly or not at all for 700-800 years
Is that true Barry? Is Zinc better than the old gal?

CHeers
Michael