Why secure a loft trapdoor in a storm?

Why secure a loft trapdoor in a storm? And what's the roof pitch to do with anything?
"Close and secure loft trapdoors with bolts, particularly if roof pitch is less than 30°"
https://www.metoffice.gov.uk/weather/warnings-and-advice/seasonal-advice/your-home/stay-safe-in-a-storm
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On 07/01/2020 20:01, Commander Kinsey wrote:

Bernouilli.
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I don't know their reasons, but mine used to come open during high winds dumping tons of old muck and dust onto my landing. Its now held shut with duct tape. No nasty whining noises or rattles either. The roof on this property is open ie it can bee seen there are gaps under the tiles, so one assumes if that hole was through to the house and a really spectacular wind came it could blow many tiles off the roof. Brian
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Ah, you mean one of those strange ones that opens downwards? Mine lifts up, a much more sensible arrangement. It simply cannot open itself, it's too heavy.

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In theory you can get a suction effect similar to what you get with an aircraft aerofoil section.
In practice it isnt seen often enough to matter.

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I wonder what the roof pitch has to do with it.

ch is

ice/your-home/stay-safe-in-a-storm

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That's what determines whether you get the aerofoil effect.
When the pitch is too high, you don't.

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But how can that get to the loft hatch?

itch

dvice/your-home/stay-safe-in-a-storm

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wrote

The suction effect in the roof space sees air moved out of the house into the roof space, lifting the hatch.

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how could that happen? Surely aerofoil works on the roof itself, lifting the tiles off.
And my hatch is made of wood, way too heavy to lift like that. And hardly the end of the world if it opened, it might give me a fright that's all.

-advice/your-home/stay-safe-in-a-storm

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wrote

The tiles arent an airtight surface like an aircraft wing so you do get considerable movement of air out of the roofspace before the tiles come off.

You're wrong when the roof is of lower slope.

Sure, but that's just as true of the other stuff like shutters that it suggests should be shut and secured.

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On 08/01/2020 00:53, Rod Speed wrote:

What of the possibility of air being able to rush from the house into the reduced pressure of the loft, so suddenly changing the differential pressure between the inside and outside of the roof and making it more likely that the roof is damaged or ripped off?
SteveW
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wrote

It doesn't work like that, essentially because the hatch lifted by the reduced pressure in the roof space doesn't see enough air moving thru the narrow gap around the edges of the hatch to matter.
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Who has a loft in a roof under 30 degrees? That wouldn't be possible.

itch

dvice/your-home/stay-safe-in-a-storm
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wrote

You do so a few mansard roofs like that.

Corse it is with a mansard roof.

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That doesn't make sense. Look at the picture in https://en.wikipedia.org/wiki/Mansard_roof There are rooms in the roof.

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wrote

Corse it does.

They arent all done like that.

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Your turn to provide a picture.
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wrote

Don't need to. Plenty of places with mansard roofs have storage with access hatches in the roof space even tho that isnt as useful as with a normal pitched roof.
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The conservatory I've half built is staying firmly put. I guess I bolted it together well :-)

ch is

ice/your-home/stay-safe-in-a-storm

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