Why secure a loft trapdoor in a storm?

[Commander Kinsey]

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°"

[GB]

Bernouilli.

[Brian Gaff (Sofa]

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

[Commander Kinsey]

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

[Commander Kinsey]

[Commander Kinsey]

The conservatory I've half built is staying firmly put. I guess I bolted it together well :-)

[Commander Kinsey]

Not done the roof yet. Hmmm, perhaps I should secure it very tightly. Mind you polycarbonate can snap easily in a wind. How powerful is aerofoil in a wind? The limit would be 14psi, which would be horrendous, but I assume that needs a lot of wind.

[Commander Kinsey]

But how can that get to the loft hatch?

[Commander Kinsey]

Who has a loft in a roof under 30 degrees? That wouldn't be possible.

[Commander Kinsey]

I wanted a more quantitative answer.

We don't get those.

[Commander Kinsey]

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.

[Commander Kinsey]

That doesn't make sense. Look at the picture in

are rooms in the roof.

[Commander Kinsey]

Somebody must have measured what wind speed is required to develop a vacuum of a certain PSI.

Because of the lack of severity.

[Commander Kinsey]

Your turn to provide a picture.

[Steve Walker]

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

[RayL12]

I agree. A low pressure from the outside will pull on a roof's attic space to try to equalise with the internal pressure of the attic/home.

If the roof opens a pathway is created. If the attic door opens, another pathway is created. This very small pathway then becomes subject to the mass of air in the home. This mass compresses and speeds up as it passes through the smaller hole and can be a very strong rush of air.

[Commander Kinsey]

For all that to happen, you have to lose part of your roof. This is unlikely, and if it did happen, an opening loft hatch would be the least of your worries. If the roof doesn't fail, the pressure of air in the loft and the house both remain at 14psi, so no hatch moving.

[RayL12]

Correct, it's all about 2 pressures wanting to equalise. Whether you lose your roof and even your loft door depends on the pressure difference and the speed it presents itself and, the time it exists. If your roof and ceilings structure cannot allow the equalisation to happen, the forces that keep them to your building can be overcome.

[Commander Kinsey]

My point is.... assuming your roof is still on firmly, there is very little pressure difference between your house and your attic. So no reason for the trapdoor to move.

[Commander Kinsey]

The amount of air travelling through the gap in the eaves isn't enough to change the pressure in the attic so it's a big difference from the rest of the house to cause the trapdoor to move.

Mine never has. But then it's proper wood, not flimsy modern plastic.