On 11/01/2020 3:06, Commander Kinsey wrote:
On Fri, 10 Jan 2020 06:24:53 -0000, RayL12 wrote:

On 07/01/2020 20:35, Rod Speed wrote:


"Commander Kinsey" wrote in message
newsp.0d1gkmz9wdg98l@glass...

Why secure a loft trapdoor in a storm?

And what's the roof pitch to do with anything?

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.

"Close and secure loft trapdoors with bolts, particularly if roof
pitch is less than 30°"

https://www.metoffice.gov.uk/weather...afe-in-a-storm



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.

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.

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.