"Commander Kinsey" wrote in message
newsp.0d1qkkgdwdg98l@glass...
On Tue, 07 Jan 2020 23:18:22 -0000, Rod Speed
wrote:



"Commander Kinsey" wrote in message
newsp.0d1n2gjawdg98l@glass...
On Tue, 07 Jan 2020 21:57:36 -0000, Rod Speed
wrote:



"Commander Kinsey" wrote in message
newsp.0d1jwqxqwdg98l@glass...
On Tue, 07 Jan 2020 20:35:23 -0000, 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.

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

You can get flatish roofs lifted right off by strong
winds. basically due to the aerofoil effects.

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?

Flat roofs regularly get ripped right off and land on the
neighbour's etc. Corse that's in cyclones and hurricanes.

I wanted a more quantitative answer.

There isnt one. What it takes to lift the roof off depends
on too many other variables, particularly how well the
roof structure is attached to the top of the walls and
the detail of how the roof decking is attached to the
purlins. And the profile of the roof too. Mine has a
curvature across the roof in one direction achieved
by purlins of different depth. The aerodynamic effect
obviously depends on the wind direction. And mine
has quite deep barge boards about 18" high which
affect the wind over the roof too.

The limit would be 14psi, which would be horrendous, but I assume that
needs a lot of wind.

Yeah, normally only in cyclones and hurricanes etc.

We don't get those.

Yes you do, only the name changes.

"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