I can'ty give you a definitive answer but a 30 foot leylandi in a neighbours garden was badly damaged in a gale a few months ago and a large ( 12 foot section) broke off.

How big is the leylandi in question?

A big part of trees blowing over is soil type and depth. There is a large conifer plantation/mixed woodland near my house and a lot of trees fell in the storm of last year. It was only conifers that fell and looking at their roots they were shallow and almost looked pot bound (clay soil area, though the woods are ancient in places, so thick humus on top). The native trees all survived, am guessing due to deeper roots systems. Trees like Leylandii are shallow rooted as many gardeners have found.

I would worry about letting any conifer in a garden get above cutting height - say 12 foot max as I do not think they the roots can support the height, especially in exposed areas and high winds.

30ft is around 9 metres - way too high for most houses and to be fair on neighbours.

Thanks for data. Tree is 8 or 9 metres high, with a cyprus fir growing beside it.

If this tree was in my garden I would be removing it pronto.

It's impossible to give you a definitive answer - many trees were blown down in the 'hurricane' in 1987 - but the cause was not only the exceptionally strong wind, but also other conditions. Had the trees not still been in full leaf, had the ground not been utterly saturated from a very wet year, many of those trees would have remained standing, or just lost some limbs rather than being completely uprooted. So the environmental conditions at the time have an impact. The ultimate height of a Leyland cypress is unknown - they appear to just keep getting taller, but the tallest one currently is around 130 feet. I wouldn't fancy its chances in a open, exposed situation in another hurricane...

A neighbour had a leylandi about 9m tall. Last spring it blew down across the fence and our vegi plot!

True I had forgotten that!