How strong are chimney stacks?

[Tim Watts]

I have a defunct stack which goes up through the hip face of the roof, is about 2x3 ft wide/deep and projects some 12 ft above the tiles including pots.

The reason I ask is tonight we have 63mph winds forecast. I'm not expecting it to fall down - it's survived worse, such as 1987. When I had my roof retiled[1], the roofers said it looked in good condition apart from some bricks have spalled a bit.

But how much margin does a 12ft high stack have against exceptional winds? I am merely wondering if some time down the line it might be worth taking it down, either to just above the tiles or even below the tiles and add the missing tiles back.

I have to admit though, as I sleep right under it, the high winds whistling around it do make me nervous as such winds almost always come in the wee hours around here.

Doing some research it seems chimneys falling over is rare, and there are some ancient houses in our village with really tall and weedly thin stacks which haven't fallen over although several do have iron bar ties to the ridge (whereas mine is tall but stout)

[1] A forward thinking person might have elected to shorten the stack then, but I was really tight on funds so extra jobs weren't really an option.

[Adam Aglionby]

Does it then create an unventilated void in the lower part of the stack?

Heard of dampness issues after chimneys have been capped, Borders has several buildings with stacks that would put the leaning tower of Pisa to shame, no one seems to panic.

[Tim Watts]

If I did, I'd leave it ventilated on top (the base already has vent grills).

Although right now, I'm only concerned with the structural integrity!

[harryagain]

They can fall down (straight through the roof sometimes or on your car) due to faulty pointing and the cement rotting away on the inside (Due to acid attack from soot) Tall ones are especially dangerous, it's only gravity holds them together. Also if there's been a chimney fire in the past it might be damaged. (Vertical cracks in brickwork) Lightening strikes cause similar damage. They should be examined by ladder) every ten years or so. So if it hasn't been done, it needs it. If faulty, fixing it can be a major expense.

Check right away with binoculars for cracks,loose chimney pots, cracked haunching and bad pointing. But you can't see all from ground level. Sooty marks indoors are another clue all is not well. Out of vertical is usually a demolition job. Don't attach high TV aerials.

If not in use, you should remove it. I removed two from my present house. Just went up there and pushed them over. They were in a really bad way. I dismantled them down to ground level, created lots of space in four rooms.

[Tim Watts]

Managed to answer my own question - hit the right google options after writing this - typical... Anyway - Part A building regs, page 33:

"The height of stack should not exceed 4.5 times its width."

The height is from the top of the stack or pots (highest) to the highest point the stack meets the roof.

Found a photo of the house, so can count bricks in the stack (old imperial)

26 brick courses plus pot so about 76mm inc joint times 26 = 2m plus pot (say 400mm), so say about 2.4m in total.

The width in the thinnest direction is 2 bricks long looking at an old photo, so about 0.46m (2 9" bricks)

The Part A modern permissible height for my chimney is 4.5 x 0.46 =

** 2.1m ** rounded.

So not far off and the top 0.4m (the discrepancy more or less) are two round pots with reduced wind load.

Actually goes to show that the 1950's build was pretty good to be this close to modern standards.

I suppose I will sleep a little happier now. if you don't hear from me tomorrow, you'll know why :)

[misterroy]

last winter one went through a kitchen extension in a moderate, for here, wind

[Tim Watts]

Any other info? Was it a very old house or was the stack thin and wibbly or in poor repair?

[tony sayer]

If it was that recent then might be made with cement mortar which always seems that bit stronger than the olde lime stuff..

Still very rare occurrence collapsing stacks..

[Sam Plusnet]

Does thermal expansion & contraction of that iron bar put any stress on the chimney it is intended to support?

[Tim Watts]

Not sure -

Here's one:

https://www.google.co.uk/maps/@50.984149,0.474942,3a,19y,292.71h,102.12t/data=!3m4!1e1!3m2!1s2xLh6dsFmPMZAIDTOdGfgA!2e0?hl=en

Stuffing that through gimp, I get a height to width ratio of about

8.2 !!!

Not surprised they added ties!

[charles]

Just before The Wrekin opened a severe gale in Shrewsbury removed many stacks since the aerial wire round the chimneys had been fitted without corner spreaders, and they were big aerials using Sutton. The Council, whose houses they were, were not amused.

[PeterC]

Don't sleep under the stack! A house in a village a couple of miles away had a stack go through the roof, but it was stopped by the galvanised cistern - and a lot of timber in the loft. Mind, middle of the night and a load of cold water and crud comes down...

[John Rumm]

While 12' is quite a projection, its also a decent girth, so I would not be worried. As you said, its been there through much worse.

[Tim Watts]

Sorry - I over estimated - more like a 8' projection after I did some brick counting...

[Tim Watts]

Short of the car, I don't have anywhere to sleep! both bedrooms are on that side - indeed the stack was for bedroom fireplaces.

Mind you the forecast has dropped back to 53mph gusts now...

[Capitol]

I removed two from my present house. Just went up there and pushed them

Get used to the idea, You're going to die. How is indeterminate!

[tony sayer]

In article , charles scribeth thus

Most all local authorities round this way, Cambridgeshire, absolutely forbade Aerials on chimney stacks, they had to be in cranked arm wall brackets to the council spec;!.

However as I 'm sure Bill W will tell you there're quite a few stacks being held together with aerial lashing wire;!...

[Tim Watts]

My aerial is on the other stack that comes up mid ridge. It's also the one that's really easy to work on as it's in the middle of the dormer flat roof :)

[Bill Wright]

Are you sure? It's hard to tell from the ground. Count the bricks in each course and the number of courses.

If your dimensions are right the chimney will be 2.5 bricks x 4 bricks. That isn't very big for a chimney 9.5 feet high plus pots.

Personally I wouldn't sleep under it until it has been checked. And I think I'd reduce it even if it turned out to be OK. I assume you won't ever use it as a chimney again.

The check is to get a ladder to it and test the condition of the mortar (and to a lesser extent the bricks). But to be honest once you get up there it's usually obvious if it's in bad nick. There's nothing magic about this. It's common sense.

On the good side, I've seen so many very dodgy-looking chimneys in my time, and really they do seem to stay up for far longer than you'd believe possible!

Here are some pics of dodgy mortar etc.

Bill

Bill

[PeterC]

All the best Tim - might be worth doing some careful checks when the wind drops. The chap who tol me about the collapsed stack was rebuilding a similar one - tall and on the hipped part of the roof - on a house about 25m away. He'd rebuilt the collapsed one. With his permission, I went up the ladder for a close look. I'd never realised how big those things were until I was by the base.