Stainless dish masts

[Windmill]

I was looking for a 2 meter cranked stainless steel mast which would (a) give me some adjustment ability (maybe allowing me to put a large dish above the bend, if forced to by clearance requirements, and (b) be strong enough to withstand large wind forces.

It's to go 75 feet up on the roof of a tenement building, where winds occasionally gust to over 100 m.p.h., so the usual aluminium masts might not be strong enough. Small Sky dishes there are usually put on very short masts just above roof level. Steel would probably be strong enough, and I could take the slightly irresponsible attitude that it won't rust through until after I'm dead, but stainless would be tougher and wouldn't corrode.

I don't see any stainless masts on roofs, and I didn't see any on Google.

Does anyone make such a thing? One metalworking place suggested that making one for me would cost well over 100.00, which seems unreasonable. (But he was a Scotty-sound-alike, only worse!)

In the meantime, I'm going to try to put the dish in the lee of the chimney stack, as near to roof level as possible, as others do with smaller dishes, and will paint a steel mast as best I can.

[Nightjar]

I suspect that the advantage of an aluminium mast would be that it is likely to fail before the bit of building it is attached to.

Colin Bignell

[Dave Plowman (News)]

You could see if you have a decent car stainless steel exhaust maker anywhere near - they can bend up tube easily - and I can't see it costing that much.

The snag with mounting a dish on a pole is movement. Even quite small movements can cause problems.

[alan]

Scaffold poles are often recommend for this type of installation.

[Bill Wright]

Cranked masts are not a good idea for dishes in windy places.

You've gone quite some distance along your thought journey, but you should have asked for directions before you set off. The best mast to use is aluminium scaffold tube. If you need a lot of stand-off use big galvanised wall brackets. If you really want it to last forever you need to protect the clamps with something. Spray-on underseal is OK.

Bill

[Windmill]

If the attachment to the building was strong enough and didn't fail, it would not necessarily be an advantage if the aluminium mast failed and as a result an 80 cm. dish and motor blew over the edge of the roof and fell 75 feet to the ground. To put it mildly.

[Windmill]

A good thought. I wonder what gauge they use? Was thinking 16 SWG / 1.5 or 2 mm. wall thickness ought to be quite strong if it's stainless.

I'm thinking that the dish mounting point on the pole will be quite close to the point at which the pole is attached to the chimney stack. (Unless dish clearance from roof etc. is a problem, but I hope not.) The upper part of the pole is only for a little 12 element terrestrial aerial. But I take the point about leverage, which is why I'm concerned. (Actually, even the little Sky dishes are on poles of a sort, but they're very small and short poles).

[Windmill]

I had thought about that, but couldn't find a single pole, just large scaffold sets. And ideally the pole's upper part would be cranked to allow some juggling.

[Windmill]

Absolutely. That's why I'm concerned to get maximum strength.

My hope is that when I have all the pieces to hand, I'll find that the dish won't need to go on the cranked part at all, but will be attached near the base of the mast. The cranked part will then just support a small 12 element antenna above the dish. Even if the dish had to be raised a little I don't think it would need to be more than a foot or two above the point where the mast is attached to the chimney. And I know that the dish needs to be as close to the roof as possible, where winds are less. (Something to do with boundary layers as I remember.) Usual practice here seems to be to put the base of the dish just a few inches above the flat roof.

Well, I know you're an expert, but believe me I did my best to think it through very carefully. Which doesn't mean I thought of everything.

Dishes can't go on the walls of the building in question (a long row of tenements); the walls are aligned almost exactly with the 2A-B-D satellites. Which weren't there in 1890. So everyone's dishes are near the edge of the flat section of roof.

I did look at scaffold tubing online, but didn't know there was such a thing as aluminium scaffold tubes. All the ones I saw were galvanised.

It still seems to me that stainless would be tougher. (Unless metal fatigue could be a problem? Surely aluminium would be worse?)

The bracket is substantial with 9" stand-off overall so the uncranked section of the mast will be about 6" from the chimney.

Doesn't have to last forever, but I'll try the underseal.

I'd like to think that it will be safe and last at least until the next major roof repair (when it's bound to be damaged and/or removed).

It wouldn't be difficult to do better than the incredible collection of toppled aerial masts, most with half a dozen different twisted antennae stacked one above the other only a few inches apart, with separate coax presumably going to different flats, which one sees on that roof.

I should post a photo!

But dishes are heavier than antennae so I have to try to be more careful.

Thank you for your help.

[Nightjar]

I wasn't thinking so much about the strength of the attachment, as the strength of the bit of building it is attached to. Make the mast and attachment strong enough and, in a really high wind, you might see the dish blowing over the edge, taking a chimney stack or a large piece of wall with it, depending which you have attached it to. OTOH an aluminium mast might simply bend before that happened.

Colin Bignell

[Bill Wright]

Is this a flat roof or a pitched roof? Pitched roofs can have horrendous winds sweeping up them. They can literally lift a man off his feet.

Oh it's flat! Thank goodness!

No, look, there's a whole industry beavering away here competing to find the best solution to these problems. No-one uses stainless. The transmission industry doesn't use stainless for stress components. I've seen what happens when people use lengths of silencer, etc. The tube wall is thin so the tube deforms under the V bolts. A hairline crack appears and eventually the tube fails. Also, the teeth on the clamps can't dig in so it's impossible to get a twist-proof fixing. So you over-tighten and the tube deforms even more. The problem with steel masts is that if you want a really strong one the tube wall has to be thick, and the weight is too much. Are you aware of the tube wall thickness of an alloy scaff tube? Believe me they are strong, yet reasonably light. The way to buy a short length is to go to a scaff hire company and ask. They often have to cut tubes to length and the offcuts normally just go for recycle. In any case, alloy scaff tubes are not pricey.

It's a pity you aren't here: I have a pile of them. There's no way an alloy scaff tube used the way you're going to use it will break. I've done hundreds of installs like that, many of them on tower blocks. Does the tube have to be 2" OD by the way?

Bill

[Bill Wright]

Bill

[Bill Wright]

Is this a flat roof or a pitched roof? Pitched roofs can have horrendous winds sweeping up them. They can literally lift a man off his feet.

Oh it's flat! Thank goodness!

No, look, there's a whole industry beavering away here competing to find the best solution to these problems. No-one uses stainless. The transmission industry doesn't use stainless for stress components. I've seen what happens when people use lengths of silencer, etc. The tube wall is thin so the tube deforms under the V bolts. A hairline crack appears and eventually the tube fails. Also, the teeth on the clamps can't dig in so it's impossible to get a twist-proof fixing. So you over-tighten and the tube deforms even more. The problem with steel masts is that if you want a really strong one the tube wall has to be thick, and the weight is too much. Are you aware of the tube wall thickness of an alloy scaff tube? Believe me they are strong, yet reasonably light. The way to buy a short length is to go to a scaff hire company and ask. They often have to cut tubes to length and the offcuts normally just go for recycle. In any case, alloy scaff tubes are not pricey.

It's a pity you aren't here: I have a pile of them. There's no way an alloy scaff tube used the way you're going to use it will break. I've done hundreds of installs like that, many of them on tower blocks. Does the tube have to be 2" OD by the way?

Bill

[Windmill]

That was certainly one of the things which worried me. Unlikely, I believe, because it's a pretty solid chimney, measured at 12 feet wide by 2-1/2 feet deep by who knows the height (much is below the roof).

I would have used lashing wire all around the chimney stack, but someone got there first to lash a pole and terrestrial antenna on the other side of the chimney, and it would be a little awkward to add another lashing. (Which may well explain the sets of 5 or 6 antennae stacked on the same pole which you see all over this roof; those who installed the untidy mess thought that adding another pole wasn't an option. Or they were just cowboys.)

I've used a mast attachment bracket with a 9" standoff, whose mounting surface is 760 mm. wide by 255 mm. high, bolted into multiple bricks by

12 satellite lag bolts.

And the dish isn't going on the end of a long pole; I abandoned that notion quite early on. It'll be as near to roof level as I can get it.

But I'll still be thinking about it, and probably checking, after the first big winter storm.

Large chimney pots (bigger than you think, looking from the street) blow off these old buildings every winter, but I don't want to see my dish fall into the back green. (Or, worse, somehow end up in the street).

[Windmill]

So about 120 m.p.h. (free fall speed for a parachutist).

It's a standard tenement building, so flat in the centre, but with 45 degree (by inaccurate eyeball) sloping sections over the loft area on either side. It'll be about 15 metres from top to gutter a.k.a. rhone. And unfortunately that is also the only place where the chimney edge is accessible from the flat roof, so that's where everyone mounts masts, dishes, etc.

Yes, I wondered about the thickness.

I have a small cranked steel mast above my own flat (this is for a tenant), and as you say it was fairly heavy. But that's only 35 or 40 feet up (again on a central flat section of roof) so it's not exposed so much to the wind.

I'll ask, but I've never seen anything but galvanised steel scaffolding here.

No, 32mm would be fine if strong enough.

[Windmill]

Bookmarked, and thank you.

[Windmill]

FEET, I meant FEET!

[Bill Wright]

Must be then. Just a few times over the years I've been on a roof when the wind has got up suddenly, and if it was coming at right angles to the ridge I have had to cling on to the chimney. Very very frightening. You can tell it would easily pick you up. It blew my glasses off my face once. I had my eyes closed but I felt them go. The customer found them for me when I finally got down. They were in the privet.

If it catches you like that all you can do is sit tight curled up as small as possible and cling on and wait until the wind drops. When it starts up very suddenly it always seems to drop suddenly after a short time. Once I was on a flat bit of a roof that was otherwise pitched and the roof ladder next to me went up in the air and landed in the street. At the same time the aerial on the other side of the chimney snapped. It's very deceptive is the wind.

If you go up a pitch roofed building from the downwind side it seems fine until your head is just above the ridge. At that point there can be an absolutely roaring gale as the wind is forced up the pitch of the roof. You can't work in it; it's impossible. The warning you get is that the aerial will be whistling, and as you shove the roof ladder up, when the hook gets to the ridge a rushing noise starts up and you can sometimes feel a bit of vibration. Often at ground level on the windward side it seems fine; just a bit draughty.

Incidentally I've worked on very tall buildings where at ground level it seems far too windy to go up, yet on the flat roof there's almost a complete calm. And vice versa.

In the days when all the chimneys smoked, I'd often be working at teatime just when everybody lit their fires. Sometimes the smoke, for some reason, instead of going up would hang about and drop down. It's impossible to stay there when that happens, unless you can find a slate to put across the top of the pot. The trouble is, that makes people come running out of the house.

The worst thing the wind's done to me was when I was about 18. I used to tie the ladders onto the roof with coax. One day it snapped because of the wind. I drove down the A19 wondering why people coming the other way were flashing their lights. The roofladder, which was a long one, about

20 feet, had turned on the roof and was across the opposite carriageway. It's a good job I didn't meet a double decker.

I was more thinking of 3" tube.

Bill

[tony sayer]

Didn't we all use co-ax;!. The cowboy outfit that I joined once had a roof rack where the ladders were on vertical as it were, sort of, on either sides of the roof flat, but vertical. One of the roof ladders was an olde wooden one with two bits of bent metal as "hooks".

One day the hooligan I was driving with overshot a red light and the roof ladder hooks did just that got themselves hooked into a large lorry and snapped some 8 feet of ladder off our vehicle, so we saw the lorry c/w 8 foot of ladder go off down the road, gave chase as best we could but fortunately instead of hitting a pedestrian or cyclist it walloped a lamp post which broke off the remains of the ladder and the hooks were still stuck on the lorry.

Anyways we decided to "leg it" and some poor driver must have wondered just how he acquired a couple bits of bent metal in the side of his lorry!...

[Windmill]

I can imagine. I've occasionally (long ago), when the weather changed unexpectedly, been in howling gales while hiking on mountain tops. The sort of thing where you're walking at a 45 degree angle. That's bad enough. I don't know how the rock climbers manage to deal with that.

Wind shear. It can cause problems for planes as they land. One moment the plane has a proper airspeed, and then a few feet lower, alhough the plane's speed over the ground hasn't chamged much, the plane's speed _through the air_ has dropped sharply and the stall warnings are sounding.

Shortly followed by the fire brigade, I would imagine :-)

Presumably clamps would be more difficult to find.

They don't seem to say anything about wall thickness of those aluminium scaffold tubes you pointed me towards, but they certainly look substantial in the picture. AIR the biggest were about 50 mm dia. so maybe 5 or 6 mm. walls.