I’ve learned a lot by reading dozens of threads on the forum. I now have a Davis station on the way and I have some questions about setting up a mast to mount the anemometer on.
I’d like to mount the anemometer on the roof. All the homes here are two story. I’m thinking of strapping a 10ft or 15ft 1.5" pipe to an existing sewer vent pipe for support. The vent pipe rises about 4-5’ above the flat roof and is about 3" across. My main questions are, given that the vent pipe is solid, is this enough support for a 10ft or 15ft mast (without guy wires), and what would be the best way to go about securing the mast to the vent pipe?
Should I use basic steel strapping? I’ve looked around online for brackets to connect two pipes together (parallel to each other) but haven’t found any – just wall mounts – but maybe I’m looking in the wrong places. It seems like brackets would be the easiest to undo to take the mast down if I needed to do maintenance on the anemometer later.
Speaking of maintenance, for something this height, and keeping in mind that I won’t be dragging a ladder onto my roof, is it worth using some sort of telescoping mast?
As for the location… I live in San Francisco. My house is definitely in an urban area, on the south-east side of Mount Davidson (http://en.wikipedia.org/wiki/Mount_Davidson,_San_Francisco,_California). I’m at about 535 ft, which is a little over halfway up. About 3 blocks down (SE) from the tree covered hilltop if you’re looking at the Google satellite image.
All the houses here are attached, and the neighbors’ roof on one side is 4 ft lower and on the other side, 4 ft higher. The vent pipe is about 7 ft from the edge of the higher neighbor’s roof line. I figure a 10ft mast would get me about 6-7’ above the neighbors’ roof, and about 32ft above street level.
Welcome to the forum and I hope you have a lot of fun with your new station.
What is your vent pipe made from and how solid/well fixed is it. In the UK these kind of pipes are often cast iron so they’re very solid, but being cast iron I’d be worried about putting too much compressive stress on them when strapping another mast on. A 15ft mast with a 4-5ft overlap, i.e. 10ft unsupported, should be OK in 1.5" thick wall aluminium…2" thick wall would be even better. I’m not sure if you’ll find any pre-made brackets…I wouldn’t think it’s something that’s done often enough to warrant anyone making them. This is based on the UK…what’s available in the US market may be totally different!
Radio Shack used to sell a vent pipe mount but I don’t see it in the catalog now (I have a couple somewhere) but maybe the drawing will give you some ideas. Not sure it would go to that diameter though.
I would be very cautious about adding that much (unguyed) onto the pipe without knowing exactly how it is supported under the roof. If it feels solid then another maybe 6 feet would be OK.
Nice location you have there, great site for an anemometer right on top of the hill, sturdy support too :lol:
Typically vent pipes are not supported by framing or even attached to the roof other than a rubber waterproof seal. Just a free standing pipe as they are not intended to be used as a support. If it were mine, I would not attach it to a 4’ PVC vent pipe. BYMMV.
Hmm, thanks everyone for the info, and the links to the vent pipe mounts!
The vent pipes here are definitely metal, probably iron, so I’d expect it’d have some sort of support within the framing just due to the weight, but that’s an excellent point to keep in mind. Hmm! I’ll have to go up on the roof to verify how solid it feels, I haven’t had cause to check it out from that perspective before (and I don’t get up there much, have to use a 28’ extension ladder).
The RadioShack pdf linked above recommends oly using it on metal pipes, and then using guy wires for over 5’. Of course,a lot of vent pipes I’ve seen tend to be much shorter, like 1’, so they may have that in mind.
I see people mount digital satellite dishes to them (the 18" elliptical kinds) all the time, but of course they’re not going for height on those.
Elderberry, what was the existing pipe you attached to?
If I were going to use guy wires, it seems like at that point I might as well just use a tripod mount with anchored feet. Are those capable of 10-15’ without guys? But I’d rather avoid penetrating the roof if possible. Would one of those non-penetrating roof mounts, the kind that uses concrete blocks for ballast, be sufficient for that height? A lot of the roof mounts of that sort I see only talk about a 60" mast, but they’re also expecting you to put a large DBS dish on it. Is there typically any concern about the load on the roof with these mounts, with all that ballast? I’d guess that not given that the distributed weight is well less than a person…
I was doing some work up the hill a few days ago, so I took a couple pictures. They’re not much use for examining the actual vent pipe due to the distance and the facade blocking, but I thought I’d post them so that it’s easier to picture where I’m imagining putting it.
(large images) http://mooism.com/roof1 My house is the center (with green paint). The gabled portion is a facade. The majority of the roof is flat, like the neighbors’ houses. Unfortunately at this angle you can’t see much of my actual roof, other than the vent pipes rising into view.
http://mooism.com/roof2 Here you can also see my neighbor’s radio mast. Not sure how he has it secured. Given the proximity to the other neighbor’s wall I’m wondering if he used that instead. He has no guy wires. For comparison, his looks to be about 12’. Probably 10’ pole + 2 ft vertical antenna.
When I get on the roof, hopefully this weekend, I’ll examine the vent pipes and take some close up pictures.
Since you have a gabled roof, I would use a tripod mount. If you buy Radio Shacks, they used to come with pads you place under the feet that get mashed down and water proof the joint.
You can see them here: www.ambientweather.com/amweeztarpad.html
If not, you can purchase these pads seperately. I’ve used tripods for years, and never had a problem.
It can handle 15ft of mast without guy wires as long as you don’t put anything up there but the anemometer.
Be sure to put blocking under the roof decking under each foot that doesn’t land over a roof rafter. Tie the blocking into the rafters. You need these to securely run the lag bolts into.
Most tripods are 3ft tall, you can also buy 5ft tall tripods, that would get you up a little higher and still only have 15ft of pole in use.
To clarify, the roof is flat. The pitched portion at the front is just a facade, like on the houses on either side. Unfortunately at the angle the photo is taken it’s not very easy to see my roof past the facade. There’s also no crawl space between the roof and second story ceiling.
If I went with a tripod I’d probably use one of those non-penetrating mounts with concrete block ballasts.
Yup, that would be my choice also. A metal vent that was solid would also work fine especially if they are mounting satellite dishes to them. Here we have PVC vents about 1 foot above the roof which would not be suitable at all.
I’ve decided to go with the vent pipe mount attached to a cast iron vent pipe, and I’ve procured some vent pipe mounts thanks to niko and Elderberry. Now my considerations are:
10ft or 15ft? 10ft will get me about 6-7’ above the next door neighbors’ roof, but probably only about 2-4’ above the roof of the neighbor after that (second house up the hill – they’re the highest on the block). A 15’ would obviously up that to about 11-12’ and 7-9’ respectively. Is the extra height important? The wind does frequently flow from that direction. The Davis guide says “4’ above the roofline” but doesn’t discuss how close other obstructions are. CWOP guide indicates 7’ feet above obstructions within 20m (the second house down is no more than 10m).
Wind-loading wise, it seems like from my poor maths that an 18" satellite dish mounted at 4’ (like I see on other homes in the area) would be a good 25% greater than an anemometer + pole, even with the extra leverage of the 15ft vs the 5ft.
Lastly, I’m guessing the top end will be open, allowing rain inside of it. Should I let the pole sit on the roof to support the weight so the vent pipe doesn’t have to, or should I be using the clamps to keep it from touching the roof? If the former, do I need to be concerned about water building up in the mast? From the mast photo it looks like the one end has a notch in it. If that end were down it’d seem like that’d be sufficient to let water drain.
On the “done” side, I’ve mounted the ISS on a ~6’ foot fence post and it’s been working great so far. http://mooism.com/iss
I’m looking forward to wind data to go with it!
They are guidelines. Just do the best you can for your location. I would need a 100’ tower to meet the CWOP requirements.
If I go 10', I'll probably just get one of those radioshack heavy-duty antenna masts. They're 1.25"
http://www.radioshack.com/product/index.jsp?productId=2062034
Even high quality TV mast is pretty flimsy at unsupported lengths longer than 10’. You need a larger diameter mast to go higher.
3. Lastly, I'm guessing the top end will be open, allowing rain inside of it. Should I let the pole sit on the roof to support the weight so the vent pipe doesn't have to, or should I be using the clamps to keep it from touching the roof? If the former, do I need to be concerned about water building up in the mast? From the mast photo it looks like the one end has a notch in it. If that end were down it'd seem like that'd be sufficient to let water drain.
Don’t let the mast touch the roof. It will wear a hole in it.
On the "done" side, I've mounted the ISS on a ~6' foot fence post and it's been working great so far.
http://mooism.com/iss
I'm looking forward to wind data to go with it!
Being conservative I would go with the 10 foot. If higher try to get a longer tube before using a joined tube, look at the fencing section of HD. I use one of those 10’ poles as sold by RS and they are pretty solid. I think you would need a lot of force on a U Bolt to clamp the swaged joint (although I haven’t tried it) and then you would probably not get it apart again. I would suggest drilling and using a couple of sheet metal screws (drill a clearance hole in the outer tube).
As talbert1952 says don’t let it contact the roof.
Best to keep the water out, mine is capped with a suitably sized PVC pipe cap. You may not get one that fits exactly but a bit of silicone will take care of it.
I’m not sure I’d cap it. I’ve never capped any of my antenna masts and I’ve not suffered from any problems from it. Capping at the top will still allow water vapour in at the bottom and any that does get in will condense on the cold metal. The cap will trap the condensation inside the tube which means it may be pretty continuously damp on the inside surface of the mast (depending on local weather conditions of course). If you don’t cap it then you’ll get some air flow through it (probably convective on a sunny day) and that will tend to keep it drier.
I second not capping it.
Also, for what it’s worth, I had an anemometer up on 15’ of non-guyed Radio Shack mast for many years, and it held just fine. Survived many a 50+ gusts.
The two pieces of masting are made to fit firmly together. You don’t really need anything extra.
Just use a rubber mallet to make sure they are seated together well before you install it.
I capped it because it appeared to be a great condensation generating device in the ocean fogs we get here. Now it’s capped I don’t see water running out of it when we have fog - which is often.
Chris, are your masts steel or aluminium? Hard to get aluminum here but I remember using it all the time in the UK.
Seems like it’ll be fine capped or uncapped, as long as water can get out the bottom.
looney, nice to hear someone’s experience with the RS masts. That encourages me. I think I tend to lean towards using them simply because I’m so undecided on 10’ vs 15’, I feel like with the joined bit, I can postpone the decision until the last possible moment.
I like that idea, the question just becomes whether that’ll encourage any kind of moisture buildup issue under it. And I guess porous or non-porous. The former might absorb some water, but it seems like it’d also allow easier evaporation, whereas the latter seems like moisture might just collect in the shade under it.
Can anyone recommend techniques for plumbing it, should I find it necessary? If I have to shim it, it seems like even a few mm of shim can cause a major angle of change in a tall mast, thus I’d need some sort of shim that I can be durable outdoors even when very thin. If I’m lucky the vent pipe and the mast after mounting will all already be plumb…
I’m also looking into setting up a webcam, but that’s a different project.
