MTC_Wes
join:2004-04-22
Fergus, ON
|  Roof Top Mast
Hi all,
I need to figure out what's the best way to anchor a mast / tower on a flat roof.
The roof is part of a industrial building, and is actually the top side of a very large beam crane, so I would think that weight is not an issue.
Ideally I would like to have a 20 to 30 foot mast on top of the flat roof (roof top is already approx. 100'), the mast should be able to handle antenna standoffs and 3 large (6') sectors plus 1 or 2 ptp systems.
So... do I use a Rohn "type" mast and lots of ballast at the base plus at the guy points? Suggestions are appreciated.
P.S did I mention that the roof is 100' off the street level, and all the materials will have to be hauled up several sections of scaffolding 
Thanks |
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp | What is the roof surface...tar, membrane, other?
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Believer
join:2002-07-04
Baltimore, MD
| reply to MTC_Wes
Take a look at FDDI Systems. (»www.fddisystems.com/products/wir···unts.htm) They have a roof mount that can be taken apart in pieces and reassembled. Supposedly you can have a 20' mast attached that can be guyed to the base without any additional weight! It's in the $500-$600 range if I remember correctly.
Although three 6' sectors will definitely add some wind load plus the PTP antennas. You might want to look into guying it into the building. |
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superdog
I Need A Drink
Premium,MVM
join:2001-07-13
Lebanon, PA
| I imagine You read this: »erecting a tower or mast& to bridge or route?
I had a 30 footer on a roof 80 ft up with no problems. It does have to be guyed, but that is no big deal as long as You remember to NEVER penetrate the roof if it is rubber or flat with SIS rolled roofing on it. You can purchase non-penetrating roof mounts that just need a couple of cement blocks for ballast. You can then attach Your guys to the SIDE of the building using straps. If You put 3 guys per 10ft section, that mast will take anything You can give it, and then some!(I actually climbed mine like a monkey for something to do:D ). If You go to Lowes, they have 30 footers in stock. That mast with the mounts works well, and will give You years of service providing You install it correctly.:)
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MTC_Wes
join:2004-04-22
Fergus, ON
| reply to John Galt
John,
The roof surface is Tar..
Others: So could I simply have the shop weld up a 4'x4' frame that the bottom of the tower would bolt into, and that we could stack concrete blocks on? Then figure out how to tie the guy wires to points at the edge of the roof?.. Or is there an "proper" way to do this?
Thanks |
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
 ·CenturyLink
| said by MTC_Wes  :Others: So could I simply have the shop weld up a 4'x4' frame that the bottom of the tower would bolt into, and that we could stack concrete blocks on? Then figure out how to tie the guy wires to points at the edge of the roof?.. Or is there an "proper" way to do this? That would work...make sure that there are no sharp edges or points that could gouge the roofing. Roofers also have "pads" that they put under roof-top mounted equipment so that the equipment does not rest directly on the roof surface. They also use these to make walkways along the roof to HVAC units and such. You could probably get some of these to put under your ballast platforms.
Otherwise, this thing is going to "get stuck" to the tar like there is no tomorrow...both good and bad!
If it gets "stuck" it will cause the roof to leak. This tower is going to move. You may not see it but the tiny movements that it does are going to cause it to leak if you don't put the pads under it.
Concrete blocks, as such, may not be enough. Sand bags might be better. Do not load the roof to more than 125 pounds per square foot. make sure that your platform is set over structural members such as posts and glulams so that the weight is transferred to those elements. Try and avoid having the weight sit on the purlin's only.
As far as the guy wire anchor point...do the same thing for them. My suggestion there is that you have some type of connection point that allows the force to be applied to the ballast at the same angle as the guy wires.
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
·CenturyLink
| Typical roof structure details...
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
·CenturyLink
| Guy wire attachment...
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robbin
Premium,MVM
join:2000-09-21
Leander, TX
| reply to John Galt
said by John Galt :...As far as the guy wire anchor point...do the same thing for them. My suggestion there is that you have some type of connection point that allows the force to be applied to the ballast at the same angle as the guy wires. I'm not at all sure that this is a good idea. A guy wire can put a lot of force on the attachment point. I question if ballast is enough unless there is a solid connection (rod, pipe, etc) going back to the central mount to keep the guy attachments from sliding inwards. I would much prefer to see solid attachment to the sides of the building for the guys. |
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
·CenturyLink
| said by robbin :I'm not at all sure that this is a good idea. A guy wire can put a lot of force on the attachment point. I question if ballast is enough unless there is a solid connection (rod, pipe, etc) going back to the central mount to keep the guy attachments from sliding inwards. I would much prefer to see solid attachment to the sides of the building for the guys. Oh...I agree, but he may not be able to make that kind of attachment due to the structure. I have seen this arrangement work before and the only reason that I suggested it was because he said the tower was going to be "short"...
Many times the parapet wall is a "scabbed on" element, and not suitable for structural attachment. It is possible to tear this entire portion of the building loose...and that is a bad thing. Unless a person understands how buildings are constructed, they may THINK that they have attached to something structural, but that is not always the case.
There is no reason not to make a roof penetration...if it is done with care and in an appropriate manner with an understanding of the issues involved. Simply bolting into the roof sheathing and globbing some silicone rubber on the top of that is not sufficient. Consulting an expert is always advisable, especially if PVC membrane roofs are involved.
We did over a 1,000 roof penetrations at the Seagate facility in Milpitas, CA. This was an existing building and we had to do them all ourselves...but we had direct previous experience and had watched the roof guys for years. So I don't get too freaked out about doing them...
Funny story...
I am sure that you are familiar with the "Black Jack" line of roof sealing products. We were on the roof and the engineers came up there, just walking around like there was no problem. Not to make disparaging comments, but they were the typical "office engineer" types...jackets, shirt and tie, slacks, loafers...plan set. We are the typical totally sunburned "low-life" installers...what do we know??!!
I saw them and walked over and they are like 'what do YOU want?'
"Well, you might want to be careful where you walk..."
"And why is THAT?"
"Because there is 'Black Jack' on the roof..."
"SO...??"
I looked at him and said "well, that is what you say NOW, but that is NOT what you are going to say when you see it all over the carpets in your BMW."
After that you would think that they were stepping on little crystal fairies the way the were tip-toeing around. They left shortly thereafter and we never saw them again.
That was back when I was young and foolish...now I'd probably just let them step in it...!
Next drawing...!! More to follow...I'm in "the mode" today, for some reason.
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
·CenturyLink
1 edit | Alternate guy attachment method...more involved but much better.
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amp13
join:2002-10-14
Mountainburg, AR
| reply to MTC_Wes
The FDDI mast is similar to what we simply weld up in the shop. I've done three. All 3 are 20ft masts in 5ft tripods, which are mounted to 8x8 frames made from 1½" angle. The mast is guyed to all four corners of the frame from 12ft up. We used 12 sand bags, 3 on each side of the frame, approx. 600lbs. These sit on top of 1" thick rubber stall mats pulled from some empty horse stalls in my barn and can be purchased at Tractor Supply. |
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MTC_Wes
join:2004-04-22
Fergus, ON
| reply to MTC_Wes
Thanks guys for all the info...
FYI the roof is what i would consider aluminum siding with tar and pebble stones on top (no rubber membrane as far as I can tell).
Although it would be nice if we could secure the guy wires to the roof by means of bolting through it... The roof is about 80 to 100 feet high, and the only way to get close to that hight (on the inside)is a service elevator that has been locked out by the TSSA. I'm going to climb to that roof sometime this week, and try to spec out what to mount the guys to. It's possible that i could attach to some existing structural point on the outside... I'll just have to see what I find. |
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chiefbmr
join:2003-04-08
College Station, TX
| reply to MTC_Wes
said by MTC_Wes :Ideally I would like to have a 20 to 30 foot mast on top of the flat roof (roof top is already approx. 100'), the mast should be able to handle antenna standoffs and 3 large (6') sectors plus 1 or 2 ptp systems. How do you plan on installing the PTP systems and aligning them on a mast? What if you have to switch an AP on one of the sectors, will you have to bring down the whole mast? I have seen it done with an extension ladder and lean it against the mast, and it's not fun.
If I were doing it, I would spend a little more money and get a Rohn 20G tower. You can probally find 10' sections for around $70. They do weigh a bit more so you would need to make sure you have a sturdy support on the roof to install on. You would still have to guy it, but the maintanence is much easier. |
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MTC_Wes
join:2004-04-22
Fergus, ON | cheifbmr,
Sorry, bad wording... Mast really = tower... that one could climb and work off of.... and yes, it's a Rohn 25G tower.. |
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wispman
join:2004-12-21
USA | You could always strap two extension ladders together and you've got a step ladder. I would seriously try it. As long as I had a guy at the bottom on each side to hold the legs. |
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superdog
I Need A Drink
Premium,MVM
join:2001-07-13
Lebanon, PA
| said by wispman :You could always strap two extension ladders together and you've got a step ladder. I would seriously try it. As long as I had a guy at the bottom on each side to hold the legs. You are insane!:D(But I like it, cause' so am I!!):)
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MTC_Wes
join:2004-04-22
Fergus, ON | reply to MTC_Wes
wow.. that's a bit crazy.... |
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PersComp
Premium
join:2005-08-17
Cayce, SC | reply to superdog
That sounds like a "Hey ya'll, watch this" moment...lol |
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