[TowerTalk] Metal Roof

Grant Saviers grants2 at pacbell.net
Thu Jan 11 20:31:56 EST 2018

For the old style galvanized corrugated panels on my barn and shed I 
added a stainless SMS about every 2' on the overlapped seam.  It was 
time to paint the rusting panels so a coat of aluminum roof paint went 
on top for leak protection.

For modern prefinished painted metal roof systems I think the screws may 
or may not make electrical contact between the panel overlaps given the 
rubber washer intended to prevent a leak.  OTOH, these seams whether 
overlapped corrugations or standing seam (no screws at the seam) have a 
lot of capacitance per foot of seam.  About a 2" wide overlap with two 
layers of paint (10 mils each) makes a capacitance of about 500pf per ft 
assuming a dielectric constant of 2.  So any diode junction rectifying 
effect is shunted by this capacitance.    The screws on my new style 
roof to steel purlins and wall to girts average about 1' of overlap per 
screw or 500pf.  One advantage of a metal framed building is every screw 
has a decent chance of grounding the panel.  They are all self drilling 
so produce a good burr in the roof or wall panel and for sure are in 
solid electrical contact with a purlin or girt.

How well the seam capacitance suppresses diode rectification RFI is a 
good question as is what is the likely power output of such a diode.  
Unsoldered copper seams sound like the devil's playground.

How to "fix" an existing roof?  The seam edge is accessible at the 
gutter on my roof so a SS screw and nut, serrated lockwashers both sides 
to bite thru the paint would make an electrical short at that point.  
How that affects a screw a few wavelengths away at VHF/UHF is ??.  For 
HF it might help.

PIM (passive intermod) avoidance is a big deal on cell towers. Amateur 
radio stations excel at opportunities for PIM, so everything needs to be 
considered if it is a problem.

Grant KZ1W

On 1/11/2018 11:10 AM, Kent Olsen wrote:
> Jim
> How would one "bond" a metal roof joint? The joints crimped together all
> along the joint?
> Thanks
> 73
> Kent
> N6WT
> On Thu, Jan 11, 2018 at 10:59 AM, Jim Brown <jim at audiosystemsgroup.com>
> wrote:
>> Hi Skip,
>> The "interference" is probably passive intermod (rectification at one or
>> more joints in that roof) that is generating a spur on the input frequency
>> of the repeater.  If this is the cause, the joint(s) must be bonded at
>> multiple points.
>> 73, Jim K9YC
>> On 1/11/2018 10:29 AM, Gilbert Kauffmann wrote:
>>>    My repeater is desensed.
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