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If you wish to put up a repeater, keep in mind that location and height is king. Getting the height (180' minimum for decent coverage) isn't cheap - even if you can find such a unicorn at all...
 
Several years ago I had invested in an off-the-shelf repeater (Bridgecom). I should have checked out possible locations first though. The least expensive I could find was a 332' tower about fifteen miles across the Indiana/Illinois border at "only" $1/foot. The only spot open was at the very top for a bargain cost of $332/month. Ouch!
 
On the other hand a friend of mine has a lovely setup at his mobile home park (he's the owner!) on a 185' tower he erected himself. His repeater has about a 30 mile footprint for mobiles, about 15 miles for HTs.

The more I learn about the FCC's General Mobile Radio Service (GMRS), the more I am fascinated by RF in general. It touches everyone's lives every second of every day, yet few people pay much attention. How cool is it that after 42 years and 14 billion miles away, the 22-Watt radio on Voyager 1 is still sending data back to earth every day (how important is that antenna?). So why aren't more people interested in GMRS or RF in general? There's no test to get a license like amateur (ham) radio. It's $70 for a 10-year license, and is really easy to get started. So here are some theories.
 
People don't know:
GMRS exists  Why GMRS exists  Why they'd want to use GMRS  How to envision themselves participating in GMRS  What it takes to get started with GMRS  What it takes to continue and improve with GMRS Technical information they should know about GMRS Etiquette when transmitting (TX) on GMRS What a GMRS "Net" is and why Most people within the GMRS community are helpful once you're in, but the community is very inviting in a go-figure-it-out-yourself way, and no one has assembled everything you need to know about GMRS all in one place.
 
We GMRS people are putting the onus on outsiders to sift through thousands of painfully esoteric webpages with a winnowing fork, separating useful info from useless, poorly written, or incomplete info.
 
When I first heard about GMRS on a 4x4 trip, I arrived home and Googled around and settled on a Midland MXT-275 because it seemed perfect for mounting on my truck dashboard. At the time, I had no idea that Midland doesn't manufacture a GMRS radio capable of operating on split-tone repeaters (Dear Midland, I know you're reading this: why do you squander so much potential?). Edit 6/28/2021: Midland heard our cry! They just updated the MXT-275 to include split-tone programming on repeater channels. So for example, now this radio is able to reach a repeater that receives (RX) incoming transmissions on 467.550 with a PL tone of 103.5 and repeats the transmission (TX) at 462.550 with a PL tone of 88.5.
I didn't even know what "split tone" was or even what "tone" meant, or carrier or squelch or hundreds of other little things you all take for granted. Since then, I realized that if the big, bad manufacturers like Midland, Kenwood, Motorola, and iCom can't even invite the public to learn more and provide useful content for each stage of the customer journey—Awareness, Consideration, Purchase, and Loyalty—the 2-way radio industry has much deeper problems and aren't there to help guys like me.
 
As we get involved deeper into GMRS, there's little-to-no hand-holding going on at each level of knowledge. I ending up relying on the Ham community here and there and a guy who is basically a saint at a little radio shop in Phoenix, Arizona (hope Tim over at Procomm and the others at the nonprofit AZGMRS.org don't mind me giving them a shout-out).
 
Edit 4/14/2020: By the way, AZGMRS made this awesome list of FCC-approved GMRS radios that they recommend. If you live anywhere near Arizona and are reading this, now would be a great time to become a member. Their repeater network covers some 100 miles around Phoenix with more and more repeaters joining the network (check out their sweet coverage map).
 
But it's still frustrating.
 
I wish a GMRS expert—presumably a manufacturer—would just come out and say exactly what a total newbie needs at each stage of their involvement or level of need. Two-way radio manufacturers should stop wasting time trying to sell, and start marketing and branding, which means educating the public about the 5 W's (Who, What, Where, When, Why [+how, +how much]) without trying to sell to them.
 
Most new users don't know what they need because they don't know what's possible. For example, I wish I knew that manufacturers don't typically include the best antenna on their radios right out of the box. It took me over a year to realize that the best bang for the buck for a portable base antenna to include in my go bag is N9TAX's Slimjim and that Smiley Antenna makes the best bang for the buck antennas for hand-held radios (which by the way, everyone just assumes newbies are supposed to know that hand-held walkie-talkie radios are called "HT" for "Handy-talkie" and what a "QSO" is).
 
How would a newbie know that the cheap Nagoya 771 "upgrade" antenna for Baofengs that everyone talks about actually isn't the best bang for the buck for the GMRS frequency band of 462–467? How would a newbie know that antennas work best when tuned exactly to what they call a "center" frequency that accommodates 5 Megahertz in each direction (+5 and -5 Megahertz) at the expense of hearing other frequencies?
 
I learned the hard way that in order to properly install an NMO antenna mount on the roof of my truck, I would need a drill bit specifically made for drilling NMO antenna mount holes, and that yes, it is worth the money to do it right the first time.
 
I'm still in the middle of learning how a "quarter-wave" or "5/8 wave" antenna works, the difference between dB gain vs. dBi gain, mic gain, antenna gain, because again, everyone seems to just assume I already know what all this means. I still don't understand what antenna "tuning" means and why you have to "cut" an antenna to "tune" it. Can I make my own antenna right now in a pinch with a copper wire in my garage? Ok, show me! How do I measure it or test it? What is SWR? Can I measure it myself? What do I need in order to measure it? Is one SWR meter better than another for my level as a newbie? Is there something I should learn to make it worth buying the better meter that opens up a whole new world of capability? Is it worth learning all that?
 
This graphic did a great job beginning to explain what dBd gain means for those of us who know next to nothing about it, but now I need to go find out on my own and sift through a thousand webpages to find out if dBd is something new I need to know. Speaking of dB, I know that "dB" is a decibel, but is it the same as my stereo volume? Why do I see manufacturers saying that the microphone and cable have a dB rating? What is going on here? This is madness! We can Google things all day, but which info is true and correct and the most helpful?
 
I think that the entire industry is sitting on a Gold Mine of consumers sitting at home for weeks on end who would love to buy GMRS equipment and communicate via GMRS to friends, family, neighbors, and other GMRS users. Whoever provides the most useful, relevant, and engaging content that stops making assumptions about what people know or don't know will win.
 
STOP ASSUMING. START EDUCATING.

Well, kt seems you have actually answered your own question.  Since rooftop antennas are allowed where you live, the real issue is getting the antenna mounted on the roof.  It is understandable you do not want to go climbing around up there, so why not hire someone to do it for you?
 
Mounting the antenna is the simple part. There are all kind of antenna mounts that might work. But, there is one very simple approach you can try just to test: Mount your candidate antenna, I'd start with a simple ground plane 1/4 wave, on one end of a 10 ft piece of 1" PVC (Sched 40 is good). Then, about 3 or 4 feet up from the base of the PVC pipe, attach something to the pipe to increase its diameter so it won't fall in to a plumbing pipe vent. Then, just drop the pipe into a vent pipe. Done.
 
Sort of like this (I think they actually drilled a hole through the mast and one side off the vent pipe and used a bolt. But, you could use a pipe-clamp and some pieces of bent coat hanger just as well)....
 

I was doing some searching for anybody that may have tried the idea and stumbled across this gem. Got me a bit interested in the idea.
 
https://az276019.vo.msecnd.net/valmontstaging/vsna-resources/microflect-passive-repeater-catalog.pdf?sfvrsn=6
 
While not exactly a water tower the idea is along the same direction, a passive reflector repeater.

While marcspaz's response if mostly accurate, signal reflection, while difficult to achieve, is possible. In the days when microwave was in common use for long haul links, mountain top reflectors were fairly common (see photo below).  The same principal will work on UHF,  IF the antenna is properly positioned at the right angle between the two transmitter locations. It is possible, though not likely, the water tower could act as a reflector. But, the transmitters would have to be well positioned and the antennas would have to be very high gain, probably a dish, and well aimed.  So, I'd say if you can borrow two nice dish antennas, give it a try. But, don't expect too much.
 
 
http://www.rfcafe.com/references/electronics-world/images2/radio-mirrors-communications-electronics-world-may-1969-2.jpg

Note. I obviously know nothing about your furnace. But, with modern high-efficiency furnaces, the exhaust gases are not very hot. In fact, a fan is usually required to use the exhaust fumes out the vent. So, if you do have a modern furnace, heat damage to the PVC is not likely to be an issue.

Another data point:
I have antenna approx 25' off the ground (10 feet mast on top of single-story house). In flat suburbia reliable communication range between base (TK-880H) and HT is around 5 miles. Anything in excess of 5 miles drops to nothing very quickly. Same base station can talk to mobile (also TK-880H), with antenna on a flat roof of SUV, to 8 miles reliably. And I can't check distance longer than 8 miles because I'm in the valley. Get some altitude and 20 miles between HT and base becomes easy (Mt Diablo to Livermore, for locals).

So, I started my personal GMRS and ham journey in January of this year for reasons that are not relevant here. Among my goals was to be able to have simplex RF comms with my wife at her office. (2.1 miles as the crow flies, through suburban northern Virginia. Small rolling hills, but plenty of RF interference along the way.)
 
Frankly, I was disappointed by the limitations of HT<-->HT around here. I was lucky to make it 3/4 - 1 mile. Clearly, line of sight matters enormously.
 
I got a simple j-pole antenna and tried that at home from my attic. (HOA issues.) Still HT to HT, but no luck.
 
I got permission from the HOA to put up two 5 foot j-pole antennas and got them up yesterday. (See attached photo. Both are Ed Fong designs. One ham and one GMRS tuned.)
 
Simultaneously, I got a 50 Watt (UHF) / 40 Watt (VHF) mobile radio.
 
This morning, I ran a quick and informal series of quick tests with the mobile radio set up in the ham bands (70 cm and 2 m) as a cross-band repeater. (I use a 70 cm simplex channel to my home mobile, which then repeats it on 2 m out to my local repeater (W4AVA).)
 
In short, VICTORY! The repeater connections had several flat spots for me. They're 99% gone now. I can trivially tx/rx to my wife's office location with 5 by 5 audio signal on a simplex connection.
 
I know many folks here dismiss the Ed Fongs, but I have to say those reviews are not consistent with my experiences. In particular, the Warrenton GMRS repeater that is some 40 miles from here was completely out of range for me in the past. With the Fongs on the roof, I'm getting 5x5 signal reports consistently on that repeater -- which serves a vast footprint of NoVA and MD. YMMV.
 
So, yeah, I'm pretty happy with where I am on my personal journey. 
 
Cheers,
 
Ken van Wyk
WRFC318 / K0RVW
 

The family aspect of the licensing (vs individual) is a definite plus, as is the fact that you can find some decent plug and play equipment off the shelf.
 
Similar situation here, was eyeing ham, but the family aspect made gmrs a better fit for an immediate need; finding how much is really out there looks like a great complement alongside ham, which i still plan to pursue

Update.  Installed yagis on both ends.  We can understand every word on every channel now.  Next phase, raising both by 6 feet and make sure the aim from mine is good.  I was actually out on the roof in my underwear re-aiming it just now haha.  I did the map aim and drew a straight line but didn't have a vantage point for beyond my house, garage roof mount on existing dish antenna j mount with a piece of toprail that probably barely clears the apex of the main roof, and it may not, didn't get on main roof to look.  I think I'm close to aimed but I don't think I can get much improvement with a little more height.
 
Stay tuned 

I've been using this for simple line of sight.  https://www.scadacore.com/tools/rf-path/rf-line-of-sight/   I like it.  I'm sure the gmrs yagi is the way to go, I was just trying to save some time and money finding a dual band.  The tram showed pretty low SWR for gmrs frequencies but the performance on gmrs is so so, I think.  I did try to hit a standard uhf repeater in the area and was able to hit one about 25 miles away with it.  If it stays sealed up, I think it's a keeper for a standard dual band omni.

They must have changed it since I posted it as none of that info was there, In fact they had claimed it was 3W output at the time while the picture and their ad showed 8W (The pic still says 8W on it). Thanks for finding the data on them though

I scrolled down the page at the link above and the spec's given were:
-------------------------------------------------------------------------------------------
Technical Specs:
 
License Free FRS Radio
462 - 467 MHz
Exceptional conditions: Approx 20 Miles range
Everyday use: Approx 3.2 Miles range
Rugged city use: 50 floor penetration
One-piece back clip
9-level noise reduction
80 decibel speaker output
16  Memory Channels
8,000 mAh internal battery
Type-C fast charging
Only 1.5CM thick (as thin as an iPhoneX)
Under 5oz weight
15 day standby
LED dot matrix display
Headphone charging interface
 
Transmitter
 
    Output Power: 2W/0.5W
    Modulation: FM (F3E)
    Max.Frequency Deviation: ≤5KHz
    Sparious Radiation: ≤7.5μW
 
Receiver
 
    Sensitivity: 0.16μV(12dB SINAD)
    Audio Power: ≥300mW
    Audio Distortion:
    80 decibel speaker output
 
 
Receiving Current: <300mA
Standby Current: <200mA
Emission Current: <1800mA
Audio Distortion: <5%
Intermodulation: >60dB
Max Frequency Deviation: <5KHz
Supply Power: DC3.7V
CTCSS/DCS: The transceiver has 50 CTCSS and 208 DCS, also non-standard subaudio can be programmed
----------------------------------------------------------------------------------------------------------------------------------------------------
 
To me it just looks like a cheap DSP based "radio on a chip" type hand-held. This is common in the "Baofeng" type Chinese radios. They use the RDA1846, or a derivative of it.
 
You can get the datasheet and programming guide for the chip here:
 
https://github.com/phishman/RDA1846/tree/master/Datasheets
 
I would like to see the FCC ID, which it should have to be sold in the US. From the FCC web site can you see what parts the radio has certification for by looking at the grant.

If they're 900 MHz, I'm all over them.

Actually, I was more curious about the radios specs. All I could find was that it was thin, output 3W, and transmits F3E (FM). I am thinking its a GMRS radio, though the ad doesn't say, so it could also be 900MHZ, though if thats the case I think the radio would be locked in at .5W. Given the other falsehoods (Digital transmissions, 8W power output) on the site I wouldn't purchase the radio myself. But as a radio geek (otherwise known as a ham) it still makes me curious about how the radio works lol.

I recommend 400-470 if you think you'll eventually get your Ham license or would like to monitor Ham.
 
If not then 450-512 is fine, but I've never had a need to go past 467.
 
They both work the same for GMRS since GMRS is just 462-467.

I am hearing on line, that a new repeater list is being generated. Many people claiming MyGMRS list is stale and has many old listing that no longer exist. Anybody have any idea, out of all the repeaters listed, how many are actually active?

5.1 watt is good on UHF, they only preform at anything close to 8 watts on VHF,

Just one quick search showed the following statement in Georgia Bill 673 which went into effect in July 2018 - - -
 
"The hands-free law does NOT apply to the following electronic communication devices and the following devices can be used by the driver when on the road:  radio, citizens band radio, citizens band radio hybrid, commercial two-way radio communication device or its functional equivalent, subscription-based emergency communication device, prescribed medical device, amateur or ham radio device, or in-vehicle security, navigation, or remote diagnostics system."

Other states may have differing wording of their laws and should be researched individually.

Hmmmmmm, if you dont like your neighbor??....................,!!

Probably to prevent “like-flation”

Thanks for all your input guys, I saw on the directory that I have 2 repeaters in mind, Mt.Wilson(says its open) and hanger 575 (Long Beach) 
 
WRAF213- Thanks for clarifying the difference on CTCSS / DCS & PL / DPL.. berkinet- These chatters on channel 22 might have been from Monterey Hills but now I saw the Mt Wilson so it could be anywhere.. thank you.. 

Yes, the repeater will not go online w/o a tone to open it up. I do not know Midland radios, but I think you need a translation sheet to set the tone. it will give you the midland setting for a given tone. Keep in mind the output tone of said repeater may not be the same as the input tone.

I have a very limited radio (Midland MXT400). I did turn on the repeater channels that takes me to channels 22 (RX 462-725  TX 467.725Mhz)  I do hear you guys(I'm assuming)  with full strength from Whittier.   
 
Not sure what PL/DPL..and not sure what a transmit tone is.. do I need this to be able to connect with repeaters? 

Those radios are narrowband, and may not produce enough PL/DPL deviation to open a wideband repeater.