panel mounting

Hi all,

I'm planning a solar hot water system install for my home in Oakland, CA. I couldn't find any decent discussion forums on solar hot water systems, so I figured I'd post here since my question is fairly generic.

My roof is tar & gravel over modified bitumen, sloped at 2.5" / 1' (approx 12 degrees). The roof sheathing is supported by TJI beams spaced 2 feet on center. The two collector panels will be mounted parallel to the roof plane. I understand the tilt is less than optimal, but tilt racks are not really an option due to aesthetics and extreme winds at our ridgetop location.

My current plan is to put the collectors on two parallel rails. The rails would be on standoffs with lag bolts through the rail and standoff, and into the roof sheathing and TJI below. My question is, how do I waterproof these penetrations? On the web I see lots of mounting details/diagrams for shingle roofs and tile roofs, but almost nothing for built-up tar & gravel roofs.

I am especially concerned about getting the waterproofing correct because the 14" space between the roof sheathing and ceiling is completely filled with blown-in cellulose insulation. If there's a leak, we likely wouldn't notice until the insulation was completely saturated, at which point we'd have a major problem on our hands.

Any recommendations/suggestions welcome.


Jim Meehan
Oakland, CA


  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    Re: panel mounting

    From what I have seen, you have two methods... One it to put the same thing used around roof penetrations (like your sewer pipes, skylights)--usually metal flashing or just built up asphalt flashing... The other type (used on my asphalt shingle roof) was just bolts through the sheathing and lots of good quality roofing goo... Not sure I like this for the next 40 years, but has not leaked in the last two years.

    If your roof is still relatively new (should be as you don't want to pull up your 20+ year collectors in the next dozen years to repair your roof)--You might want to contact your roofer and ask for their help--especially if they are guarantying your roof against leaks.

    But why I really am posting is that, from what I have read, is that you may end up getting too much heat in the summer (overheating collectors/tanks is not a good thing)... I am thinking of placing my collectors (if I ever do it) on the side of my home (I used my good roof space for solar electric grid tie inverter system). Biased more towards winter heat collection--specially when it is more difficult to gather the heat in the first place (low sun, more clouds).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: panel mounting

    The roof -- and the rest of the house -- are brand new. Just completed in January of this year. We were going to install the solar systems (PV and hot water) as part of the original construction, but budget got in the way and we had to cut those out for the time being.

    I'm not sure if there's a warranty specific to the roof. The roofing contractor was a sub to the general contractor that built the house, so we never had any contract directly with the roofer. Not a bad suggestion to consult the roofer though.

    On the overheating issue, I've seen recommendations of 1.5 - 2 gallons of storage per sq. ft. of collector area, with the higher end of the range being for warm, very sunny climates like FL and AZ. I will have 80 sq. ft. of collector area and a 120 gallon storage tank, which is at the lower end of the range. However, my collectors will be derated somewhat due to orientation (12 degree tilt toward SE 120 degree heading). Also, here in Oakland, we are prone to foggy mornings and late afternoons in the summer. I'm thinking with those two factors, I should be in the safe zone as far as overheating goes.
  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    Re: panel mounting

    I am just across the Bay in San Mateo (and grew up on the coast in Pacifica)--so I know what you mean about fog...

    You can look here (solar irradiance for California in PDF) and decide if you are building the system more towards summer or all year long...

    Given that saving energy in Summer (with solar hot water) is just as good as saving a similar amount of energy all year round--it may not make much difference in the overall heating bill.

    A couple of points... You probably will want to make sure your panels are tilted to 15 degrees from horizontal (12 degrees may be close enough) to ensure that they are, more or less, self cleaning (leaves and dust). Horizontal panels will mean a lot of cleaning.

    From looking at the solar data--I would suggest that you mount at latitude or latitude plus 15 degrees (~37 - 52 degrees). It won't affect your summer collection that much--but will really increase your "dead of winter" solar collection by 50% or more.

    In California, there is state law that says that you can install solar panels anywhere without regards (to much) to local zoning and association requirements...

    If you are or will be mounting solar pv panels eventually, you will definitely want to get those panels tilted up.

    Roughly speaking, your thermal panels are about 80% efficient and cost around $0.50 per collected watt... Solar PV are about 20% efficient and cost $5.00 per collected watt. So--50% increase in performance by just tilting your panels (for a nominal cost in framing, and probably almost no additional installation costs), you will want to tilt your Solar PV panels (because of costs).

    And since your solar thermal panels will be, roughly, 1/4 the size (area) of your solar pv panels--I would suggest bitting the bullet and tilting all of your panels to latitude (or up to latitude +15 degrees)... That loss in collection (during the winter months) is hard to give up...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,108 ✭✭✭✭
    Re: panel mounting

    I second the idea of getting ahold of the original roofer. If he does not have a satifactory answer that he will stand behind, I suggest you find a roofer who specializes in commercial flat roof buildings. Modern EPDM and torch down product are well regarded if installed properly. If you go on the roof of any strip mall, you will find a miriad of pipes, wires, vents, hvac unit that are on stand offs on the roof.

    I think that this comes under the catagory of NOT being penny wise and pound foolish as my late mother would say.

  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: panel mounting

    I worked as an electrician for most of my working life in and around Oakland. When installing equipment on tar and gravel roofs we glued redwood 4x4's to the roof with mastic for mounting conduit runs or junction boxes (things not subject to being blown away by wind).

    Objects with enough surface area to have windage problems were mounted on standoffs bolted to the roof structure.

    The bolts passed through roof jacks (or pitch pockets) that were filled with sealant.

    Roofers were always called in to fill the pitch pockets so I don't know what the sealant was. But it looked like some kind of tar that hardened.

    When gluing down the deadmen (the 4x4's) make sure you remove all the loose gravel under them.
  • VolcanoSolarVolcanoSolar Solar Expert Posts: 56
    Re: panel mounting

    For a sealant I recommend polyuretane roofing caulk over most others, it seems to stick best, not get hard, and be durable. (I'm equally nervous about numerous roof penetrations for my solar and hot-water panels here in Hawaii.)

    I once had to do parapet-wall penetrations through the EPDM roofing material atop a brand new Home Depot store (I used to be in the sign business). There were endless negotiations with the roofer as to what would or wouldn't void the warranty.

    Check out the following products:
    - Thaler: liquid tight flexible conduit flashings
    - Firestone Building Products: they make the EPDM material and have online instructions for how to handle penetrations

    - Ted
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