Far before you bring any panels onto the roof, you need to have decided the location and orientation of your solar panels. Here are a few tips:
- Avoid roof valleys: snow and ice will build up there and the existence of panel corners can prevent sliding and melting.
- Avoid peaks: panel tops should fall a foot or more below the ridge peak to minimize wind exposure from the north side.
- Allow room all around the array of collectors -you and others will need room to install and maintain panels. Leave at least 18 inches on the sides and the bottom to move a bout.
- Try to center the array on the roof-it will give the roof a more balanced look.
- Consider Structural members location: your mounts need to connect to rafters or purlins for support and uplift restraint. Moving the array a few inches either way can make a ll the difference for your mounting screws.
Your options for mounting panels to the roof include:
Flush mount: parallel to the roof pitch.
Tilt mount: a frame or strut on the top of t he panel that tilts the panels steeper than the roof pitch.
- Free standing mount (for flat roofs only).
Much of your time off the ground, you'll be on a wood-framed roof, held up by rafters, and covered in plywood sheathing and then shingles. To securely anchor panels to the roof, you need to make sure you are attaching them directly into the rafters (the sheathing is too thin). Tying your mounts into these rafters demands care and ingenuity, because electronic "stud finders" don't work on shingles or tiles. The common rafters used in most roofs are 1 1/2 inches wide. Modern stick framing places rafters 16 or 24 inches apart. Older homes will have spacing that will challenge your imagination. You must hit the middle of the rafter with your lag or structural screw, otherwise the strength of the connection will be compromised.
Method 1: Use a wide faced hammer to strike the roof at different places. the sheathing will sound hollow, and the hammer head will bounce a bit. when you hit a rafter, you'll feel the firmness and hear the lack of reverberation. This method only approximates the location of the rafter. Finding the center line requires more careful methods.
Method 2: Use very strong permanent cylindrical magnets taped to each side of the rafter (you should be able to get into the attic). A small compass on the roof will spin when you are centered over the magnets.
Method 3: Pry up a shingle tab and use a small drill bit (maybe 1/8 to 1/4 inch) to probe the roof for the rafter edges. That drill bit piercing the sheathing will serve as a reference point on the roof to measure to the exact location of the center line of the rafter.Caulk holes afterwards.
Method 4: For roofs where attic access is impossible, the position of the rafter can be inferred from nail or screw depressions in the soffit fascia at the bottom and similar clues beneath the ridge vent on top.
Method 5: Hire a roofing specialist to provide guidance or to do the roof attachment and penetration installation himself. There is nothing like years of professional experience to create a structurally sound and leak proof roof installation.
Method 6: Put the panels on a ground mount. The roof will never leak!
We suggest using G R K's Rugged Structural Screw and Simpson's strong drive screws. These make long-lasting connections without drilling a hole into the rafter first. All you need is a portable drill with a Torx or nut driver, and you can just zip them in. You save time by not having to drill pilot holes and grab the ratchet. The labor difference is so large that by the time you finish drilling pilot holes and ratchet wrenching in just a few lag bolts, you could have finished the whole job with structural screws.
Pull Out Strength
Structural screws and lag bolts screwed into wood will pull out if uplift is large enough . Pull out strength is determined by several considerations:
- Size and tensile strength of the screw or bolt. Consult the manufacturer's technical data.
- Characteristics of the bolt or screw threads: fine threads will not hold as well as coarse ones.
- The size of the "lead" pilot hole that you must drill for the lag bolt's threads (structural screws do not need pilot holes). Guidance on the size of this hole varies, but a reasonable value is approximately 75% of the total fastener diameter. For a 5/16 inch lag bolt, that works out to be about 7/32 inch .
- The final torque you place on the bolt or screw: don't over-torque it or you will strip the thread connection inside the wood.
- The characteristics of the wood into which the bolt or screw is attached: spruce pine fir (SPF) is one standard wood material that designers base their analyses upon with densities ranging from 0.42 to .0.5.
- The location of the screw or bolt in the wood should be in the middle one third of the cross-section.
- The orientation of the grain of the rafter a nd the presence of any inconsistency in strength like a knot right where you intend to drill the hole.
Each bolt manufacturer offers structural ratings that you can consult for your project site and application. Structural screws can handle hundreds of pounds of allowable uplift, depending on the screw size and type of wood. Lag bolts can resist a bit more uplift per inch of thread length, but that resistance can be compromised by wood quality and the diameter of your lead pilot hole (too small, and the wood will crack, too large and the threads won't h old). A simple wood screw can't offer this kind of pull-out strength!
Flashing (or Roof Pipe Penetration Fitting)
Flashing is a general term that describes small sheets of aluminum, galvanized steel, or copper (and pre-manufactured shapes like roof boots) which help roofers cover holes in the roof and/or surround penetrations to prevent water entry into the building.We use a lot of flashing materials which we slide under shingle tabs to reduce deformation of shingles under our panel mount brackets and to leak-proof the holes we have to make in the roof. Step flashing is required to waterproof shingle roofs around panels that are mounted directly on the roof sheathing. We keep a roll of copper flashing material in the truck with a pair of metal shears.
Roof Attaching and Penetration
For pipe and control cable roof crossing, we suggest using one of the 2 flashing types mentioned in the previous section. An EPDM pad shall be installed under the roof shingle and between the roof and the bottom of the flashing. Caulk sealant under EPDM Pad and around the flashing.