Flat Tile Hook Spacing

Your designer is probably concerned with the point loading on the rafters or trusses in your roof structure. The longer the span, the more weight gets loaded on a single spot in the roof. Even if the rail system is strong enough, many roofs are not. Your roof was no doubt not designed to carry a solar system and we generally rely on the excess load margin that truss companies build in. Almost all mobile (or manufactured) homes though are designed right to the exact minimum snow load that is required and is why we generally can't do solar on manufactured homes (when you have an AHJ that is on the ball). Solar arrays put about 3 pounds/sq ft of additional unplanned loading on a roof which most site built homes can handle but if it is concentrated into a heavy point load because the span between mounts is long, there can be problems.

Also worry about the type of screw being used to fasten to the top chord of your roof trusses. A lot of generally accepted solar mounting systems use a basic 5/16" stainless lag screw that can really damage a typical 2x4 top chord. If the installer doesn't pre-drill properly, that big screw can split the wood sometimes. (I'm sure your builder didn't use cheap wood right?) And some solar mounts even use two of these screws about 3" apart which will almost certainly split the top chord of a truss. (go ahead and try it, I have)
We use a special screw engineered just for this application. Either a SPAX brand or a GRK brand, 1/4" shank with special treads that chew their way through the wood instead of wedge their way through. Comes with a specified pullout spec instead of the unspecified, your guess is as good as mine one on regular lag screws. And don't buy regular zinc plated lag screws from your home center. Absolute cheap, mystery metal junk.

We also don't like using tile hooks because they have a set height where the hook is supposed to slip through between the tiles. The problem is tile installers don't always use batten boards under the tile that are compatible and a lot of the time, your hooks won't fit your tiles. Yes, you can use a tuck-point wheel on an angle-grinder tomake them fit (have fun with that...) We prefer the tile-trac system that works on any tile but you do drill holes in the tile (get a good hammer drill)

Thanks for the response. He was concerned with the actual strength of the hook itself. I have a 2x6 @ 24” OC roof. You bring up a good point about the lag bolts splitting the lumber, especially since there are 2 of them spaced only 3” apart. I’m familiar with the GRK or SPAX crews. Care to share where you purchase them from, if not the big box stores?

Only place that sells SPAX is HomeDepot. I contacted SPAX once to see about ordering in bulk - they said HD is their only distributor of this German product in the USA. You can order them by the pail from the HD online store and get substantial savings though.

Looks like you can get a box of 5/16 x 4” GRK in stainless online at Menards. Do you still run a pilot hole with these or no?

I used tile hooks on my flat tile roof. I don't know if they are the same design as yours, but I wouldn't consider going with more than 4' spacing with mine because I think it's an inferior system in uplift conditions. The panels pull upward on a lever that pulls up on the fastener rather than pulling directly on the fastener centerline. And, even though each hook uses two fasteners, the design doesn't look like it would load both fasteners equally. The uphill screw doesn't see load unless the downhill fastener fails. If you try to save a few dollars by going with 6' spacing, good luck.

I can find them at my regional home center chain in Northern California. I would refer to SPAX or GRK information first especially about the size of a pilot hole. Generally if screwing into old dry rafters I like to drill a pilot hole to reduce splitting the rafters and weakening them. I also have more control over the angle of the hole with a drill than I do over the angle of the bolt with a wrench or driver.

Right on everyone. Thanks for all the quick responses and good info. I had the tile hook manufacture (quick mount) send my some more details. They have ran tests with their product and all the different failure modes. On the uplift test the mode of failure was lag pull out at something like 1400 lbs. now determining what the wind uplift on an array is a bit more convoluted and uncertain but some conservative numbers are around 25 PSF during 90 MPH gusts (as long as your placement has an offset from the very edge of the roof). If you figure my array is around 300 SF then my total load would be around 7500 lbs. Theoretically 7500 into 1400 gives you 5 and that would be the min. number of attachments needed. Obviously that is way over simplified but it gives me a good enough gut check to go with 6’ spacing. That along with evey installer here locally that I have talked to is doing the same, and has done the same for many years without issues.

at the end of the day, with 6’ spacing the weak point will definitely not be the hook or any component of it.