Finding the joist in thw rood

[Iamsteve]

Does anyone have a good way to locate the joist in the roof for mounting PV support channels? The stud finder I'm testing don't seem very reliable really.

[Ampster]

Follow the rafter tails from the gutter. Then use the On Center spacing. If using the classic sheet metal design you an drill a couple of pilot holes to find the center of the rafter. Those holes will be covered by the sheet metal.

[Iamsteve]

[QUOTE) Follow the rafter tails from the gutter. Then use the On Center spacing. If using the classic sheet metal design you an drill a couple of pilot holes to find the center of the rafter. Those holes will be covered by the sheet metal.[/QUOTE]

Thank you for this reply Ampster. I was just up on the roof where I was checking out your idea. Because I have a gutter system in place I can't view where the rafters are from my vantage point on the roof. I'm going to have to place an object at the bottom, where the rafter tails are to help me with this task. I see on my 40 year old house they are spaced around 24" apart, give or take a 1/2".
Best wishes.

[Ampster]

If it has plywood sheeting it shouldn't drift too far off 24" centers.

[Iamsteve]

If it has plywood sheeting it shouldn't drift too far off 24" centers.

Thanks again, your a great resource.

[J.P.M.]

Follow the rafter tails from the gutter. Then use the On Center spacing.

That works pretty good until the rafter span gets long enough to need overlap and it's not possible to determine if/where/how it occurs. Such a situation may well shift the O.C. position by 1.5 ".

[Ampster]

Happy to help. What attachment system are you using? Flash foot, or other system?

[scrambler]

Thank you for this reply Ampster. I was just up on the roof where I was checking out your idea. Because I have a gutter system in place I can't view where the rafters are from my vantage point on the roof. I'm going to have to place an object at the bottom, where the rafter tails are to help me with this task. I see on my 40 year old house they are spaced around 24" apart, give or take a 1/2".
Best wishes.

I used a laser level from in front (slightly below) the extended rafters, so the line is visible along the rafter and onto the gutter.
Align the vertical laser beam with the underneath edge of visible rafter extension (you do need the rafter to extend at least a foot or two for precision), then you can mark the line on the gutter.
Then you can propagate these location to the top of the roof.
But I am not sure how precise that is, as I could imagine the old rafters not being all quite parallel underneath.

Of course there is also the Bosch D TECH 150 deep scanner.

I read online the easiest method is the hammer method, I am wondering how precise that really is and how much damage to the asphalt shingles

[Iamsteve]

Happy to help. What attachment system are you using? Flash foot, or other system?

I'm using the Unistrut system, what are your thoughts on this Ampster?

[Iamsteve]

I used a laser level from in front (slightly below) the extended rafters, so the line is visible along the rafter and onto the gutter.
Align the vertical laser beam with the underneath edge of visible rafter extension (you do need the rafter to extend at least a foot or two for precision), then you can mark the line on the gutter.
Then you can propagate these location to the top of the roof.
But I am not sure how precise that is, as I could imagine the old rafters not being all quite parallel underneath.

Of course there is also the Bosch D TECH 150 deep scanner.

I read online the easiest method is the hammer method, I am wondering how precise that really is and how much damage to the asphalt shingles

Thanks Scrambler, I'm going to have to look into this.

[Iamsteve]

I used a laser level from in front (slightly below) the extended rafters, so the line is visible along the rafter and onto the gutter.
Align the vertical laser beam with the underneath edge of visible rafter extension (you do need the rafter to extend at least a foot or two for precision), then you can mark the line on the gutter.
Then you can propagate these location to the top of the roof.
But I am not sure how precise that is, as I could imagine the old rafters not being all quite parallel underneath.

Of course there is also the Bosch D TECH 150 deep scanner.

I read online the easiest method is the hammer method, I am wondering how precise that really is and how much damage to the asphalt shingles

I see the Bosch D TECH 150 deep scanner is over $500 depending on the exact model you get. Some are as high as $1000. Think I'll pass on these. I don't trust my hearing with the hammer test really. The laser may have merit for me.

[Iamsteve]

The plywood is an inch thick. Would it be an international crime if I missed the joist and just went into the plywood, here and there? Thanks everyone for your help.

[Ampster]

I'm using the Unistrut system, what are your thoughts on this Ampster?

I have no experience with it for solar, but if you can get the engineering drawings you need for your permit then it should work. Because it is slotted on the bottom it may save you from having to use L feet.

[Ampster]

The plywood is an inch thick. Would it be an international crime if I missed the joist and just went into the plywood, here and there? Thanks everyone for your help.

What do your engineering drawings say how much thread that your lag bolts need to penetrate the rafters?
It might not be an International crime but you could hurt someone if one of your attachment point fails. Is there any reason you can't move the location of the bolt to pick up a rafter. If you are using Flexfoot or similar the mistake hole would be covered by sheet metal anyway.

[foo1bar]

I used a laser level from in front (slightly below) the extended rafters, so the line is visible along the rafter and onto the gutter.

I didn't have a laser level.
I used a ladder by the tails and sidewalk chalk to mark.
And measuring from the edge at the tails, then measure the same near the peak (and hope it was the same (IIRC it mostly was. I think I had to redo one because I missed the rafter.)
I also used a chalk line reel for making some lines. I figured a few rain storms would take care of any visible chalk marks, and it did.