Using a Hilti PS38 to locate roof rafters

[JFinch57]

Hilti PS38.jpgHilti PS38a.jpgA company that I bought inverters from on eBay had previously listed a Hilti PS38, which they referred to as a "studfinder on steroids". I was able to borrow one yesterday for my current installation's roof layout. Although it did not work as well as I had hoped, it was pretty amazing. Had to switch between concrete and general modes (drywall is for looking for wood but didn't work) but was able to pinpoint the roof rafters through the shingles once I had a close point with a hammer test. You will see in the one picture the dot on the cardboard, where the 48" center falls, and the dot I put on the roof, which is where the Hilti narrowed the center to. Haven't done any drilling yet but will post an update on how well the Hilti did once I do that. Note: the Hilti PS38 has been updated by the PS50 but is not available yet.

[Sunking]

Worked with a lot of carpenters and roofers. None of them use any gizmo to find studs. Just a light tap with whatever tool they have at hand. Works great for me.

[inetdog]

Worked with a lot of carpenters and roofers. None of them use any gizmo to find studs. Just a light tap with whatever tool they have at hand. Works great for me.

Practice helps!

And what is good enough to make sure a short nail lands somewhere in the area of a 2" member is a lot easier than locating closely enough for a large diameter bolt or lag screw that has to go the full depth of the member.

[JFinch57]

Worked with a lot of carpenters and roofers. None of them use any gizmo to find studs. Just a light tap with whatever tool they have at hand. Works great for me.

I did 80 roof mounted panels on my last house (was actually 2 houses tied to one meter) and used PV Mounts. I can tell you with using a hammer we hit an acceptable spot 19/20 times. With a large flashing, which I'm also using, it's no big deal to miss the hole, but a lot better to hit 20/20! If you can get your hands on one of these it's worth it.

[JFinch57]

Practice helps!

And what is good enough to make sure a short nail lands somewhere in the area of a 2" member is a lot easier than locating closely enough for a large diameter bolt or lag screw that has to go the full depth of the member.

Where is the "like" button (for your post)? I'm an engineer and if you can locate all studs exactly with an extra hour of work it's well worth it! It's an expensive tool (~$1500) so not all will have the benefit of using the technology. What's hard is if the sheathing isn't nailed down firm against the rafters, then you're into a guessing game and this tool eliminates that.

[inetdog]

What's hard is if the sheathing isn't nailed down firm against the rafters, then you're into a guessing game and this tool eliminates that.

FWIW, that part is less of a problem if the person who is doing the testing stands or sits on the line in question and is very heavy!

[foo1bar]

FWIW, that part is less of a problem if the person who is doing the testing stands or sits on the line in question and is very heavy!

Maybe...
My house (and others in the area) have horizontal boards (skip sheathing) going across the rafters between the rafters and the plywood.
So I can be directly over a rafter, but it's 3/4" of plywood, then a 3/4" space, then the 2x6/2x8 rafter. And 4-6" upslope on the roof it's 3/4" ply, 3/4" solid board, and then rafter.
So finding the rafters by tapping didn't work well - but looking at the rafter tails under the eaves worked well for the most part. (1 spot the visible tail was offset by 1.5" from the real rafter, and in many spots getting on the rafter 8' away from the edge required a try or two.) Chalk lines, measuring out 3'x4'x5' right triangles to get square, and measuring from one rafter location to the next helped a lot.

[inetdog]

Just tap above and below the point you are interested in at places where the furring strips are present, then connect the dots?

[JFinch57]

Maybe...
My house (and others in the area) have horizontal boards (skip sheathing) going across the rafters between the rafters and the plywood.
So I can be directly over a rafter, but it's 3/4" of plywood, then a 3/4" space, then the 2x6/2x8 rafter. And 4-6" upslope on the roof it's 3/4" ply, 3/4" solid board, and then rafter.
So finding the rafters by tapping didn't work well - but looking at the rafter tails under the eaves worked well for the most part. (1 spot the visible tail was offset by 1.5" from the real rafter, and in many spots getting on the rafter 8' away from the edge required a try or two.) Chalk lines, measuring out 3'x4'x5' right triangles to get square, and measuring from one rafter location to the next helped a lot.

foo1bar (or is it fubar?!), you beat me to it. On my last house I installed 1" rigid foam insulation then 3/4" plywood over the rafters. It was impossible to locate anything with a hammer test. We had to get a rough dimension by measuring in through an attic window, then zero in on the first one, measure top & bottom from there. Luckily the PV Mount flashing is forgiving!

[solarfrank]

Try Bosch D-tect 150 it's kind of pricey but it's the most accurate scanner which we use it. On the roof we use just on flat roof and we have good results, just remember to have a piece of cardboard 3 ft x 1ft . For tile or shingle roof just use a rubber hammer and drill pilot holes and then use a cooper wire to check the location of stud.

[JFinch57]

After drilling all of the top rail mounts and most of the lowers I can say that the Hilti PS38 did an amazing job, was on the money 19/20 times. Not sure why it occasionally located dead space, but thanks for flashing type mounts that are forgiving. Here's the top rail on and part of the lower. The lower flashings are not notched for nails and are just covering the drilled holes in case it rains. Roof Layout Top Rail.jpg