How to locate trusses?

[where2]

I need to precisely locate 17 trusses in my roof so that I can install two lag bolts in 102 places to mount solar racking. The composition of the roof is as follows from the top down: decra 24 gauge steel tile (can be removed), Mylar faced 1/8" foam radiant barrier, 30# roofing felt, 3/4" tongue and groove roofing deck, and 2x4 roof trusses.

At the peak of the roof, I have a partial ridge vent. The roof deck was cut for a ridge vent but the tar paper doesnt have sufficient opening to actually function like a ridge vent. So, I may be able to push a paint stick or something up through the peak to use as a guide.

Based on what I have described, does Garage Journal have any thoughts how to locate 17 trusses accurately enough to drill two pilot holes for 3"x5/16" lag bolts into the trusses in 102 locations?

My structural engineer recommended the most expensive version of the stud finder. It is commonly used for windstorm mitigation inspections in Florida.

 

[timewarp]

Either buy the fancy stud finder or,

Get a long 1/8" drill bit that you could drill straight up through the Truss from the bottom to give you a pilot hole in the roofing, you would only need to do this in a few locations, then you could measure on the underside to where you want the other bolts and transfer those measurements to the roof.

Just measure from known positions in the attic then transfer the measurements to the roof, like plumbing vents, end walls, any other location that is transferable from the attic to the roof.

 

[brewchief]

Either buy the fancy stud finder or,

Get a long 1/8" drill bit that you could drill straight up through the Truss from the bottom to give you a pilot hole in the roofing, you would only need to do this in a few locations, then you could measure on the underside to where you want the other bolts and transfer those measurements to the roof.

Just measure from known positions in the attic then transfer the measurements to the roof, like plumbing vents, end walls, any other location that is transferable from the attic to the roof.

You don't even need a drill bit, cut the bottom section of a coat hanger(the straight piece) at an angle with dikes or linemans pliers and put it in your drill, you have to go slow but it will drill right though a 2x4 pretty easy.

With a 3/4" thick deck I wouldn't be too concerned if I had a couple brackets that missed the truss and ended up right next to it.

 

[ripper70]

If its a straight gable run then find 1 truss closest to gable. Use a fine drill bit to locate the center of it..drive a nail and measure 2 ft. centers all the way to the end. If its a truss roof it will be 2 ft. centers 95% of the time.

 

[akpolaris]

Try going from the bottom so you don't make any more holes that the 102 tht you need. Find the tail end of the truss at the eves abd follow them back up