Finding the joist in thw rood

[NewBostonConst]

Thermal cameras that connect into your phone work good for this and can be bought for $200.....great use for other projects in the house like sealing air leaks and insulation.

[littleharbor]

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.

If you do use a chalk line use white or blue chalk. Red is permanent.

[emartin00]

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

Well that depends where you are. Here in the North East, pretty much everything is 16" centers to handle the snow load

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.

Yes, the plywood is not going to give you anywhere near the strength that the rafters are.

My preferred method is the use a piece of coat hanger wire bent in a "C" shape. Drill a hole and if you miss the rafter use the C wire as a pointer to show you where the edge of the rafter is.
With a little practice a hammer will tell you where the rafter is as well.

[Ampster]

Well that depends where you are. Here in the North East, pretty much everything is 16" centers .......

Where I am standing, 16" OC lays out well with plywood also. Six rafters per sheet of plywood in portrait mode instead of four.

[Iamsteve]

You know what, I just tested the metal function on my stud finder. It seems to work pretty well. The stud finder constantly alarms though .I think the metal function is the way to go. I must ask myself now, are 3 inch lag screws long enough for this job?

[Ampster]

I must ask myself now, are 3 inch lag screws long enough for this job?

It depends on the spacing between lags and the type of rail. Your engineering drawings submitted with your permit is where the answer is.

[Iamsteve]

My plan, that I submitted to the city didn't go into that much detail. They gave me a permit to proceed based upon my modest, general plan. I guess that gives me wide latitude to do as I think right. This is where this forum comes in handy.

[Ampster]

My plan, that I submitted to the city didn't go into that much detail. They gave me a permit to proceed based upon my modest, general plan. I guess that gives me wide latitude to do as I think right. This is where this forum comes in handy.

I would not take the advice of an anonymous poster about something like that. Go to Iron Ridge or UniRac calculators and at least enter some values for wind or other specifics for your area. They will give you a way to see what the spacing and depth of your lags should be. Be sure to stagger your splices of your racking. In most cases wind uplift is the biggest issue driving design..

[J.P.M.]

I must ask myself now, are 3 inch lag screws long enough for this job?

Go to ANSI/ AF&PA NDS 2017, or a PDF of the 2005 ed., chap. M11 for some possible guidance..

That's the National Design Specification for Wood Construction.

Kind of a plug/chug cookbook.

You'll need the design uplift per bolt as calc'd from the wind loadings to compare to the max. allowable pullout loads as calc'd from the ANSI ref.

FWIW, I calc'd. the max. sustainable, non impact and non cyclical sustainable pullout load on a 5/16" dia. lag screw embedded in douglas fir normal to the grain to be ~ 275 Lbf. per inch of thread embedment (So, for my example only, a 500 lbf non cyclic imposed pullout load will require a min. embedment depth of 500/275 = 1.82" w/factor of safety = 1.0. Your results will most probably be different. Cyclic loads and factors of safety > 1 as well as embedments parallel to grain of the wood or for embedment in OSB will need consideration.

I used an older version of that spec for my 2013 roof array design array, and the 2017 version this past May when I was designing and calcing' the pullout loads for the railings on my replacement deck.

Take what you want of the above. Scrap the rest.

[Iamsteve]

Thanks Ampster and J.P.M, I'll look at this.