Flash Foot 2 Installation on a purlin

[LPG]

While inspecting my roof, I found something quite odd. On one side of the roof instead of having rafters I have purlins (essentially horizontal rafters than run from hip to hip instead of eave to ridge pics attached).

After seeing this it got me thinking if I can use traditional lag bolts to go into the purlin. Since the purlin run perpendicular to the roof and they're square they only touch the roof at the corner leaving a triangular gap, if i put a lag bolt down it'll pull the OSB on the roof against the purlin and won't fully tighten until the gap has been closed which will break the OSB.



Here's what the roof looks like



Anyone ever run into this issue?

[Mike90250]

aren't you the lucky one. At least you looked first.

I've got no idea, except set the lag's but don't torque, and fill from the attic side, some of the dense spray foam (low expanding) which should help fill the void. Don't know if it would be structural enough.

[J.P.M.]

I've never dealt with that situation, but if I did I'd consult an old timer who's seen it all. There may be an easy fix that's been around since roofs were invented

After that, and off the top of my head, and only after failing to find the wisdom of experience, I'd use a spacer in the gap with the same cross sectional shape/profile as the gap, and that's of a harder (less flexible or deformable) material than either the OSB or the purlin material.

I'd also use washers with a slightly but not too much larger collar width on the outside (top) surface of the roof to spread the force on the OSB a bit so as to perhaps lower the likelihood of tearing/crushing the OSB from the likely stress concentration when the joint is tightened and the bolt may not wind up be normal to the OSB.

[emartin00]

That's a tough one. If you don't drive the lag in perpendicular to the roof surface, then it probably won't seal properly. If you drive it in perpendicular, then you aren't going to be in the purlin.

The only 2 options I can think of are: Run rafters between each purlin wherever you need to put a mount.
Or use the deck mount style roof anchors. I've never used them before, but they seem to be gaining in popularity. You'd probably want to keep them as close to the purlins as possible though.

[ImInPhxAZ]

This is usually when one would do internal blocking like emartin00 describes. Block between purlins and attach to the new material. Or "sister" against the purlins with the correct angle cut and attach to sister.

You'd want to have a quick chat with your local building code office if they'd accept that and what calculations if any they'd need for it.

[LPG]

So to bring this thread to a close. I spoke with IronRidge and they also pointed out that my crude drawing of the issue had a fault. I put the lag bolt perpendicular to the joist instead of the roof, which means that the lag bolt would in at 45 degrees to the joist making the whole ordeal even worse as even adding a sister to the joist would still not provide me the thickness necessary.

I then went and spoke with my AHJ and they said that blocking in between the joists would be fine, So I'm adding an addendum to my permit to show the necessary blocking for that roof.

Thanks for the replies.

[J.P.M.]

So to bring this thread to a close. I spoke with IronRidge and they also pointed out that my crude drawing of the issue had a fault. I put the lag bolt perpendicular to the joist instead of the roof, which means that the lag bolt would in at 45 degrees to the joist making the whole ordeal even worse as even adding a sister to the joist would still not provide me the thickness necessary.

I then went and spoke with my AHJ and they said that blocking in between the joists would be fine, So I'm adding an addendum to my permit to show the necessary blocking for that roof.

Thanks for the replies.

Well, depending on the width of the joist and the length of the lag, and the angle (pitch) of the roof which, BTW, unless it's a 45 degree roof pitch won't put the lag at 45 degrees to the joist, the lag might just break through the long side of the side of the joist and wind up with insufficient embedment length. Keeping the lag perpendicular to the joint will help avoid that situation. As for the lag not being perpendicular to the roof: Most of the problem there is one of creating a stress concentration or stress riser by the lag on the OSB that it isn't perpendicular to and compressing the OSB more over some portion of the lag circumference than on the other portions of the lag. That's why I suggested using washers on the outside surface as a way of spreading the load and reducing the likelihood of a stress concentration.

[LPG]

Well, depending on the width of the joist and the length of the lag, and the angle (pitch) of the roof which, BTW, unless it's a 45 degree roof pitch won't put the lag at 45 degrees to the joist, the lag might just break through the long side of the side of the joist and wind up with insufficient embedment length. Keeping the lag perpendicular to the joint will help avoid that situation. As for the lag not being perpendicular to the roof: Most of the problem there is one of creating a stress concentration or stress riser by the lag on the OSB that it isn't perpendicular to and compressing the OSB more over some portion of the lag circumference than on the other portions of the lag. That's why I suggested using washers on the outside surface as a way of spreading the load and reducing the likelihood of a stress concentration.

I read your reply but it dawned on me that even with a sister I would have to catch the joist right at the corner where it touches the OSB in order to have a solid base and not deform the OSB itself. the FF2 has a built in base that acts as a washer but even with that since it is FL and hurricanes are the norm here I feel safer blocking in between.

[solarix]

Yep. Internal blocking required. The trusses are not spaced right for where you need to run the solar mounting rails anyway.
Another example of how the architectural community is clueless about "supporting" solar.

[LPG]

Yep. Internal blocking required. The trusses are not spaced right for where you need to run the solar mounting rails anyway.
Another example of how the architectural community is clueless about "supporting" solar.

Hell i'd be happy with just proper spacing between joists, but apparently using a measuring tape is hard.

[J.P.M.]

Hell i'd be happy with just proper spacing between joists, but apparently using a measuring tape is hard.

Especially when the user doesn't know how to count or do basic math - and don't laugh - it's getting that bad.

[emartin00]

Hell i'd be happy with just proper spacing between joists, but apparently using a measuring tape is hard.

Welcome to my world. Found out the hard way that stud spacing in my house ranges from 13-17" along the same wall...

[LPG]

Welcome to my world. Found out the hard way that stud spacing in my house ranges from 13-17" along the same wall...

I think I can top that, I have sections where they decided to stagger 1"-3" to either left or right, it's as if they got to the roof plate and cut off the joist, then they said "**** we forgot about the overhang, lets just put the overhang in offset to the rafters". then on my west facing roof joists go from hip to hip until you get 3/4 of the way down (you know where you can't reach inside the attic unless you crawl on your stomach in fiberglass) where they then changed back to standard eve to ridge rafters. :sigh: