[RE-wrenches] How many times can a rafter have a lag screw put in and out before it is no longer structurally sound? Some roofs getting their 3rd set of new shingles

[William Miller]

Scot:



Fascinating question.  It’s a bit ironic that the changing climate is
damaging the equipment we are utilizing to combat climate change.



A lag unscrewed will damage a wooden member less than a lag ripped out.
>From your post it seems we are dealing with the former, rather than the
latter.



If your layout isn’t changing there is a likelihood that the new mounting
locations will be at our near the old mounting locations.  This will be
true for a rafter lag or deck mount screw.  It seems to me the process of
re-screwing in the same spot in a rafter is just as questionable as
re-screwing in the same spot in sheeting.  I see no benefit in deck
mounting and many disadvantages.



If severe weather is an issue I would avoid deck mounting unless you have
vetted the decking and the applicability of the chosen fasteners to that
material.  I would not standing-seam mount without knowing how the sheeting
panels are secured and finding a method to calculate how many clips are
needed given the increasing wind loads.



I see two questions about reinstalling screws repeatedly:

1.   Is the rafter less able to support loads?  Or,

2.   Is the rafter (or decking) less able to retain the fasteners holding
the PV to the roof?



Speculation on the above:

1.   Wood fibers adhere to each other.  They are not like a bundle of
straws.  If you damage a fiber in one place, it is supported on either side
of the point of damage by this adherence.  My guess is by just withdrawing
screws a few times you do not compromise the load handling ability of a
rafter member.  Site built rafters require different considerations than
trusses.  Site built are a wild-card and need to be considered on a
case-by-case basis.

2.   If you penetrate wood with screws multiple times in a small area, you
are going to make that area “soft.”  Screw holding capabilities will be
drastically reduced.



Options that I see:



1.   Flashed stanchions.  The lags never need to be removed.

2.   Blocking, to attach in new locations.

3.   Sistering, again to attach in new locations.

4.   Not deck mounting (IMHO).



Anecdote:  We had a tile roof mount inspected one August afternoon.  The
inspector insisted on looking in the attic.  We asked why and he said this
city has a particular concern about structural integrity of wood framed
roofs.  I was aware of this because I have had some silly arguments about
this with the lead inspector.  Apparently they have seen a lot of big-box
solar company installers drive lags carelessly and had the lags blow out
the side of the rafter.  If they spot this they make the installer hire an
engineer and design a system to sister the rafters to engineering
standards.  I totally get this concern.  We use a rafter locator and are
really careful.  This is not to say we have not had a blow out now and
then.  You gotta have a feel for the torque resistance of a lag while
seating.



Thank you for bringing this up.  As I said, fascinating (to this wonk).



William



Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com

CA Lic. 773985





*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *Scot Arey via RE-wrenches
*Sent:* Wednesday, May 29, 2024 3:16 PM
*To:* 'RE-wrenches'
*Cc:* scot.arey at solarcentex.com
*Subject:* [RE-wrenches] How many times can a rafter have a lag screw put
in and out before it is no longer structurally sound? Some roofs getting
their 3rd set of new shingles



With all our Texas hail storms, we now have roof solar systems that might
have to come down for the third time as hail-pockmarked shingles need
replacement. We’re at the 11-year mark of doing business and we have some
customers ready for another insurance roof claim,



So that means a new set of lag screws drilled into rafters…after how many
times is that 2x6 rafter not 2x6 strength?

I’ve asked this to my 3rd party structural engineer and instead of an
answer got a “wow, hadn’t thought of that.” We’ve considered going to deck
mounts to avoid rafters at the 3rd “detach and reset.”

Any of you guys consider this and have your techniques? I suppose in
perfect world, the mount would stay in place and roofer would shingle
around it but we still use old-school metal flashing so that is really not
possible. I look forward to hearing your thoughts on this.
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