FRT plywood, air barriers, and fasteners

Author Message Ann Sobiech Munson, AIA CSI CCS
Senior Member
Username: acsobiech

Post Number: 9
Registered: 12-2014
Posted on Monday, April 10, 2017 - 12:51 pm:    Apologies for a long one:
We are seeing more frequent use of plywood sheathing due to continuous insulation requirements and rain screen exterior wall assemblies. (Cladding needs substructure; substructure needs to fasten back to structure but allow for continuous insulation, which means continuous substructure members are held 3-4 inches from face of sheathing; plywood provides structural base for fasteners, reducing risk of missing studs in back-up wall.) We often use Type 2 construction so plywood sheathing needs to be FRT to be used in these assemblies.

Issue: we have had difficulty with manufacturers confirming that their products will bond properly to FRT plywood, and we have had many parties express concern about fastener degradation due to FRT chemicals. For instance, one building wrap rep told me they had to re-install their product over FRT plywood on a project in NE using special gaskets; stainless steel staples did not work with FRT chemical. Added complication: we work on many public bid projects, so specific product info (exact FRT plywood product, exact air barrier, etc) is not known until after the bid.

Options seem to be:
1 - require all spacers, clips, etc to fasten back to studs through glass-mat (check for issues before interior face of wall is closed?);
2 - provide additional blocking in walls (if wood, it would still be FRT); or
3 - cover the plywood sheathing with glass-mat to address the adhesion or bonding issues.

Is anyone else running into this? Additional suggestions? Thanks Mark Gilligan SE,
Senior Member
Username: mark_gilligan

Post Number: 813
Registered: 10-2007
Posted on Monday, April 10, 2017 - 03:14 pm:    In California wood buildings are almost exclusively sheathed with plywood or OSB which is not treated. I am not aware that California has more problem buildings than elsewhere.

You may want to challenge your basic assumptions. John Hunter
Senior Member
Username: johnhunter

Post Number: 145
Registered: 12-2005
Posted on Monday, April 10, 2017 - 04:56 pm:    For a similar applications for Type II buildings, we have specified Sure-Board panels (http://www.sureboard.com) using glass-mat sheating as the substrate to which the steel was laminated. My understanding is that it is more expensive and more difficult to work with, but the installations have been successful for cladding systems where the fasteners would not align with the wall framing. There were no concerns regarding compatiblity of substrates and issues associated with the FR treatment. I believe both fluid-applied and self-adhered air barriers were used. J. Peter Jordan
Senior Member
Username: jpjordan

Post Number: 955
Registered: 05-2004
Posted on Monday, April 10, 2017 - 07:22 pm:    In our market, we use plywood or OSB to provide shear values for exterior walls. Exterior sheathing is typically glass mat faced. Ties that need structural support are connected to framing (either cold-formed steel studs or wood framing). There are problems getting the subs to adequately treat the penetrations when their fasteners don't "hit" the studs, but they do seem to find the studs eventually. I have not heard of any projects that would require FRT Plywood for this application. J. Peter Jordan, FCSI, AIA, CCS, LEED AP, SCIP
Don Harris CSI, CCS, CCCA, AIA
Senior Member
Username: don_harris

Post Number: 295
Registered: 03-2003
Posted on Tuesday, April 11, 2017 - 11:58 am:    We do a bunch of wood-framed 3A podium projects. The exterior bearing walls need to be FRT (not so in 5A construction). We have found a couple of OSB products with a coating that have tested as fire retardant. These may be more friendly than the FRT chemicals. Louisiana Pacific has a product called FlameBlock and there is one called FlamedXX. These could be possible solutions depending on your specific conditions. Full disclosure...I work for neither. Mark Gilligan SE,
Senior Member
Username: mark_gilligan

Post Number: 815
Registered: 10-2007
Posted on Wednesday, April 12, 2017 - 05:39 pm:    There is much concern regarding the corrosion associated metal with FRT wood products which means that all of the fasteners used to connect the panels probably need to be stainless steel along with the structural hardware in contact with the FRT plywood.

Talk to your structural engineer.

While I claim no expertise related to fire ratings my experience suggests that others have found solutions that do not require FRT plywood. Anonymous (Unregistered Guest)
Unregistered guest
Posted on Thursday, April 13, 2017 - 10:55 am:    We don't use plywood as a structural base for fastening the substructure that supports cladding. We expect the contractor to be able to find the studs--option 1 from the original post. We haven't run into any issues on our projects that I'm aware of using this approach. Perhaps it depends on the choice of cladding and the requirements for how often it needs to be fastened. Brett Scarfino (Unregistered Guest)
Unregistered guest
Posted on Monday, April 17, 2017 - 11:31 am:    1) I understood wood sheathing products can vary widely in their chemical makeup. The 90% wood content is probably not the issue (probably the resins and other organic additives, as well as the phosphate salts and other inorganic stuff that makes others nervous). The info in the SDS/MSDS might be helpful to some.
2) custom stud spacing is always possible if needed.
3) A word of caution - not all stainless steels are created equal. There is generally a trade off between corrosion resistance and strength (in addition to cost). Mark Gilligan SE,
Senior Member
Username: mark_gilligan

Post Number: 816
Registered: 10-2007
Posted on Tuesday, April 18, 2017 - 09:09 pm:    Brett

With the increased awareness of the chemistry of the materials and products specified there is the need to put this information in context. Yes there are some significant health and environmental issues but it is also easy to obsess over non issues. Brett Scarfino (Unregistered Guest)
Unregistered guest
Posted on Wednesday, April 19, 2017 - 03:22 pm:    Sorry for any confusion. Context for above-
1) Yes, there was an air water barrier manufacturer who expressed a lot of concern with wood sheathings. I do not remember the specifics, beyond the potential for adhesion issues and chemical incompatibility with their adhesives and fluid applied membranes. This was many years ago, and most all of the AWB products have since evolved for NFPA 285. The designers moved to glass-mat faced sheathing.

I like the idea of putting the ultimate onus on the manufacturers/contractors to select/review/verify/confirm their proposed collection of products agree with one another; coordination (ditto for load path from cladding to building structure). Technically, specified or proposed/in-use products could "change" tomorrow (they usually state as much in literature) and do you think they notify everyone of their new resin, pigment or catalyst supplier, assuming the color didn't change and it still met applicable QA/QC/performance? I had no intention of stirring health/envinronmental matters. I do not believe an MSDS always lists 100% of the ingredients, the possible end-of-reaction product(s), nor how the product might change over 20 years, but someone knowledgeable may be able to quickly assess the potential for incompatibilities (or potentially salts, acids, and other stuff that might attack metals and their protective coatings).

3) "stainless steel" is a very vague description that I commonly see loosely used. In the context of corrosion, there is distinct difference between the alloys. I think 400 series stainless alloys (Steel +Chromium) if I hear about or see heavily corroded "stainless". For enhanced corrosion protection, 300 series alloys (400-series +Nickel) is the step up and higher grade 300 alloys such as 316 (300-series +Molybdenum) have superior resistance for chlorine/salt/natatorium/coastal areas. Unfortunately, 300 series is soft and weak relative to 400 series in addition to being more pricey (especially 316, so I'll only mention duplex alloys in passing).
Rhetorical questions - What do you think will be provided if "stainless steel" is called for? Who is responsible for determining if it will remain "stainless" in its service environment? If the stainless staples disappeared but the sheathing fasteners did not, what was special about the sheathing fasteners? What was/is recommended by the sheathing manufacturer?
By the way, one can check 300 v. 400 with a magnet; I carry one in the field. I bet most of the stainless in your kitchen that touches food/water/salts/acids is non-magnetic. If it has rust, my guess is its magnetic, and is a ferritic/martensitic alloy in the 400 series family.