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Not Field-Treating Lumber Cuts on Decks? Here's Why It's Time to Start.


By Mike Guertin

Do you apply topical wood preservative to cuts and drill holes you make on the jobsite in pressure-treated wood ? If you answered 'Yes' then you're likely building decks and outdoor structures west of the Rockies.  If you answered 'No' you're likely building to the east.

So what's so special about the Rockies that I can pretty reliably tell where you build based on whether you apply preservative to cuts or not? 

It has nothing to do with the Rockies actually but rather the species of  pressure-treated lumber that is available in your area.  In the west, the dominant species group is hem-fir and to the east it's yellow pine, and there's a difference in the penetration of treatment chemicals into the different species.  Hem-fir doesn't take up the chemicals readily — they barely penetrate 1/4 in. below the surface even when the lumber is incised (dimpled with tiny knife cuts to aid the treatment process). 

So when you crosscut or rip pressure-treated hem-fir on a jobsite (the 'field') the core of the lumber is exposed.  The stark contrast in color between the treated perimeter and the untreated core is obvious — and it needs to be coated with a topical preservative chemical.  Western builders have taken field-treating cuts in treated lumber for granted for a couple generations; it's a well-established practice but not so much by eastern builders.  

Why isn't field-treating a common practice by users of treated southern pine?  There's no research to explain but my take is it's a legacy of the old CCA (chromated copper arsenate) treated lumber commonly used prior to 2003.  CCA was extremely decay resistant and field-treatment wasn't required in the building codes at the time. 

After 2003 when the new family of treatment chemicals replaced CCA they were less decay resistant  at the treatment levels applied in lumber used for deck construction. But deck builders stuck with the same practice of not treating cuts.  And it was new territory for code inspectors too.  Plus it took the treatment industry and Southern Pine Council a while to recommend and then require field-treatment in their own literature.

Southern pine lumber found at building material dealers east of the Rockies takes up the treatment chemicals very easily.  The penetration is obvious when you cut into a piece — the green or reddish color of the chemicals usually reaches all the way through.  But the chemical concentration is often less in the wood fibers near the center of a board than it is near the surface.  You'll sometimes notice the hue of the wood near the center is lighter than closer to the edges. 

While the penetration of treatment chemicals into southern pine is better than in hem-fir, wood near the core is still more prone to decay than wood near the surfaces — and that's why field-treatment is necessary.  Plus field-treatment is a requirement in the International Residential Code  — but the code makes you work just to find out what you need to do.

Section R318.2 Field Treatment states "Field-cut ends, notches and drilled holes of pressure-preservative-treated wood shall be retreated in the field in accordance with AWPA M4." 

Ok, so what's this AWPA M4? 

It's titled "Standard for the Care of Preservative-Treated Wood Products" and it will cost you $25.  I bought a copy so I could see what secrets it held — secrets that I needed to know in order to comply with the building code.  The M4 standard is only a few pages long and only a couple of parragraphs apply to field-treatment. 

The basics are that all cuts, holes and other 'injuries such as abrasions' in pressure-treated lumber have to be coated with an acceptable preservative. For cuts in lumber to be used outdoors the options are Copper Naphthenate or Oxine Copper based preservatives.  For drill holes, the standard gets pretty explicit with one option ".. pumping coal-tar roofing cement... into holes using a grease gun or similar device."   

I work in New England and have been field-treating cuts in pressure-treated southern pine since I first noticed section R318.2 in the IRC 15+ years ago. Our local code officials don't check to see that deck builders are field-treating cuts, so very few contractors do.   

So why do I bother field-treating cuts and holes if the inspectors aren't checking?  Because it's my responsibility.  Just because an inspector doesn't verify that my cuts have been treated doesn't relieve me of the responsibility to follow the building code.  There are lots of things inspectors don't check during inspections — take the size of the nails used to frame the deck and the compressive strength of the footing concrete. It's the deck builder's responsibility to follow the building code. 

The code aside, my main reason for field treating cuts and holes is to reduce the chance for decay.  I've seen numerous incidents of decay in treated lumber since 2003 when CCA lumber treatment was phased out and newer less toxic chemicals began widespread use. I've seen rot in stair stringer tread cuts, post ends and joist ends where field treatment chemicals weren't applied on projects built by others — and on projects I built.  

When I bring up the field-treatment topic with other east-of-the-Rockies builders they often discount the risk of decay.  Most of their local code inspectors aren't checking that they're field-treating cuts so they figure there's little benefit to the extra labor and materials cost. 

Yes — coating the lumber cuts does take more time but it's also an opportunity to show prospective clients your thoroughness and professionalism. I discuss my field-treatment practice with prospective clients — it's a sales tool.  I point out what the code requirement is, why field-treatment is important, and suggest that they ask other contractors they meet how they treat field cuts.  Plus it gives me peace of mind that I'm doing all I can to ensure my work lasts a long time.

And one tip — don't treat drill holes by pumping roofing cement in with a grease gun.  I tried and found it extremely messy and slow.  Instead, make an applicator from a small balled-up piece of cotton rag and a coat hanger or other stiff wire.  Like a PVC cement applicator, dip the rag ball in copper naphthenate or oxine copper and swab it through the hole a couple times to saturate the wood fibers.  

The AWPA has a useful information page directed at building code officials that's worth a look.  It has a treated lumber use category table and a section explaining the field-treatment requirements.  

Mike Guertin has written hundreds of articles for publications such as Fine Homebuildng, the Journal of Light Construction and Professional Deck Builder as well as authoring several books. He's also known for his how-to videos and live events at venues such as JLC Live, the Remodeling Show, IBS, IRE and other trade shows along with numerous lumberyard events.