Post Base Question for Elevated Deck Adjacent to Residence

[Michael Cobb]

I am discussing wood deck posts with a structural engineer her in California. We are reviewing CRC 317.1.4

They believe this code section allows pressure-treated wood to be closer than 6” or even in contact with the ground.

It is their sense the “exceptions” listed in this code section are exceptions that allow wood to NOT be pressure treated. It was my sense the exceptions are listing proximities where even if something is pressure treated it is NOT permissible to use wood.

In my experience, typical wood post bases for decks surrounded by open ground are required to 6” off the ground. Certainly 317.1.2 allows for ground contact wood to be pressure treated, but it is my understanding this only pertains to wood that is not otherwise functioning in the structural capacities listed in the other sections.

Could someone help clarify this? I'm questioning my learning here. If we use pressure-treated posts, can we cast the posts directly into the ground? I didn't think we could, but it is being suggested otherwise. Any help is greatly appreciated. I am attaching a typical detail put out by our local building department also.

 

[bill1952]

Well, I'm probably wrong, but the opening paragraph says all pt or naturally decay resistant. The exceptions allow non treated non decay resistant wood under those specific conditions. But in California, who knows.

 

[e hilton]

Well, I'm probably wrong, but the opening paragraph says all pt or naturally decay resistant. The exceptions allow non treated non decay resistant wood under those specific conditions. But in California, who knows.

That’s how i read it. Seems very plain. Much more so than many other sections.

 

[mtlogcabin]

The key word is approved pressure-preservative-treated wood

R317.1 Location required.
Protection of wood and wood-based products from decay shall be provided in the following locations by the use of naturally durable wood or wood that is preservative-treated in accordance with AWPA U1.

For Code Officials

American Wood Protection Association (AWPA) develops standards for wood preservatives and treated wood products.

awpa.com

The table below identifies the types and their use based on the level of retention of preservatives.

Types of Pressure Treated Lumber and Where to Use Them
Type	Location	Uses
UC1	Interior Dry	Stud framing & baseboards
UC2	Interior Wet	Sill & bottom plates, damp locations, basement framing, bathrooms, flooring, baseboards
UC3	A	Exterior Above Ground, Coated with Rapid Runoff	Protected decking, fascia & trim
B	Exterior Above Ground, Uncoated or Poor Runoff	Cedar shakes, exterior stairs, joists, beams, decking, railings and fence boards/pickets, any wood within 6” of soil & vegetation
UC4	A	Ground Contact, General use	Fence, deck & structural posts, ledgers, retaining walls, garden boxes, any wood within 6” of soil, vegetation, or freshwater, or poor air circulation
B	Ground Contact, Heavy-duty use	As with UC4A plus posts set into the ground, retaining walls, wood foundations & supports, freshwater contact, saltwater spray, garden & utility posts
C	Ground Contact, Extreme duty use	Pilings or wood in direct installation in soil, gravel, concrete, freshwater, or extreme weather exposure
UC5	A	Marine Use, northern waters, Salt or brackish water	Contact & immersion in cold ocean waters north of Long Island & San Francisco, docks, piers, wharves, shore walls, boardwalks, & buildings
B	Marine Use, central waters, Salt or brackish water	Contact & immersion in warmer ocean waters south of San Francisco and Long Island, and similar to UC5A
C	Marine Use, southern waters, Salt or brackish water	Contact & immersion in warm Gulf Coast waters & south of Georgia, and similar to UC5A
UCF	A	Interior Above Ground Fire Protection	Interior construction, no ground or moisture contact, fire partition walls, kitchens, hallway & stairwell framing
B	Exterior Above Ground Fire Protection	Exterior construction, exposure to weather but not ground contact, porches, exterior stairs, window treatments

 

[Mark K]

look to the IBC. We are asking the IRC to address an issue it never intended to address.

 

[bill1952]

look to the IBC. We are asking the IRC to address an issue it never intended to address.

I don't understand. It was a question on the IRC, and e hilton and I agreed on interpretation. If you're referring to mtlogcabin comment, well maybe.

 

[mtlogcabin]

We are asking the IRC to address an issue it never intended to address.

The IRC does address which type of pressure treated wood is treated for a specific use through the AWPA. No need to go to the IBC.

R317.1.2 Ground contact.
All wood in contact with the ground, embedded in concrete in direct contact with the ground or embedded in concrete exposed to the weather that supports permanent structures intended for human occupancy shall be approved pressure-preservative-treated wood suitable for ground contact use, except that untreated wood used entirely below groundwater level or continuously submerged in fresh water shall not be required to be pressure-preservative treated.