Pressure‑Treated Lumber: Durable Solutions for Code Compliance in Los Angeles

Los Angeles projects have to meet strict building codes aimed at preventing rot, insect damage and structural decay. Those codes don’t leave room for guesswork: if wood will touch soil, sit close to the ground or rest on concrete, it must either be naturally durable or properly pressure‑treated. Pressure‑treated lumber is infused with preservatives under high pressure to resist rot, fungi and termites. This makes it indispensable for sill plates, decking, fence posts and exterior framing, particularly in coastal areas such as the South Bay where untreated wood would deteriorate quickly. 

In Los Angeles County, foundation plates or sills and sleepers on a concrete slab in direct contact with earth must be preservative‑treated, and any field cuts or drilled holes must be re‑treated. In short, if wood touches moisture or soil, treat it.

What Is Pressure‑Treated Lumber?

Pressure‑treated lumber is wood that has been infused with preservatives inside a sealed cylinder. A vacuum removes air from the wood’s cells, the cylinder is filled with preservative, and then pressure forces the chemicals deep into the wood. This process leaves the surface dry and paintable while protecting the core against decay and insect attack. Since the preservatives penetrate so deeply, treated wood is expected to provide decades of service in structural applications.

When and Where Should You Use Pressure‑Treated Wood?

Local codes and the International Residential Code (IRC) spell out exactly where treatment is mandatory. In general, any wood that is likely to stay damp or that could wick moisture from soil must be either naturally decay‑resistant or pressure‑treated. Key situations include:

• Low clearances – Wood joists within 18 inches of soil and framing members resting on concrete foundations less than 8 inches above grade require treatment. Keeping non‑treated wood further from the ground allows air circulation and drying; where that isn’t feasible, pressure‑treated lumber provides the only safeguard.
  
  

• Contact with concrete slabs – Concrete and masonry slabs soak up moisture from the earth. Sill plates or sleepers placed on slabs directly contacting soil must be pressure‑treated. A vapor barrier between slab and soil may eliminate the need for treatment, but without it, unprotected wood will rot.
  
  

• Embedded members – Wood projecting into concrete or masonry walls with less than ½ inch clearance must be treated. The same applies to posts and columns encased in concrete or exposed to weather.
  
  

• Exterior wall framing – Wood siding, sheathing and wall framing with less than 6 inches of clearance from soil must be treated or naturally durable. In high‑moisture areas, even materials above ground can face ground‑contact conditions.
  
  

• Moisture‑permeable roofs and floors – Wood supporting concrete or masonry roofs or floors exposed to weather must be treated or isolated by an impervious barrier. Without treatment, rain and snow can saturate the wood and cause decay.
  
  

• Furring strips on masonry walls – Interior furring strips attached directly to below‑grade masonry walls must be treated or separated by a vapor barrier to prevent moisture from rotting the strips.

Understanding Use Categories

Supply Ontime offers both Above Ground lumber for exterior projects that stay elevated and can dry between weather cycles, and Ground Contact lumber for posts, sills, or framing that touch soil, concrete, or areas with constant moisture:

• UC3A/UC3B – Above-ground exterior: UC3A applies where water drains quickly, such as coated siding and trim. UC3B covers decking, railings, and fencing that get wet but have enough airflow to dry out between exposures.
  
  

• UC4A – Ground contact: Designed for wood in direct contact with soil, concrete, or fresh water. Posts, sill plates on slabs, and deck beams near the ground fall into this category, since they face higher risks of decay and termite attack.

Installation Tips

1. Maintain proper clearances. Follow code‑mandated minimum distances: 18 inches from soil for joists, 8 inches between grade and wood on concrete foundations, and 6 inches from grade for exterior siding. Adequate clearance promotes air circulation and reduces moisture; when you cannot maintain it, choose a ground‑contact UC4 rating.
   
   

2. Seal field cuts. Any cuts, holes or notches in pressure‑treated lumber expose untreated core wood. Building codes require field treatment of those surfaces with a preservative such as copper naphthenate or an oxine copper solution. Brush or spray the preservative on after cutting to maintain protection.
   
   

3. Use the right fasteners. Chemicals in waterborne preservatives can accelerate corrosion. The IRC requires stainless steel, hot‑dipped galvanized, bronze or copper fasteners for treated wood. Avoid uncoated steel nails and screws, especially for critical connections like joist hangers.
   
   

4. Prevent moisture traps. Design decks and cladding to shed water. Avoid decorative skirts or fascia boards that trap leaf litter against deck boards. Provide ventilation under decks and between cladding and sheathing. In coastal Los Angeles, consider UC4A deck boards or posts for structures built near grade or in salty air.
   
   

5. Match treatment to climate. For inland valley projects with dry summers but occasional irrigation, UC3B may suffice for decking and fence pickets. In foggy coastal zones or damp canyons, choose UC4A posts and UC3B or UC4A boards to withstand constant moisture and termites. Always check the tag to confirm the Use Category.

Sourcing and Delivery in Los Angeles

Supply Ontime provides reliable delivery across the Greater Los Angeles area. Customers can count on:

• Same‑Day or Scheduled Delivery
  The company provides same-day delivery service for urgent framing needs and allows customers to schedule deliveries in advance for bigger projects. The system delivers materials at the exact time construction workers need them which prevents delays and maintains project timelines.
  
  

• Flexible Unloading Options
  Deliveries can be handled in multiple ways depending on the site. Truck Bed delivery works for lighter items; Curbside delivery keeps drop‑offs simple within 5 feet of the truck; and White Gloves service places materials at the doorway for added convenience.
  
  

• Clear Delivery Rates
  Pricing is weight‑based and transparent, so there are no surprises. Customers know up front what it will cost to have materials delivered, whether it’s a small order under 20 lbs or a large shipment requiring specialized handling.
  
  

• Convenience at Drop‑Off
  The delivery process needs to remain simple for all delivery options that customers select. The materials can be unloaded from the truck either by placing them at the curb or by staging them at the first entrance of the building. The system provides flexibility which enables construction teams and homebuilders to concentrate on building activities while avoiding logistical concerns.

Final Thoughts

Pressure‑treated lumber is a workhorse for Los Angeles builders. Codes demand it wherever wood touches soil, concrete or moisture. By selecting the right Use Category, preserving field cuts and installing with proper clearances and corrosion‑resistant fasteners, you can create structures that endure decades of exposure without rotting, warping or attracting termites. Pair those practices with reliable local sourcing and you’ll build decks, fences, and foundations that stand up to Los Angeles’ diverse microclimates while staying compliant and keeping occupants safe.