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Quick Answer
A treated 4×4 post can last 15-20 years or more in the ground depending on the type of wood, the preservative treatment used, and exposure conditions like soil contact and moisture. Cedar, redwood, and pressure treated pine are common 4×4 post woods that can have a long service life when properly treated and installed.
What Factors Affect 4×4 Post Longevity?
There are several key factors that impact how long a treated 4×4 post will last when installed in the ground:
Wood Type
The natural decay resistance of the wood species affects longevity. Cedar and redwood contain natural preservatives called extractives that make them more rot resistant. When properly treated, pine can also have good longevity. Less naturally durable woods like fir may not last as long even with preservative treatment.
Preservative Treatment
Pressure treating wood with preservatives protects against fungal decay and wood boring insects. The common treatments for residential 4×4 posts are:
– CCA (chromated copper arsenate) – provides long-term protection, up to 30 years for posts. CCA has been phased out for residential uses since 2003.
– ACQ (alkaline copper quaternary) – the main CCA replacement for residential posts and lumber. Offers 15-20 years of protection.
– Copper azole – another CCA alternative using copper and tebuconazole fungicide. Similar longevity to ACQ of 15-20 years.
– Creosote – derived from coal tar, effective for 30+ years but limited to industrial/commercial uses.
Higher retention levels and deeper penetration of preservatives increase longevity. Look for 0.40 psf retention and at least 0.60″ (15mm) penetration.
Environmental Conditions
How long the post actually lasts depends on the installation site and exposure:
– Direct ground contact – Posts last longer if not buried and surrounded completely by soil which retains moisture and accelerates decay. Elevating on concrete piers extends longevity.
– Freshwater vs. Saltwater – Saltwater exposure is extremely corrosive so longevity will be much less near the ocean or saltwater sources.
– Temperature and humidity – Warmer, humid climates cause more rapid decay and deterioration.
– Wood damage – Any untreated scrapes, cuts, or cracks expose unprotected wood and decrease service life.
– Post size – Larger 6×6 or 8×8 posts last longer than 4×4 posts simply due to greater wood volume.
Proper installation like pre-drilling holes and anchoring with post bases also prevents splitting and mechanical damage.
Estimated Service Life of Common Woods
Here are general expectations for service life of treated 4×4 posts:
| Wood Type | Treatment | Estimated Service Life |
| Cedar | Naturally durable | 15-25 years |
| Redwood | Naturally durable | 15-30 years |
| Pressure Treated Pine | CCA, ACQ, Copper Azole | 15-20 years |
| Pressure Treated Fir | CCA, ACQ, Copper Azole | 10-15 years |
These are general estimates only. Actual post life depends on installation, maintenance, and environmental factors. Higher preservative retentions and proper field treatment of cuts or holes will extend post life.
Installation Tips for Longer Life
Here are some best practices during installation to get the maximum lifespan from treated 4×4 posts:
Choose the Right Wood and Treatment
Select naturally durable species like cedar or redwood, or pressure treated pine for longer service. Make sure the treatment retention and penetration levels meet the recommended minimums. Get a treatment certificate from the supplier.
Elevate Out of Direct Ground Contact
Install posts on concrete footings or piers to prevent decay. This allows air circulation and minimizes moisture contact. Anchor securely with post bases.
Allow End Grain to Dry Before Installation
The cut end grain of posts absorbs the most moisture. Allow several weeks of drying time before installation. Coat exposed end grain with a wood sealer or preservative.
Pre-drill Holes for Fasteners
Predrill slightly undersized holes before nailing or screwing. This prevents splitting that would expose unprotected wood.
Apply Field Treatment to Holes and Cuts
Seal any cuts, notches, or holes made on site with a concentrated preservative like copper naphthenate. This minimizes exposure of untreated wood.
Inspect and Maintain Regularly
Check posts at least annually for signs of rot, insect damage, cracking, or mechanical damage. Make repairs and apply supplemental treatment promptly to extend life.
Conclusion
A properly installed treated 4×4 post can last 15-20 years or longer depending on the wood species, preservative treatment, and exposure conditions. Maximum longevity requires using durable wood, adequate treatment levels, elevating out of ground contact, sealing field cuts, and regular inspection and maintenance. With proper selection and installation, treated 4×4 posts can provide long service lives for backyards, fences, mailboxes, and other exterior uses.
The purpose of this article was to provide instructions and a general guide for selecting, installing, and maintaining treated 4×4 posts in residential settings. Always check your local building codes for specific requirements. With careful attention to best practices, you can get the maximum desired lifespan from posts supporting your outdoor structures and features.
Post longevity is affected by many variables, so it is impossible to provide an exact service life span. However, following the recommendations here will help maximize the life of your 4×4 posts and provide long-lasting support for your projects. Be sure to regularly inspect posts and make repairs to any damage promptly. Your diligence will be rewarded with sturdy, lasting 4×4 posts.
For extremely critical structural supports like a deck or porch, you may want to consider using 6×6 or 8×8 posts, which have more wood volume and preservative chemicals. While more expensive, these larger posts can last 25-30 years or longer. Consult your local building codes for structural requirements. Homeowners should also check with their HOA or jurisdiction for any restrictions on post materials or chemicals permitted in their area. With some planning and proper installation, a treated 4×4 post can provide reliable support for many years.
Choosing the optimal wood species, preservative treatment, installation method, and maintenance for your particular application will ensure you get the maximum lifespan from 4×4 posts. Consider having valuable structural 4×4 posts professionally installed if you are unsure of the best practices. It is a small upfront investment that can pay off for decades with posts that stand the test of time. We hope these recommendations provide helpful guidance to get long service from your 4×4 wood posts. Let us know if you have any other questions!