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Home Improvement--General/Deep logituinal cracks in deck posts


I have deep logituinal cracks in 3 deck posts supporting my deck. The cracks run down the middle on all 4 sides of each of the posts. I have 3 questions please: (1) Is there a structural issue with the cracksing, or is this a common problem new treated timber expanding in the heat of summer, (2) what based product (silicon(?)) can be used to fill/repair the deep cracks. The filer product will expand/contract with the weather, and still remain fully functional, (3) what product should be used to preserve the wooden posts.

Many thanks,

Hi Duncan,

Treated Lumber, especially pine, is prone to checking and cracking.  This is generally caused by rapid and uneven drying with the majority of cracks occurring in a short period of time between milling and installation.  While these cracks may detract from the appearance of the wood, it is rare for them to be severe enough to impact the structural integrity of deck posts. Structurally, it is unnecessary to take any steps to prevent or repair these cracks.

The cracks may be filled to improve the aesthetics of the deck posts but it is unlikely that any method or product will last more than a few years without the need of maintenance.  Silicone is a poor choice for most home repairs or maintenance including filling cracks in treated lumber.  Silicone itself is very long lasting but will loose it's adhesion to most materials over time, additionally, paints or stains will not adhere to it, making it impossible to achieve uniform coverage in areas where silicone is applied.

The best product to fill cracks is dependent on if or how the deck posts will be finished.  If the deck posts will be left unfinished no filler is required or recommended.  If they are to be painted (which I don't recommend due to peeling issues associated with treated wood) a bondo type wood filler,, will provide the most uniform surface.  A slightly better alternative to painting decks is a solid color latex stain, (please see: for more insight on staining and sealing decks.)The above mentioned bondo product will also work well if latex stains are used.  

If using an oil based stain over bondo test an inconspicuous area before filling all cracks to see if the stain color will be uniform enough over the bondo. Latex caulks may be used in conjunction with latex or oil based stains but should be color matched to the stains as they will not absorb the pigment. Latex caulks will adhere well but shrinks as it dries making it almost impossible to achieve a smooth level surface.

Treated deck posts are rated for ground contact and require no additional treatment to resist decay and insects for the life of the product.  To keep unpainted treated lumber looking good, over a longer period of time, deck sealants, applied on a regular basis, are recommended. (Please see: for more insight on sealing decks)

I hope I have been of some service in this matter.

Best regards,

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Tony Wood


I can answer a wide range of home improvement and maintenance questions. My expertise is in carpentry, painting, framing, siding/boxing, doors, windows, decks and screen and glass enclosures. Wood's Home Maintenance Service


I am a life time resident of North Carolina with over 30 years experience in multiple facets of the industrial, commercial and residential construction industry. For the past 23 years I have owned and operated Wood's Home Maintenance Service , providing services primarily in Johnston, Wake and Sampson Counties of North Carolina.


Several Home improvement articles published on Angie's List and other online forums.

High School graduate ~ proficient in CAD (computer aided design), Open Office program suite.

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