Most deck beams are supported by posts resting on concrete footings. The deck post elevating the beam to the correct height. Just like fence post, is it a good practice to set the deck post in concrete, stabilizing the deck?
Deck posts set in concrete increase rot and make it impossible to replace the post in the future if it is damaged. Also, in many jurisdictions, setting deck post in concrete is illegal; instead, deck posts need to be placed above the concrete footing extending below the frost line to 6″ (150mm) above grade.
It may seem like a good idea to add strength to the deck posts by setting it in concrete. The concrete and dirt holding the post upright, preventing the deck from swaying in the wind or being pushed over. But this advantage is lost with the increased post rot.
Deck Posts Will Rot When Set in Concrete
Deck posts set in concrete will deuterate quickly as the concrete traps water around the wood post. Concrete around the post increases post rot.
The concrete around the post is acting as a bowl holding water. Just like a concrete swimming pool holds water, so does concrete around a deck post. The rain falls and collects on top of the concrete with no place to go except into the post. Which acts as a natural sponge soaking up the water, rotting out the base of the post.
I have replaced many a fence post. Many snap off right at the ground level. With the post rotten clear through. Just above the concrete but right below the ground level. Setting a deck post in concrete creates the same problem. Rot at the base of the post just above the concrete.
Not only does the post rot out at the top of the concrete but also the water wicker alongside the post trapped by the concrete. The water running down the post with nowhere to go except rot out the side of the post.
Deck Post Set in Concrete are Hard to Replace
Not only will setting the deck post in concrete increase rot, but once the post rots, it is harder to replace. With the bottom of the post stuck in the concrete. But the top of the post rotten, needing to be replaced.
Setting the post on top of the footing but not in the concrete makes replacement much simpler. The beam then needs to be temporarily supported. Then the rotten post can be removed and replaced with a new post. Allowing the deck to be used for many more years.
Setting Deck Post in Concrete Is Not Permitted in Many Jurisdictions
In Calgary, where I build decks but is not unique for deck footing requirements, burring deck posts or setting in concrete is forbidden. Instead, the deck post must be supported with a 10″ (254mm) diameter concrete pile 4′ (1.2m) below grade. This concrete footing must extend 6″ (150mm) above grade, removing the wood from exposure to the fungi in the soil. Limiting rot extending the deck’s life.
To increase the deck post’s strength and prevent uplift, the deck post should be anchored to the concrete pile using a post saddle treated for contact with ACQ contact. The saddle also brings the added advantage of raising the post base out of the water pooling on top of the concrete footing. Protecting the post while preventing uplift.
Instead of Setting Post in Concrete, Brace the Post from the Deck Frame
Deck post still should be stabilized even if not set in concrete. Bracing is the best way to keep the post plum under the deck, on the footings. Braces should be run from the beam to the base of the post at forty-five degrees. Locking in the base of the post in the correct relationship to the deck substructure. Ensuring the load of the deck is transferred directly from the beam to the footing.
Braces should be made out of 2×4 or larger dimensional material. Running at least a third the length of the post or two feet, whichever is greater. The braces should be attached to the beam and post with structural screws or bolts to ensure that they stay in place for the deck’s life.
Conclusion of Not Setting Deck Post in Concrete
Not setting the post in concrete will decrease rot at the base of the post. In the event the post does rot or is damaged not being set in concrete, it can be replaced without removing the entire footing under the deck.
Along with being a good construction practise, many cities require deck post not to be set in concrete. Just one more reason to set the post above the concrete footing, not in it.
To compensate for the loss of stability with the post on top of the concrete not set in it, it is a good idea to run 2×4 braces from the deck substructure to the post base.
Properly braced, the post will be more robust than when set in concrete. But protected from rot lasting much longer. Giving you many more years to enjoy your deck.