How to Level a Deck Like a Pro

Decks often sag over time and need to be leveled. Here’s how to re-level a sagging ground-level deck like a pro.

Sagging decks almost always result from faulty support systems, usually those farthest from the house.

Decks that are small to medium in size typically are supported on two sides.  One side is attached to the house.  The opposite side generally is supported using beams near the outer edge and posts. 

Larger decks have an intermediate beam, held up by posts mid-way between the side attached to the house and the outer support system. 

Check Ledger Board

The first step in leveling a deck is to check the ledger board.  These anchor the deck to the house. Ledger boards seldom are the cause of sagging but are vital to a safe and solid deck.   

Typically, ledger boards are pressure treated 2×6s or 2×8s.   They are attached to the house with galvanized or stainless steel bolts or lag screws that hold it securely to the house’s rim joist.  The deck joists generally are attached to the ledger board using joist hangers, although some are toenailed in place.

Before starting to lift the deck, be certain the joists are secured to the ledger board.  Otherwise, the process of lifting could pull one or more of the joists away from the ledger board. 

One way to secure the joists is to install large galvanized “L” brackets where each joist meets the ledger board.  These will hold the joists in place as the deck pivots upward during lifting.  These brackets should be at least six inches and secured in place with ceramic-coated deck screws or lag screws.

The next step is to check the condition of the beams and joists.  If beams or joists have rotted, they will need to be replaced before proceeding.  Trying to level a deck with faulty beams or joists is a waste of time and money. 

The next steps are to check the condition both of the support posts and whatever is underneath supporting those posts.  

Be Safe

If the deck is far enough above the ground to work beneath it, always be mindful of personal safety.  Nothing should be disassembled unless proper temporary supports are installed first.

Decks that are close to the ground will require the removal of decking boards in order to access the support posts, and check the condition of beams and joists.  Other decking will need to be removed next to the house to check the security of ledger boards. 

Likely Culprit

Sagging is almost always caused by the failure of supports under the posts holding up the deck.   These may be concrete footings, pavers, concrete post holders or sometimes even just rocks or gravel. Usually, the supports have sunk into the soil as it has settled over time.
The most reliable support systems are footings of poured concrete that go down below the frost line and fan out.  These rarely fail, but are expensive.  

A more economical system almost as good is reinforced pre-cast concrete pavers, at least two feet square, set on six to eight inches of ¾-inch packed gravel.   On top of the pavers are concrete post holders into which the support posts are inserted, if wood posts are used. 

In common use, but much less reliable, are posts in concrete post holders placed directly on the soil or on a bed of gravel.  These are almost certain to fail within a couple of years, even with well-drained soil.

Choose Posts

Posts will vary with the size of the deck.  Some decks are supported by 4×4 or 6×6 wood posts.  These should be pressure treated, or at least painted or stained.

An even better choice is steel teleposts, the same as in many basements.  They are stronger and more durable than wood, but best of all they can be adjusted easily should the soil settle over time. 

Raise Deck

Once the support system is selected, the deck needs to be gradually raised back into its original place.  Be careful not to raise it too far.  The deck should have a gentle slope away from the house to shed water.  A slope of one inch in 10 feet is adequate.

The first step in the raising process is to install a temporary support, one post at a time.  This will make it possible to remove the existing posts and do the ground-level preparation.  In the case of a large deck with an intermediate beam, install temporary supports in pairs, under the outer and intermediate beams.

To do this, rent a hydraulic jack or jacks, called a bottle jack, from a local rental store, about 10 to 15 tons in size.  Raising a low deck may require only a block of wood under the jack, placed beneath an adjacent beam or joist. 

If the deck is high, a temporary support, or ‘jack support’ post, will need to be fashioned to jack up the deck.  For larger decks, do this in pairs.  One option is to use a 4×4 or 6×6 post. Cut it to accommodate the distance between the top of the jack when it is fully retracted, and the beam under the deck. 

Jack Support

Attach short pieces of wood (15-20 inches) to all four sides of one end of the jack support, leaving a small lip of about two inches.  This will fit over the top of the jack to prevent the post from sliding off.

Slip this end over the jack, making sure the jack support is perfectly vertical; raise it until the opposite end is tight up against the beam.  Attach this end to the beam with a couple of four-inch deck screws toe-nailed in place.  Before doing this, have the temporary post ready. 

Raise and replace one post, or one pair of posts, at a time.  As each is lifted, install and secure the temporary post as close to the old one as possible, leaving room to work preparing the new base system.  If necessary, for safety, install additional temporary posts and add bracing and cross bracing to ensure the stability and safety of the deck. 

Ground Base

The next step in this process is to prepare the ground-base portion of the system.  It’s the Achilles heel of decks.  If the ground support is inadequate, the deck will fail.

The ground base can be prepared in a number of ways.  If, for example, the plan is to use pre-cast pavers and gravel, the existing posts will need to be removed in order to replace the old base support, and then clear away loose dirt, dig down six to eight inches to accommodate the gravel, pack it, and level the pre-cast paver on the gravel. A concrete post holder will go on top of the paver. 

Should the choice be poured concrete footings, then holes need to be dug and forms built or sauna tubes installed to hold the concrete in place.  Make sure post-brackets to hold the bottom of the posts are installed while the concrete is still wet in the sauna tubes or forms.

Prepare Posts

Next, prepare the new posts.  Measure the required distance for the adjusted height of the deck, from the top of the new footings or supports to the bottom of the beam.  If pavers are used, so should concrete post holders, so measure the distance including both.

If teleposts are the support of choice, and they are the best, make sure the size chosen is long enough to fit under the deck but leave most of the adjusting head for later use.  The measurement should include eight-inch by eight-inch by two-inch concrete blocks under the foot of the teleposts and on top of the paver.  These spread the weight of the teleposts. 

Whatever posts are selected, make absolutely sure they are installed perfectly vertical.  Use a good four-foot level to be certain.

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  1. An important element in this process you did not cover is the potential effect that lifting the joists will have on the attachment points along the ledger board. As you raise joists or the beam under them to correct the sagging problem, you are effectively prying the joist ends away from the ledger. Depending on the original attachment method, i.e., toe-nailing or joist hangers, there can be a pulling away from the ledger that should be watched and possibly followed by adding adding reinforcement. Removing old hangers and nails is likely not possible, so you face adding new brackets or hardware over the old to ensure that you have not dangerously weakend this critical connection.

  2. Thank you Don. A good point. I did just that on a deck that I leveled for a client, and for the reason you noted. I installed large ‘L” brackets where each joist connected to the ledger board. Somehow overlooked that point in the article. Will add a correction. Thanks again for your vigilance. Much appreciated.
    James Osborne

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