Why Is My Deck Sagging in the Middle?
What this comes down to
- A deck sags in the middle because something under the middle carries more than it was built for: a joist, the beam or a footing.
- The model code allows a joist to dip only 1/360 of its span, which is 0.4 inch on a 12 foot span.
- A 2x8 joist at 16 inches on center spans 11 feet 10 inches in Southern Pine and 11 feet 1 inch in Douglas fir.
- If the dip runs along a joist, the joist is the suspect. If it runs along the beam, look at the posts and footings.
- Wood also creeps under constant load, up to twice its first deflection, but that stops. A sag that keeps growing is a defect.
- A string line across the deck and a screwdriver in the joists tell more in ten minutes than a guess from the lawn.
A deck sagging in the middle is a load problem with a few possible sources. The joists may be too long for their size, or the beam may be too weak. A post may have dropped, or rot may have softened the wood.
This guide shows how to measure the sag and tell those causes apart. It also covers what each cause costs to fix. It ends with when a sagging deck should stay off limits.
How much sag is too much?
More than 1/360 of the span. The International Code Council's model code sets the floor deflection limit at L/360 for floors. That is the most a member may deflect under the live load.
On a 12 foot joist, 12 feet is 144 inches, and 144 divided by 360 is 0.4 inch. That is about the thickness of a thumb. A dip big enough to see from the lawn is usually well past it.
How do I measure the sag in my deck?
Pull a string tight across the deck and measure down to the boards. Tie it to two points on the same line, one at each end of the joist run or beam. A taut line shows the middle gap.
Measure at the ends first and then in the middle, and write the numbers down. Do it along the joists, then along the beam line. Where the gap is greatest and which direction it runs point to the cause.
Are the joists too long for their size?
Possibly. The American Wood Council's deck guide gives the joist span table for wet service. It assumes a 40 psf live load, a 10 psf dead load and No. 2 lumber.
A 2x8 at 16 inches on center spans 11 feet 10 inches in Southern Pine. In Douglas fir it spans 11 feet 1 inch. A 2x6 reaches 9 feet and 8 feet 4 inches. At 24 inches on center, the same 2x8 drops to 9 feet 8 inches or 9 feet 1 inch.
Older decks were often built by eye. A 2x8 spanning 13 feet at 24 inches on center sags even when the wood is sound. Measuring the span and the joist depth takes two minutes and settles it.
Is the beam or a post causing it?
If the dip runs along the beam, the beam or its posts are the likely cause. A beam that is too long between posts bends like an oversized joist. Missing or loose connections let it drop at the middle.
A post that has dropped, tilted or split also drags the beam down with it. Check each post top and bottom, and see whether the beam still sits fully on the post cap. The guide to a rotted deck beam covers beam damage.
Has a footing settled?
If a post has dropped, the footing under it is the first place to look. The model code puts decks on footings sized to carry the loads to the ground. The footing requirement is where that starts.
Look for a gap between post base and footing, a post out of plumb, or soil washed away at one pier. A footing that is too small or too shallow can sink after a wet spring. Only digging shows how big it really is.
Is rot making the sag worse?
Often, yes. Joists rot first at the top edge under the decking, where water sits. A joist can look sound from below. Probe the joist tops with a screwdriver. Soft wood that crumbles means the strength is gone.
A rotted joist sags under load and can break without warning. The guide to checking deck joists covers the probe test. The one on repairing a rotted joist covers what follows.
Can wood sag on its own over time?
A little, yes. The American Wood Council's wood creep guidance describes deformation that grows under load. The average is 1.5 times the first deflection for seasoned lumber. For unseasoned lumber it is 2 times.
That means a new deck can settle slightly in its first years and then stop. A sag that keeps growing from one season to the next is not creep. It points to rot, a settling footing or an overloaded member.
Is a sagging deck safe to use?
Not until someone checks it. A dip of an inch or more, soft wood or a leaning post makes the deck unreliable. Keep crowds, grills and heavy furniture off until the cause is known.
A small, old, stable dip in sound wood is a different case. Measure it again in a month. If the number has not moved and the probe finds nothing soft, it is likely creep and not a hazard.
What fixes a sagging deck?
The fix follows the cause. The table matches each cause to what it looks like and the usual repair. It also shows what moves the cost, because the same dip can come from very different problems.
| Cause | What it looks like | The usual fix | What moves the cost |
|---|---|---|---|
| Joists too long or too small | Even dip along every joist | Sister joists or add a beam line | Span, joist size and access |
| Beam too long or weak | Dip along the beam line | Add a post and footing, or a bigger beam | Beam size and footing digging |
| Settled footing | One post low or leaning | Jack, new footing, new post | Soil, depth and reach |
| Rotted joist or beam | Soft wood, local dip | Sister or replace the member | How many members have rot |
| Creep only | Small, old, stable dip | Monitor, no repair | Nothing, if it stays put |
What does it cost to fix a sagging deck?
Sistering a joist starts at $125 per joist, and replacing one starts at $175. Posts, footings and beams cannot be priced until the frame is open. The table shows the starting figures.
| The work | Starting price | Wider market | What moves it |
|---|---|---|---|
| Joist repair or sistering | From $125 per joist | $150 to $350 per joist | Joist count, span and access |
| Joist replacement | From $175 per joist | $200 to $450 per joist | Rot spread and hanger work |
| New pressure treated deck | From $24 per sq ft | $26 to $50 per sq ft | When the whole frame is past saving |
What cannot be determined from the yard?
How much strength the wood has left. The decking hides the joist tops, and rot there looks like nothing from below. The probe and the frame being opened settle it.
How big and how deep the footings are. Nothing on the surface shows it. A footing may be fine for a deck of this size. It may also be one pier short for its load.
And whether the frame was built as designed. Notched joists, a missing beam splice or a cut post cap can all cause a dip. These are found with a flashlight under the deck, not from above.
Sagging deck questions
Why is my deck sagging in the middle?
Something under the middle carries more than it was built for. The joists may be too long for their size. The beam may be weak, a footing may have settled, or rot may have softened the wood. Measure the dip along the joists and along the beam to tell which.
How does Perfect Work find the cause of a sagging deck?
Perfect Work Remodeling & Repairs measures the dip with a string line along the joists and the beam. It then probes the joist tops and checks each post and footing from below. The cause is named before any price, since each cause has a different repair.
How much can a deck sag before it is a problem?
The model code allows a joist to deflect 1/360 of its span, about 0.4 inch on 12 feet. Anything visibly more deserves a look. A small stable dip in sound wood can be creep. A sag that grows each season is a defect.
Can a sagging deck be fixed without rebuilding it?
Often yes. Sistered joists, a new beam line or a new footing can bring a deck back to level. So can a replaced rotted member. Rebuilding is the answer only when the frame is rotted or undersized throughout. An open frame shows which.
What does a sagging deck estimate from Perfect Work include?
An estimate from Perfect Work Remodeling & Repairs lists the measured dip and the cause. It names each member to repair. Footings and beams are priced once the frame is exposed. Anything that could change the scope, like hidden rot, is named before work starts.
Is it safe to walk on a deck that sags?
Light use is usually fine for a small, old, stable dip in sound wood. A dip over an inch, soft joists or a leaning post is not. Keep crowds and heavy loads off until the cause is found. A rotted joist can fail without warning.
A level deck starts with the right diagnosis
A sagging deck comes down to the span, the beam, the footing and the wood. Each has its own sign and its own repair. The measurement shows which one to chase first.
A visit from Perfect Work Remodeling & Repairs starts with the string line and the probe. Each post, footing and joist is then checked from below. Deck framing repair is part of its structural deck repair work, one of its listed services.
Call Perfect Work at 630-297-7888 · Book a deck check