Overview: The weak deck problem and solutions
A well-built deck will last for decades. But a deck that's rotting or missing
fasteners, or that moves when you walk on it, may be dangerous.
Decks built by inexperienced do-it-yourselfers, not inspected when
they were built, or more than 15 years old (building codes were different back
then!) are susceptible to serious problems. Every year, people are severely
injured, even killed, when decks like these fall down. This has usually happened
during parties when the deck was filled with guests.
Now for the good news. Most of the fixes are quick, inexpensive and easy.
Home centers and lumberyards carry the tools and materials you'll need. Or visit
strongtie.com to find local stores that stock anchors, post bases and connectors.
In this article, we'll show you the warning signs of a dangerous deck—and how
to fix the problems. If you're still not sure whether your deck is safe, have it
inspected by your local building inspector.
Problem 1: No lag screws in ledger board
The ledger board holds up the end of the
deck that's against the house. If the ledger
isn't well fastened, the deck can simply fall
off the house. A building inspector we
talked with said the most common problem
with DIY decks is ledger boards not
properly fastened to the house. For a strong
connection, a ledger needs 1/2-in. x 3-in.
lag screws (or lag bolts if you have access
from the inside to fasten the washers and
nuts) driven every 16 in. This ledger board
was fastened mostly with nails instead of
lag screws (and no washers).
Starting at one end of the ledger board,
drill two 1/4-in. pilot holes. Offset the
holes so the top isn't aligned with the bottom
hole. Then drive the lag screws (with
washers) using a drill and an impact socket
(you'll need a socket adapter that fits in
your drill). Don't countersink the screws—that only weakens the ledger board.
Problem 2: Missing nails in joist hangers
Granted there are a lot of nail holes in a
joist hanger—but they all need to be filled.
Otherwise, the hangers can pull loose
from the ledger board or rim joist. Deck
builders sometimes drive a couple of nails
into the hangers to hold them in place,
then forget to add the rest later. This deck
had only a single nail in some joist hangers.
In other areas, it had the wrong nails.
Joist hanger nails are the only nails acceptable.
These short, fat, galvanized nails are
specially designed to hold the hangers in
place under heavy loads and resist corrosion
from treated lumber.
Problem 3: Rotted posts
Deck posts that rest directly on footings
soak up water and then they rot, especially
posts that aren't pressure treated
(like this one, which is cedar). As the post
rots, it loses its strength and can't support
the deck's weight. Newer decks keep
the concrete footings a few inches above
ground and use a special base bracket to
keep the posts dry. Replacing a rotted
post is the best solution. Before removing
the post, be sure you have everything
you need for the replacement, including
a wedge anchor.
Clear grass or stone away from the bottom
of the deck post. Prod along the bottom
of the post with a screwdriver or an
awl. If the wood is spongy or pieces easily
peel away, you'll need to replace the post.
Start by nailing 2x4s or 2x6s together to
use as temporary braces. Place scrap wood
on the ground for a pad within 3 ft. of the
post being replaced, then set a hydraulic
jack over it. Cut the brace to size, set one
end on the jack and place the other end
under the rim joist. Slowly jack up the
brace until it's wedged tight. Be careful not
to overdo it. You're just bracing the deck,
not raising it. If you hear the joist boards
creak, then stop. Then place a second brace
on the other side of the post (Photo 1). (If
you don't have jacks, you can rent them.) Or you can set your temporary
braces directly on the pads and drive
shims between the posts and the rim joist.
Mark the post location on the footing,
then remove the post by cutting through
the fasteners that tie it to the rim joist. Use
a metal blade in a reciprocating saw (or
knock out the post with a hammer). If
there's already a bolt sticking out of the
footing, use it to install a new post base. If
not, you'll need to add a 3/8- by 4-in.
wedge anchor. Do this by placing the post
base at the marks where the old post sat,
and then mark the center. Remove the
post base and drill the center mark with a
3/8-in. masonry bit. Drill down 3 in., then
blow the dust out of the hole.
Tap the anchor into the hole with a hammer
(Photo 2). Install the post base over
the anchor. As you tighten the nut on the
anchor, the clip expands and wedges tight
against the hole's walls to hold itself in place.
Cut a treated post to fit between the
post base and the top of the rim joist. Set
the post into place and tack it to the post
base with 8d or 10d galvanized nails
(Photo 3). Place a level alongside the
post. When it's plumb (straight), tack it in
place to the rim joist. Then install a connector
and drive carriage bolts through
the rim joist (see Problem 4 below).
Problem 4: Wimpy post connections
Ideally, posts should sit directly under the
beam or rim joist to support the deck. If
the posts are fastened to the side of the
beam or rim joist, like the one shown
here, the weight is put on the fasteners
that connect the post to the deck. This
deck had only three nails in the post—a recipe for collapse. Nails alone aren't
strong enough for this job, no matter how
many you use. For a strong connection,
you need 1/2-in.-diameter galvanized
Add two of these bolts by drilling 1/2-
in. holes through the rim joist and post.
An 8-in.-long 1/2-in. drill bit costs $10.
The length of the bolts depends on the size
of your post and the thickness of the rim
joist (add them and buy bolts at least 1 in.
longer than your measurement). We used
8-in. bolts, which went through two 1-1/2-
in. rim joists and a 3-1/2-in. post. Tap the
bolts through with a hammer, then add a
washer and nut on the other side.
Problem 5: Wobbly deck syndrome
If your deck gets a case of the shakes
when you walk across it, there's probably
no reason for concern. Still, in some
cases, the deck movement puts extra
stress on the fasteners and connectors.
Over time, the joists can pull away from
the rim joist or ledger board and twist
out of their vertical position, which
weakens them. Fastening angle bracing
under the deck will stiffen it and take
out the sway. The braces are mostly hidden
from view and let you walk on your
deck without feeling like it's going to fall
down at any moment.
Run a treated 2x4 diagonally from corner
to corner, under the deck. Drive two
16d galvanized nails through the brace
into each joist. If a single board won't span
the distance, use two, overlapping the
braces by at least two joists. Cut the bracing
flush with the outside edge of the deck.
Problem 6: Missing ledger flashing
The area around the ledger board should
be watertight. Even small leaks can lead
to mold inside the walls of the house and,
even worse, the house rim joist (which
supports the ledger) will rot and the
ledger will fall off. Stand or crawl under
the deck and look at the ledger board. If
you don't see a metal or plastic lip over
the top of the ledger board, add the flashing.
Flashing was completely missing
from this deck.
To add flashing, first remove the deck
board that runs alongside the house. If
the boards run diagonally, snap a chalk
line 5-1/2 in. from the house, then set the
blade in a circular saw to the depth of the
decking boards and cut off the board
ends. (Replace the cutouts at the end of
the job with a 5-1/2-in.-wide board
installed parallel to the house.)
For vinyl, wood or other lap siding,
work a flat bar under the siding and gently
pull out the nails (Photo 1). Insert
the flashing behind the siding (Photo 2).
If you have a brick or stucco house, you
probably won't see any flashing because
the ledgers are often installed directly
over brick or stucco.
We used vinyl flashing, but you can
also use galvanized metal or aluminum
flashing. At each joist location, make a
small cut in the flashing lip with a utility
knife so it'll lie flat over the joists. The rest
of the lip should fit over the top edge of
the ledger board.
You should have flashing under the
bottom edge of the ledger too. But since
there's no way to add it without removing
the ledger board, run a bead of acrylic
caulk along the bottom of the ledger
board to seal out water (Photo 3).
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Problem 7: Rickety railing posts
Loose railings won’t lead to your deck
falling down, but you could tumble off
your deck. Railing posts attached only with
nails are bound to come loose, and no matter
how many new nails you drive into them, you
won't solve the problem. Instead, add carriage bolts.
Measure the thickness of the post and rim joist, then buy 1/2-in.-
diameter galvanized carriage bolts that length plus 1 in. Also get a
nut and washer for each. Drill two 1/2-in. holes through the post
and rim joist. Offset the holes, keeping one about 1-1/2 in. from the
top of the joist and the other the same distance from the bottom
(make sure to avoid drilling where a joist abuts the rim joist). Tap
the carriage bolts through the holes, then tighten the nuts until the
bolt heads are set flush with the post.