Are cracks in swing set wood safe?

The short answer

Most cracks in a wooden swing set are checks, and they are safe. A check is a lengthwise separation of the wood that "commonly results from stresses set up in wood during seasoning", in the words of the USDA Forest Products Laboratory's Wood Handbook⁷. It runs along the grain, it shows on the face or the end of a post or beam, and every maker compared below calls it normal.

A crack is not safe when it does something a check does not: crosses the grain, runs from a bolt out to the end of the board, sits where the swing beam meets the legs, goes deeper than a quarter of the wood's thickness, or has soft wood inside it.

Owners hear two things: "wood cracks, it's fine", and "any crack in something children hang from means replace it". Both are half right. The makers say checking is normal and large cracks are not, but none of them draws the line in one place. The decision table below does.

Why wood checks in the first place

Wood dries from the outside in. The Wood Handbook explains that early in drying the outer shell shrinks while the wetter core holds it, so the shell goes into tension, and "sometimes surface cracks, or checks, occur"⁷. End checks form the same way on the cut end, where moisture leaves the wood fastest⁷.

CedarWorks describes what that looks like on a playset. The sapwood around the heartwood "dries more quickly than the heartwood" and checks, but "checking does not pass through the heartwood itself"¹. Most of it happens "within the first several months" and subsides once the wood has adjusted¹. That is why a set that looked perfect on assembly day shows open seams on its posts by the end of its first season. The materials comparison covers why wood moves outdoors in ways steel and plastic do not.

What five makers and the CPSC say is normal

Gorilla's warranty excludes checks "that do not affect the intended function of the part"². A crack that does is the one to photograph and send to the maker.

The one number in the table is Jungle Gym's: cracks "smaller than ¼ of the wood's length or cross-section will not affect the strength or stability"⁶. Applied to common playset lumber:

The decision table

Why the beam to leg joint is different

In 2006 the CPSC announced a repair recall of about 3,600 swing sets after 22 reports of "cracks at the connection between the swing's horizontal top beam and the vertical end support post"⁸. Consumers were told to stop using the sets immediately and to "remove the swing's chains and seats from the top beam" until a repair kit was fitted⁸. Those were metal frame sets, not wood, so the recall says nothing about checking. What it shows is where a swing set cracks dangerously: the connection that carries every swing's load into the legs.

The Wood Handbook makes the same point for wood. Separations along the grain cut a bending member's resistance to shear, so grading rules restrict them "most closely in those parts of a bending member where shear stresses are highest", and checks and splits are limited on the same principles⁷. In a beam resting on two supports, shear is highest at the ends, beside the legs. The same check that is harmless mid span matters more where the beam sits on the A-frame. The swing set weight limit page shows that load reaching two to three times body weight at the bottom of a high arc.

The handbook also notes that its connector strength figures come from tests on joints "whose members were clear and free from checks, shakes, and splits"⁷. Published joint capacities assume sound wood around the bolt.

Cracks you caused: overtightened bolts

Gorilla says to check bolts twice a month in season, "but not so tight that you crack the wood"². Rainbow gives the same warning and a number for its acorn nuts, 5 foot pounds of torque⁴.

A fresh crack radiating from a washer you have just cranked down is crushed or split wood, not seasonal checking. Back the bolt off to snug. If it no longer holds, the hole needs repair: repairs and replacement parts covers the dowel and through bolt fixes, and hardware and fasteners covers what to replace it with.

How to check a crack in five minutes

1. Look at the direction. Along the grain is a check. Across the grain is a break.
2. Measure the depth. Push a thin plastic card into the widest point, mark it at the surface, and compare with the quarter limits above.
3. Mark the ends with a pencil line and the date. Checking mostly settles after the first several months¹, so a crack still growing past its marks in later years is outside what the makers call normal.
4. Probe it with an awl. Hard wood is a check. If the point sinks in, it is rot, and timber rot and repair takes over.
5. Load it. Have an adult swing gently while you watch the crack and the nearest bolt. Nothing should open, flex or creak.

Add it to the spring list in annual maintenance, or the longer inspection checklist.

Sealing, sanding and what not to do

Gorilla says that treating the set with stain regularly "will help prevent severe checking/splitting and other weather damage"², and Rainbow says to spot sand any area that is checking or splintering⁴. That is the whole treatment for a normal check. Painting and refinishing covers products and timing. The Wood Handbook says shake, the related separation along the grain, lets in moisture that leads to decay⁷, which is reason enough to keep checks sealed.

Two exceptions. On older sets built from CCA treated wood, EPA and CPSC advice is to avoid sanding. And do not fill a check with rigid filler or caulk: it opens and closes with the weather, so the filler cracks out or seals water in. Round holes beside a check are carpenter bees.

Where this stops applying

The quarter rule is one maker's, written for its own timber; no standard read for this page gives a crack limit for home playsets. The makers' "normal" applies to their own designs, so a homemade set built from DIY plans carries no such assurance, and neither does a secondhand one: buying a used playset covers what to check before you pay. Cracks in metal, welds and plastic are never checking.

What to do next

Walk the set with a card and an awl and sort every crack into a table row. Seal the checks. For anything in a stop row, take the swings off and work through repairs and replacement parts, including its retire test if the beam or two legs are involved. Then recheck the swing safety clearances and the anchoring.