Vinyl Siding Bulging or Wavy? Causes, Checks, and Fixes

Pick a hot afternoon, stand back, and sight along the wall at a low angle. If the bulge is strong in the sun and nearly gone on a cool morning, tight nails are the probable culprit, and a siding zip tool and a hammer may be all the fix takes. If the bulge is permanent, soft, or stained, the cause is different and the repair probably isn't a DIY one.

What does a bulging or wavy vinyl siding panel mean?

A bulge or wave usually means a panel can't slide as it expands, though a permanent or soft bulge can point to moisture or damaged sheathing behind it. A 12-foot vinyl panel can change length by up to roughly half an inch across a big temperature swing, and dark colors in full sun move the most. Vinyl is meant to hang loosely on the wall, not be clamped to it.

Why do tight nails make vinyl siding bulge?

A panel nailed tight has nowhere to go when it grows in the heat, so it buckles outward. Set vinyl siding nails in the center of the slot with the head standing off the panel by about 1/32 inch. A nail gun makes it easy to drive nails snug against the vinyl, or to nail at the end of a slot where the panel can't shift at all.

The giveaway is a wave that repeats along a course and looks worse after a hot spell. We'd check this first on any wavy wall, before spending money on panels or a contractor.

How do you check for pinned vinyl siding?

You can check for pinned siding in about ten minutes with your hands and a siding zip tool. Do it on a mild day.

1. Press the flat of your hand against the panel and slide it side to side. A properly hung panel moves easily, roughly half an inch or a bit less. If it won't budge, it's pinned.
2. Check the end gaps where the panel meets J-channel or corner posts. Manufacturers typically call for about 1/4 inch in warm weather and up to 3/8 inch in cold weather, so check your siding's instructions. A panel jammed against the trim has no room to grow.
3. Hook the zip tool under the bottom lip of the panel above the bulge and slide it along to release the seam. Then look at the nails. Tight ones have heads pressed flat against the vinyl or sit off-center in the slot.

Can a missing starter strip or jammed ends cause waves?

Yes. A missing or wrong starter strip, or panels jammed at the ends and corners, can cause waves even when the nails are fine. Without a proper starter strip, the bottom course can sag and ripple, and panels cut too long and forced into corner posts push outward as they warm.

Look at the bottom course and the corners. A panel pressed visibly against trim, or a bottom edge that hangs loose, is a likely suspect. Trimming about 1/4 inch off a jammed end solves many of these.

What if water behind the siding is causing the bulge?

Water behind vinyl siding causes bulges that stay put in cool weather and often come with stains, soft spots, or a musty smell. Vinyl itself doesn't absorb water, but the wall behind it does. Typical sources are failed window flashing, gaps in the house wrap, clogged gutters, a sprinkler hitting the wall, or a leaking dryer vent.

After releasing a panel, press the sheathing with a screwdriver tip and read it with a pin-type moisture meter. Readings above roughly 20 percent mean the wood is wet enough to risk rot. Don't caulk over it or nail the siding back down and hope. Find the water source first, because everything behind the siding keeps rotting while the outside looks fine.

Could the sheathing underneath be the problem?

Yes. A bulge that lines up with a swollen or soft spot in the wall sheathing means the siding is just following what's underneath. Oriented strand board swells at its edges when wet, plywood can delaminate, and rigid foam boards can come loose. Any of these will push the vinyl out no matter how loosely the nails are set.

If the wall behind a released panel feels spongy or crumbles, stop. That's a job for a siding contractor or carpenter, and it may involve structural framing. On homes built before 1978, old painted wood under siding or trim can contain lead, so hire a contractor certified under the EPA's lead renovation rules before disturbing it. Very old homes can also hide asbestos-containing materials, such as cement shingles, under newer siding. Don't cut or break anything you suspect and have it tested first.

How do you fix a heat-related bulge step by step?

To fix bowed vinyl siding caused by tight nails, release the panel above the bulge, back the nails off, check the end gaps, and snap everything back into place. You'll need a siding zip tool, a claw hammer, a flat bar, tin snips or a utility knife, and a few corrosion-resistant nails if any need replacing.

1. Work on a mild day, ideally 50 to 70°F, when the panel is neither fully expanded nor shrunk.
2. Run the zip tool along the bottom lip of the panel above the bulge to release it, then hold it up out of the way.
3. Find each tight nail. Back it out until the head stands about 1/32 inch off the vinyl, or pull it and drive a new one in the center of the slot. Nails should reach at least 3/4 inch into solid wood.
4. Trim any panel end that's jammed against J-channel or a corner post until you have a gap of about 1/4 inch.
5. Push up on the bottom lip of the released panel until it snaps along its length, then slide it side to side to confirm it moves freely.
6. Recheck on the next hot afternoon.

If a panel stays wavy once it's free, heat may have stretched it permanently. Swap in a replacement vinyl panel. Old siding fades, so take a sample to match color, or pull a spare from a less visible spot.

When should you call a siding professional?

Call a siding professional when the bulge stays after you loosen the nails, the wall behind it is wet or soft, or the problem covers more than one wall. Add any home built before 1978 and any wall that needs a tall extension ladder.

A siding contractor typically charges about $150 to $500 to loosen nails and replace a few panels. Opening a wall to replace rotted sheathing can run $500 to $3,000 or more, depending on how much is damaged and how high it is. Structural repairs may need a permit, so check with your local building department, since many local codes are based on International Code Council model codes. For hiring, the FTC's consumer advice site offers guidance on vetting contractors and avoiding repair scams.

Your next step: on the next hot afternoon, photograph the bulge, then slide the panel by hand. If it's stuck, you've found your fix.