Waves along whole runs: check the nailing
Vinyl gets longer in heat and shorter in cold, so it is hung loosely, not pinned. The Building America Solution Center's vinyl siding installation guide calls for nails in the center of each slot, no more than 16 inches apart, with about 1/32 inch left between the nail head and the panel. It also calls for a gap of about 1/4 inch where panels meet trim when installed above 40°F, and 3/8 inch below 40°F.
Signs the nailing is the problem:
- Waves or bulges run along long, straight stretches, often worst on the sunny side.
- A panel won't slide side to side at all when you push it gently along its length.
- Panel ends are jammed tight against corner posts or window trim, with no gap.
- The rippling started soon after new siding went up, or after a repair.
Fixing this means removing and rehanging the affected courses with proper slot nailing and end gaps. Panels that have stayed distorted for a while may need replacing.
Siding was painted darker: the paint is the cause
Sherwin-Williams' vinyl siding warping note says the likely cause of buckling on repainted vinyl is a paint darker than the original color. Dark paint absorbs the sun's heat, the vinyl expands too far, and it may not shrink back. Their advice is to use a vinyl-safe color line or a shade no darker than the original, and to skip oil-based coatings, which aren't flexible enough for vinyl. For panels that are already warped, they say to have a contractor assess them, because they may need to be replaced.
One wavy patch: look across at the windows
A localized patch, sometimes a ripple only a foot or two wide, is the classic sign of reflected sunlight. NAHB's builder paper on low-e window reflections says vinyl begins to distort at around 160 to 165°F. Reflected sunlight from double-paned low-e windows has been measured at more than 200°F at the point where it focuses. It notes the low sun of late fall, winter and early spring makes this more likely. A builder survey it cites found most damage happened when the window was 30 feet or less from the siding.
The Polymeric Exterior Products Association says most cases involve double-pane glass that has bowed slightly inward and focuses the beam like a lens. That means both light and dark siding can be affected, and siding color is at most a secondary factor.
This matters in Philadelphia, Bucks and Montgomery County neighborhoods where twins and closely spaced houses sit within a few yards of each other. To check it:
- Note the time of day and season the patch gets hit, then look for a window, sliding door or storm door that faces it, on your house or a neighbor's.
- Look for a bright spot of reflected light on the siding at that time.
Replacing the panels won't stop it, because the reflection comes back. The PEPA solar reflection page and the NAHB paper list fixes that block the beam: a screen on the offending window, an awning or sunshade, or a tree, shrub or other barrier between the glass and the wall. If the window is a neighbor's, any fix needs their cooperation, and PEPA suggests contacting the window and siding manufacturers about your specific situation.
Don't skip the wall behind it
Vinyl is a rain screen, not a waterproof skin. The BASC drainage plane guide says most cladding lets some water through, so a water-resistive barrier behind it has to be lapped over flashing at windows, doors and other openings. When panels come off, the solution center's re-siding guide says to inspect the old cladding, the sheathing and the inside wall finish for staining, mold or rot, especially around windows and doors, and to fix the cause before new siding goes on. A musty closet on an outside wall or soft spots near a window are reasons to ask for that check. The flashing guide covers why leaks so often start at those joints.
Questions to ask before anyone replaces panels
- Which cause do you think this is, and what did you see that points to it?
- If it's nailing, will you rehang the surrounding courses or only swap the bad panels?
- If it's a reflection, what will stop it from happening again?
- Will you check the sheathing and housewrap while panels are off, and show me photos?
- Can you match the existing profile and color?
Historic properties in Philadelphia
If your house is on the Philadelphia Register of Historic Places, the Historical Commission reviews alterations to exterior appearance, including facades and trim, and there is no charge for that review. Check whether your property is designated before replacing or re-siding a visible wall.