Nothing, in most cases. Small, hairline cracks are normal and generally won’t compromise stucco’s integrity. Stucco is not waterproof; rather it is inherently breathable so moisture can leave the wall cavity. It is the tarpaper layer that protects the interior of your home from the elements while the stucco protects the tarpaper. Two things can damage the tar paper moisture barrier: ice or cracks caused by structural shifting. If cracks are more than 1/8” wide or there is an accompanying bulge, call us for a free estimate to take a look. Please note that caulking cracks won’t seal the moisture barrier and will make a re-stucco job more difficult and costly.