Buckling. A number of factors can cause shingles to
buckle: improperly applied felt, wrinkled underlayment, poor roof
ventilation or new shingles applied over an existing layer of shingles.
If buckling is caused by wrinkled felt below the shingle, a roofing
professional can simply remove the affected shingles, cut the wrinkle
and replace the shingles. If the cause is poor ventilation, you’ll need
to add ventilation to the attic space to allow heat and moisture to
escape and keep attic temperatures lower. 
If buckling shingles are left untreated, the shingles will begin to
crack and tear and will need to be replaced immediately to avoid
significant damage to the roof or roof deck which can lead to leaks and
rotting. Consult a roofing professional for this job. If your roof has
more than one layer of shingles, the top layer will eventually conform
to and reveal any imperfections of the layer below. The only solution is
a complete tear-off and re-roof.
Curling. Curled edges on shingles can be a result of improper
fastening (such as high nails or too few nails), poor roof ventilation
or lack of a back-coating on the shingles. Curling is more common in
organic shingles such as wood and will often start at the bottom edge of
the shingle. However, it’s important to note that if roofing is
installed on a cold day, some temporary curling or cupping is natural.
As soon as the temperature increases the shingles should lay flat. If
the problem persists, contact a roofing professional as soon as possible
to avoid further damage to the roof.
Rotting. Rotting is caused when the mat at the core of the
shingle absorbs moisture. Rotting is more common when the mat is made
from organic compounds. Replace rotted shingles with ones made from
non-organic compounds, such as fiberglass, which resist moisture and are
less likely to rot.
Detecting the warning signs of a rotting roof is easier than you’d
think and doesn’t require a roofing expert. By simply standing on the
ground and looking up at the roof, look for shingle irregularities.
Depending upon the roof, these irregularities appear as discoloration,
black spots or large spaces of missing color granules. Because granules
protect the shingle from the harmful affects of UV light, without
granules the asphalt is exposed to the elements and will turn black and
begin to rot.
Blistering. Blisters are sometimes found in non-organic
shingles and are caused by trapped moisture in the shingle. Not all
blistered shingles need to be replaced. But, if blistered shingles break
open they can leave shingles susceptible to other damage such as
granule loss, color fading and may even lead to water leakage and should
be replaced. While blistering shingles are rarely visible from the
ground, a punctured blistered shingle will be easily spotted because it
will likely appear to have black spots – which are actually the exposed
asphalt shingle.