Any wooden beam on the outside of a house is susceptible to rot or insect damage. How likely your deck is to have problems depends on a combination of the construction techniques used and the weather in your area. Rot and insect problems usually result from:
Some deck builders go to great lengths to prevent individual beam members from trapping water. They use thin, triangular wood spacers between built-up beam members to ensure that water can flow freely between adjacent two-by-eights in a beam, for example. This space also provides ventilation allowing for quick drying after the wood gets wet. This is a good detail, but not a common one.
To check your deck,look at places where water may be trapped in or against the beam. Probing with a screwdriver is required. Rotted wood can sometimes look just fine from the outside. Built-up beams are susceptible to rot since the water is often trapped between beam components.
Insect exit holes in the wood surface are also clues to possible damage. Paint or stain that has not been well kept up makes wood more vulnerable to rot. Where paint or stain is not in good repair, watch for rot.