There is a huge misconception that any roof vent cap available is good enough to use as a dryer vent that exits the home on the roof. Not only does Building Code require that a dryer vent termination be free from a screen, but there are several reasons it's a bad idea to have a screen in your vent cap.
A screen in your vent cap will build up lint over time and cause a blockage. This leads to the following problems,
Decreased efficiency: Running the dryer 2-3 times can cost a family of four, hundreds per year.
Extreme fire risk: All it takes is one spark and some lint and you can have a fire that spreads to your roof.
More frequent dryer vent cleanings are required: This can get inconvenient and expensive over time.
Property damage: The clogged cap traps moisture and the moisture can penetrate the sheeting on the roof causing it to rot. These types of repairs are expensive and require hiring a roofer.
Please have a professional, with proper training in the products they offer, install a roof jack. Here are some pictures of a roof vent that became clogged and caused the sheeting to rot. Several shingles had to be removed, and the roof completely opened up and repaired properly.