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 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, the roof completely opened up and repaired properly.
The roof was damaged by moisture trapped under a box-style vent cap with a screen