Winter is a great time for dirt, leaves, and other debris to pile up in your rain gutters. Leave it long enough and there will be buildup.  Leave it a really long time and those bits of dust, plantlife and other things will mush together and decompose, eventually becoming plant-worthy soil.  Add a few seeds and some rain, and before you know it, green sprouts are popping up on your roof.

Today I passed a home in Los Gatos in which this seems to have happened.  Grass and weeds were sprouting out of the rain gutters and had crept onto the roof quite a few inches.

Why is this a problem? Because the more you grow things on the roof, the more moisture you have getting trapped up there and the faster the roof decomposes.  You don't want things like fungus and dry rot to munch away at your home, but having plants grow on your roof is going to lead to exactly that.

Keep your gutters clean so problems like this do not have a chance to start.  It is infinely easier to prevent this than to cure it.

(Not a great photo - taken with my treo's camera and enlarged to show the roof but trying to keep the house itself anonymous.)