I understand your worry about the thawed slush--which is VERY heavy--sliding off the roof and destroying your deck. Not only could this heavy snow destroy your deck, though, it can also really hurt someone. My family and I had the same problem with a home in northern Idaho where there is heavy snowfall in the winter. I'll give you the low-down on how we outfitted houses out in Idaho to protect against this.
If you decide to go with metal or slate roof then you will definitely want to install some type of snow guard. There are several companies that make them--google search "snow guards for roofs"--and it is possible to install them yourself. Snow guards are devices that stop avalanching snow--sort of like avalanche barriers up in the mountains--and instead allow snow to come off the roof in safe quantities.
A lot of people say you do not need snow guards on composite roofs. However, whether or not your composite roof requires snow guards depends on many things such as the slope of your roof, average snowfall, etc. In my experience you can get away with no snow guards on a composite roof as long as you shovel off the snow regularly (which should be done anyway).
I hope this helps!
My company is<a href="http://www.masterswa.com">Masters Construction</a>.
I'm glad to answer any roofing questions you may have!