That's a rainy part of the world. Used to live in Canada. Now live in Japan. Also can be very rainy here. Just get a good tent and be dry, comfortable and safe. For the sake of 500g or even 1kg (same as 1 litre of water) there is no point fooling about with tarps or biv bags etc. Just image you have to spend a whole rainy day in your shelter. In a tent you are laughing as you read your comedy novel. Anything else you are crying on the pages of the Shakespearian tragedy.