The ROOT team provides ready-to-use binary distributions for some systems (see “Download a pre-compiled binary distribution” therein), including various Ubuntu versions, which you just need to download and unpack. For Ubuntu 16.04 / x86_64 / gcc 5.4:
https://root.cern/download/?C=M;O=A;P=*[uU][bB][uU][nN][tT][uU]*16*
BTW. Before using it, make sure that you “completely remove” (“purge”) all “system-provided” ROOT related packages.