Normally, to find the root of a verb in the subjunctive tense, you would take the nous-form and remove the -ons. In être, the nous-form is nous sommes, so there is no -ons to remove. What is the subjunctive root of être?
Être, présent du subjonctif :
You would say the root would be "soi-"
"être" is as strange in French as it is in English : I am, you are, he is, don't show any roots either.