What is the chemical composition of pitch

Bitumen or resin?

There are three main formulas for commercial pitch.  Here they are.

Madadam hot spray.  55% petroleum asphalt, 30% creosote, 15% superfine sand.  They heat this mix in a gas fired kettle and spray it on a prepared bed.  They make that out of 1/6 #12 sharp sand, 1/3 crushed rock (3/8" minus) the balance 3/4" minus.  Lay a 9" bed of that blended material and hit it with hot spray until it will not absorb more.  Cover with an excess amount of #12 sand and let it cool.

Roofing hot spray.  1/2 petroleum asphalt, 1/4 coal tailings, 1/4 fiberglass filaments.  That is by weight.  Melt it in a roofing tar kettle and either spray or mop.  Apply at least 3/4" for flat roofs.

Cold patch.  1/4 petroleum asphalt, 1/10 oxidized petroleum grease (ASTM 23 non lithium) 1/6 naphtha, and the balance either carbon black or coal tailings.  They blend it hot and dispense it into 5-gallon pails.  You will see that on rooves that need emergency repair as it will stick in the rain and dries hard in a few hours. 

Heavy polycyclic hydrocarbons of various kinds.