A process for manufacturing fiberboard by preparing a fiber slurry mixture including containing cellulose fibers and water then atomizing a liquid bituminous material, such as asphalt. The liquid bituminous material is atomized by mixing it with a pressurized gas, such as compressed air, forming a mist comprising droplets of bituminous material having a diameter between 20 microns and 50 microns. A water spray solidifies the bituminous material droplets thereby forming bituminous particles which fall into the fiber slurry within the spray chamber. From there the slurry is sheared, dewatered, and dried, forming a finished fiberboard.