A slidable cover for truck rack comprising an impermeable horizontal cover plate that is slideably attached to an existing truck rack or cargo rack on a vehicle so that the cover plate remains fixed in the vertical direction but is slidable in a horizontal direction so that the cover plate may be slid back and forth horizontally to move the protection area that the cover plate provides. Cover plate may be slid rearwards to provide shelter over the rear of the vehicle or cover plate may be slid forwards to provide shelter over the cargo area of the vehicle.