Cal. Bus. & Prof. Code § 7108.6
A licensed contractor is required to pay all transportation charges submitted by a duly authorized motor carrier of property in dump truck equipment by the 20th day following the last day of the calendar month in which the transportation was performed, if the charges, including all necessary documentation, are submitted by the fifth day following the last day of the calendar month in which the transportation was performed. The payment shall be made unless otherwise agreed to in writing by the contractor and by the duly authorized motor carrier of property in dump truck equipment. In the event that there is a good faith dispute over a portion of the charges claimed, the contractor may withhold payment of up to 150 percent of the disputed amount or an amount otherwise agreed to by the parties. A violation of this section constitutes a cause for disciplinary action under Section 7120 and shall also subject the contractor licensee to a penalty, payable to the carrier, of 2 percent of the amount due per month for every month that payment is outstanding. In an action for the collection of moneys not paid in accordance with this section, the prevailing party shall be entitled to his or her attorney's fees and costs.
This section applies to all private works of improvement and to all public works of improvement.
Ca. Bus. and Prof'l. Code § 7108.6