How is the gross valuation for a JCT interim payment built up?

The gross valuation is the running total of everything the contract says is payable at the Interim Valuation Date. The sum due is that figure less retention and less the amounts already certified. In section 4 of the JCT Standard Building Contract With Quantities 2016 (SBC/Q), it is built in layers. First comes the value of work properly executed, measured against the Contract Bills at the Interim Valuation Date. That includes Variations valued under section 5 and any adjustment for approximate quantities. Defective work not yet put right is not properly executed, so it comes out. Second come materials and goods delivered to site for the Works. They count as long as they are there reasonably, not too early, and properly protected against weather and damage. Third come listed items held off site, where the Contract Particulars list them and the contract's conditions for them are met. Then come the other amounts the contract puts through the Interim Certificate, such as loss and expense already assessed and fluctuations where they apply. Some of these carry retention and some do not; the contract says which. Retention at the Retention Percentage comes off the parts that carry it. So do any deductions the contract allows against the Contractor. One example is the cost of others doing work after the Contractor failed to follow an instruction. Finally, the total stated as due in earlier Interim Certificates comes off, and the balance is the sum due for the period. The usual mistakes are valuing to the application date instead of the Interim Valuation Date, and carrying last month's measure forward without checking what was done. Others are taking materials on site that are unprotected or arrived early, and applying retention to loss and expense, which the contract does not do. Your sub-contract valuation is built on the same layers, so check your application and your main contractor's Payment Notice against each one.