The estimation of quantities in construction is essential for the proper allocation of resources, budgeting, and planning. In the context of structural timber work, we need to determine the correct basis for its quantity estimation.

When estimating the quantity of structural timber work, the focus is on determining how much timber (wood) is used in the construction of beams, frames, or other structural components. The most accurate way to measure this is by considering the volume of timber used. Here's why:

1. Volume of Timber Used:
   • The volume is calculated using the dimensions of the timber elements, often in cubic meters (m³).
   • This measurement takes into account the length, breadth, and depth of timber pieces, making it a precise way to evaluate the material used.
   • Since structural timber is bought and sold by volume, this is the most practical and accurate method for estimation.
2. Incorrect Options:
   • Number of Rooms: Rooms vary in size and construction requirements; hence, calculating on this basis would be inconsistent and imprecise.
   • Weight of Timber Used: Weight does not directly correlate with the structural applicability or procurement of timber since different timber species have different densities.
   • Floor Area of Building: While floor area might help in estimating other materials (like flooring), it does not directly relate to the amount of timber required for structural work.

Therefore, the correct answer is based on the method which accurately reflects the quantity of material available and usable in construction: Volume of timber used.