In the single derricks the queen legs can be connected by a trussed beam similarly formed, or by a single balk carried across and laid on the top transom.
Fig. 4.—Plan of Top Platform partially covered
If the span is considerable, struts can be carried from the queen legs towards the centre of the underside of the balk to prevent sagging.
On the trussed beams thus formed, joists of 9 in. by 3 in. or ordinary poles are laid about 3 feet apart.
They are laid parallel to one another, and in a direction at right angles to the truss or single beam forming the back support of the platform.
The centre joists are continued to the ends of the balks which project beyond the king leg.
The advantage of having continued the top balks can now be seen, as it gives greater area to the platform immediately round the engine.
The boards 9 in. by 11⁄2 in. are laid at right angles to the joists.
Another way of forming the platform is to cover only partially the surface between the legs. In this case two additional joists, 6 in. by 6 in., are thrown across the king leg (see [fig. 4]), the boards not extending beyond their length.