I recently came across 150 sqm (1,600 sqf) of Iroko decking being discarded from a renovation site that was in good enough condition to salvage. I plan to use some of it for my own deck in a house I'm planning to build and use the rest for flooring. I'll have the grooves on top planed to a smooth surface for the flooring but I'm unsure of the best way to install it. It has grooves (no tongue) on both sides and the deck mounting fasteners leave a gap between each board which is not desirable for internal floors. Any suggestions welcome. Boards are 2cm (3/4") x 10 cm (4") and the floor will be installed over a concrete slab.






https://www.dropbox.com/sh/4rgwejyf8ihw ... Ojc1a?dl=0

The grooves are probably not deep enough to insert a spline, now. You can deepen both grooves and use a grooving bit to make a space for a hardwood spline.
Lay down the first board, then insert the spline. Fasten the boards down by going right through the spline with the fasteners. Spline is sold by the lineal foot at hardwood distributors.
Maybe turn the boards over and plane off the bottom of the boards. Decking gets all kinds of grit embedded in the wear surface which will dull regular planer blades quickly.