When the joists run parallel with a wall that has been removed, you may find that one floor is not level with the other. This may have been caused by slight movement in parts of the building or it may be that the floors were never intended to be aligned. Depending on the difference between the floors, a slope or step will provide a satisfactory solution to the problem of misalignment.

When the joists of the two floors are supported on the same wall plate, the chances are that both floors will be at the same level. Because wood can shrink or warp, however, it may be necessary to pack or trim the top of one or two joists slightly to allow for the infill board to sit properly between the floors.

A misalignment up to 18mm (3/4in) can be accommodated by the short lengths of floorboards cut to span the gap. Although probably acceptable, the slope will be apparent. Where the difference in level is large, it may be necessary to create a single step or make a gradual slope. The latter should be less noticeable, but cannot satisfactorily run across a door opening.

Trim the ends of the floorboards on the high side flush with the joists and nail a batten to it. Trim the boards on the low side in the same way, but screw a 38mm (11/2in) thick planed softwood riser to the side of the joist to finish level with the batten on the higher floor.

If the floors are to be covered, cut and nail short lengths of floorboards to form the step tread. Where you want a bare-wood floor, a single board running the width of the step would look better. 

In this case, skew-nail noggings flush with and between the riser and adjacent joists at approximately 750mm (2ft bin) centres — necessary for a wide board that is weak across its width.

Where a floor has been raised, make a shallow threshold step at a doorway. Prepare a hardwood threshold board to fit between the door linings and finish flush with the raised floor. Nail it to the lower floor. Trim the door to clear the step and refit it on its hinges.

Cut the floorboards flush with the joist on the high side, and nail a batten to the joist as before. Remove the skirting boards from the side walls and lift the floorboards from the room with the lowest floor. Rest one end of a stout straightedge on the batten nailed to the higher floor and the other end on one of the joists of the lower floor to make a gradual slope (1).

Take measurements between each joist and the underside of the straightedge. Set an adjustable bevel to the angle between the side of each joist and the board. With a power saw, cut lengths of 50mm (2in) wide softwood at the required angle to fill these gaps. Nail the prepared packing to the tops of the joists in descending order (2).

Re-lay the floorboards, butting their ends against the boards of the higher floor. Insert new floorboards where necessary to fill any gaps.

For a finished-wood floor, re-lay and shuffle the boards from both floors to break up the straight joint line.

Replace the skirtings, following the line of the floor, and nail to the wall.