Reverse, Switch Direction
Reversing or switching the direction may occur when the installation goes to other rooms and some areas may fall 
behind you; also called backfill (shown in the second illustration below). When entering the new area we have to establish another reference chalk line. The installation has to follow the same general alignment of the completed area so the rows will line up square with opposing parallel walls in the new area.
Establishing The New Chalk Line
Take a few scrap pieces and engage them tight (not fastened) against the last completed row with the tongue facing out. On the underlayment make two pencil marks where the tongue falls. Remove scraps and you have two reference points. With a helper at one side of the room (top) and you at the other (bottom) align the chalk line so it gets snapped over the two points. With a tape measure check to see if the new line is square with the three parallel walls in this area (illustration 2).
About Back Filling
With back filling, the installation runs backwards. In other words, the tongue will have to face the opposite direction of the completed area so we can staple it in. To do so we have to extend the next row of boards through the area (top to bottom). The already completed area can simply be stapled.
For the balance, align the boards with the tongues 
facing out on the edge of the chalk line and fasten. Yes, they will move unless you place some temporary scrap backer boards behind it. Use as many pieces as you like, attach to the subfloor every twelve inches (just a number) or so with tongue and groove engaged. Grab that nail gun and shoot three or four directly into the scrap pieces.
You could just as easily use some decking screws to attach the scraps, but they generally do not go in well without (they split) pre-drilling through the hardwood. Once completed you now have enough support that will prevent the new row from moving off the chalk line when nailing.
Slip Tongues, Splines
Most installers don't do backfill areas until the rest of the forward layout is installed. It's your option, complete or just finish off the fill in area. Now that we have that row installed with the groove facing the 
backfill area we need the use of a spline, or as some call a slip tongue. A slip tongue (shown in black) is a strip that will be inserted into the groove facing the backfill area. It essentially becomes a new tongue and is glued and stapled into place as shown in the pictures below.
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Glued spline |
Nailing after inserting spline |
Splines can be purchased, but can be hard to find. Some installers simply make their own slip tongues with the use of a table saw on site with the same material. If you're considering such, it's important 
they fit snug and not loose. Otherwise they will allow the two boards connected to rub against one another causing squeaks.
Finishing Off The Backfill Area
Finishing is just as easy as the rest of the installation. Simply rack out some boards, while grabbing those straighter ones that will go against the wall where the nailer cannot reach (discussed on previous page).
View the spline video
