Reveal, not face width
The single arithmetic error that costs people a return trip is counting courses off the board's face width. A nominal 1x6 shiplap is about 5-1/2 inches wide overall, but half an inch of that disappears into the rabbet where the next board overlaps it. The exposed reveal is 5 inches, so a 96 inch wall takes 20 courses rather than the 18 you get from dividing by 5.5. That is eleven percent more material, which is more than most people's entire waste allowance.
Tongue and groove is the same story with different numbers: the tongue is buried, so the face you see is narrower than the board you buy. Nickel gap is narrower still, since the deliberate gap between boards is part of the pattern but contributes nothing to the coverage. Measure a sample board, or better, dry fit two of them together and measure the reveal directly. Manufacturer figures for face width and coverage width are both published and people read the wrong one.
Where the last course lands
Twenty courses at 5 inches is exactly 100 inches, which on a 96 inch wall means the top course is exposed only one inch. That is a sliver at the ceiling, in the most visible plane of the room, and it is the panelling equivalent of a sliver tile at the wall.
The fix is the same idea in a different direction: rip the bottom course narrower. The bottom of the wall is where the baseboard is, and a board that has lost two inches off its bottom edge disappears behind trim. Take the awkwardness out of the top and put it at the floor where nobody looks. If there is no baseboard, split the difference between the top and bottom courses so both look intentional rather than one looking wrong.
Boards or sheets
| Individual boards | Sheet panels | |
|---|---|---|
| Speed | Slow — one course at a time | Fast — a wall in a few pieces |
| Seams | The pattern is all seams, so none of them read as joints | The vertical joints between sheets need a detail or they show |
| Walls out of level | Forgiving — each course can be adjusted slightly | Unforgiving — an out-of-level floor shows across the whole sheet |
| Material cost | Higher per square foot | Lower per square foot |
| Waste | Offcuts are usually reusable as short runs | An offcut from a sheet is often unusable |
Sheets win on a wainscot where a cap rail hides the top edge and a baseboard hides the bottom, because the only visible joints are the vertical ones between sheets, and those can be planned to land at corners or behind a stile in a board and batten treatment. Boards win on a full height wall, on a wall that is out of true, and anywhere the horizontal line of the pattern is the point of the exercise.
Fastening and the wall behind it
Panelling wants to be nailed into framing, not into drywall. Locate and mark every stud before you start, because a board fastened only to the drywall face will pull loose the first time the material moves. Where a course lands between studs and has to be fixed anyway, construction adhesive plus a brad works, but adhesive makes the panelling effectively permanent and takes the drywall face with it if it ever comes off.
Wood panelling moves seasonally across the grain. Bring the material into the room and let it sit for at least several days before installing, ideally a week, so it reaches the humidity it will live at. Boards installed straight off a cold truck will shrink and open gaps in the joints within one heating season, and a nickel gap pattern that grows to a quarter inch gap looks like a defect rather than a design.
Related
Panelling changes the depth of everything it meets, so the door and window casing usually has to be furred out or replaced — the trim calculator covers that footage. For the wall underneath, drywall sheets, and for the painted finish over the top, the paint calculator, remembering that a profiled surface takes noticeably more paint per square foot than a flat one.
Questions people ask
How do I find the reveal if the label only gives one width?
Dry fit two boards together the way they will sit on the wall and measure from the outside edge of the first to the outside edge of the second, then halve it. That is the reveal, and it is always smaller than the individual board width for shiplap, tongue and groove and nickel gap. If you cannot lay hands on the material yet, look for a coverage width or effective width on the product listing, which is the same number under a different name. Where a listing gives only one figure, assume it is the overall width and that the reveal is somewhere between three quarters and seven eighths of it, then confirm before ordering.
Should the boards run horizontally or vertically?
Horizontal is the common shiplap look and it is easier, because the courses land on every stud and you can nail anywhere along their length. Vertical boards need horizontal blocking or furring strips behind them, since there is nothing to nail into between studs, and that blocking has to go in before the panelling does. Vertical also makes a room read taller and horizontal makes it read wider, which is worth thinking about in a small bathroom. This calculator works the same either way as long as you enter the height and run consistently: the run is the direction the boards travel, the height is what you are stacking courses across.
Why is the waste allowance 12 percent rather than the usual 10?
Because panelling generates end cuts on every course, not just at the ends of the job. Each course crosses the wall and hits a corner, a door casing or a window at both ends, so the offcut count scales with the number of courses. A twenty course wall has forty end cuts. Some of those offcuts are long enough to start the next course, which is why the figure is 12 and not 25, but a room broken up by openings uses more than a plain wall. If you are running a pattern where the joints have to be staggered in a specific way, push it higher.
Does the calculator account for the boards over doors and windows?
Partly, through the deduction setting. Full deduction assumes the opening removes its full width from every course, which is only true for a full height opening such as a doorway. Half credit is the realistic middle for a window, where the courses above and below it run the full width and only the courses beside it are interrupted. No deduction is the safe choice and is what most people should use on a wainscot, where a window sill is above the panelling entirely and does not interrupt it at all. The openings are counted at three feet of wall width each, so adjust the count up or down if yours are much wider.
Can I install panelling straight over existing drywall?
Usually yes, and it is the normal approach. The drywall gives a flat backing and the panelling nails through it into the studs. Two things to check first. The wall has to be reasonably flat, because thin panelling telegraphs bumps and any drywall repair that stands proud will show as a ridge running through several courses. And everything that meets the wall gets pushed out by the thickness of the panelling, so outlets need box extenders, door casings need furring or replacing, and the baseboard has to be removed and refitted over the new surface rather than left in place.