Board direction and coffer math for a great-room ceiling

Two calculations that settle most ceiling layout arguments before a single board is bought.

Reviewed Oct. 17, 2026. Sources appear below the guide.

Ceiling design looks like taste, but a lot of it is arithmetic. Which way should boards run? How big should coffers be? Both questions have numbers behind them, and working those numbers out early saves second-guessing later. Our example is an 18 by 24 foot great room, a common size for an open living area.

How many rows of boards does a ceiling take?

Divide the distance the rows have to cover by the exposed face of one board, then round up. With a 5.5-inch exposed face on our 18 by 24 foot room:

The footage is nearly identical either way, which means direction is a design choice, not a budget one. The last row is rarely a full board; splitting the difference between both edges keeps the cut rows equal and less obvious.

So which way should they run?

Boards running the long way pull the eye down the room and make it feel longer. Boards running across make a long room feel wider and calmer. In an open plan, a useful tiebreaker is the main view: running boards toward the sliders and lanai leads the eye outside. Where a ceiling steps or has a beam, boards usually look best meeting that line squarely rather than at an angle.

How big should coffers be?

For a grid with beams only between coffers, each coffer measures the room dimension minus the total beam width, divided by the number of coffers:

Coffers that come out close to square, as these do, tend to look orderly. If one direction comes out much longer, try adding or removing a coffer in that direction and rerun the numbers. Wider beams make each coffer smaller and the grid heavier; on a tall great-room ceiling a heavier grid can read better from the floor.

What about tray ceilings?

A tray is a single large recess, so the arithmetic is simpler: the border around it sets the proportion. Many designers keep the border width consistent on all four sides so the tray reads as centered in the room, even when furniture is not. Measure from the walls, not from the light fixture, which may not be centered.