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Ship Lapping Walls

by mangorockfish
posted 07-31-2017 03:39 AM

4 replies so far

View dday's profile


180 posts in 2277 days

#1 posted 07-31-2017 01:33 PM

Do you have a compass or something similar? Lay a board on the wall and butt the corner to the arch of the ceiling. Take you compass and spread it far enough to reach past the gap at the bottom of the board. Scribe the angle on the board by running one leg along the arch and the other across the board. Now you have the exact line you need to cut.

You can do this with cardboard and transfer to a single board ( each end) if it needs to span the whole distance.

View JBrow's profile


1368 posts in 1767 days

#2 posted 07-31-2017 02:08 PM


Another method for determining the angles where the ceiling meets the walls is to use trigonometry, specifically the cotangent function, also called the Inverse Tangent function.

The first step is to measure the distance between the two walls (the RUN). Next the RISE must be calculated. To calculate the RISE, subtract the floor to ceiling height at the short wall from the floor to ceiling height at the high wall. This value is the RISE.

The RUN is divided into the RISE, resulting in a decimal value.

This decimal value is entered into a calculator set to calculate the cotangent with a result in degrees. The calculated value will be the angle at the short wall between the ceiling and an imaginary line drawn perpendicular to the short wall and ending at the high wall. This angle can be used to set the bevel on the saw to cut the ship lap planks.

To find to corresponding high wall/ceiling angle (if wanted), the short wall angle is subtracted from 90 degrees.

I suspect the description is a little confusing so the first sketch is my attempt to pictorially depict this description. In the sketch, the RUN or distance between the walls is 144”. The short wall height is 96” and the long wall height is 144”. Thus the RISE is 144” – 96” = 48”. The decimal value is RISE/RUN = 48”/96” = 0.3333. The cotan of 0.3333 is 18.4 degrees. This is the angle from the imaginary perpendicular line at the short wall and the ceiling (a in the sketch). The angle where the ceiling meets the high wall is 90 – 18.4 = 71.6 degrees (b in the sketch).

Here is an on-line calculator for cotan or Inverse Tangent…

Sketch for calculating wall to ceiling angles…

For me cutting complimentary angles at the mitre saw can be a little confusing. If it is confusing to you, perhaps this sketch showing the saw set up to cut the high and short wall planks with a single mitre saw angle might be helpful. Since it would be unusual for the math to work perfectly, making a few test cuts to find the perfect angle is probably worthwhile.

View tomsteve's profile


1064 posts in 2066 days

#3 posted 07-31-2017 07:04 PM

keep it simple.when ya get the siding up near the angle, set a full piece in place. slap a piece of siding up against the ceiling and mark the cut on the siding.

View Smitty_Cabinetshop's profile


16901 posts in 3466 days

#4 posted 08-01-2017 03:24 AM

How about a Stanley bevel gauge. Put it up to the ceiling, level the other portion, then transfer the angle to your mitre saw. Done and done.

-- Don't anthropomorphize your handplanes. They hate it when you do that. - OldTools Archive -

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