Most boats are longer than a standard sheet of plywood. While you
can purchase plywood in lengths longer than 8', it can be expensive
and difficult to find.
This essay shows three methods of splicing shorter pieces of wood
into one longer piece.
Method 1: The Butt Joint
The easiest way to join two pieces of plywood is the simple Butt
Joint. It is fast, cheap, and easy, but some people find the joint
The butt strap usually has the same thickness as the plank (it
is most often of the same material) and should be 16 to 20 times
as wide as the plywood is thick. 1/4" ply will require a butt
strap 4" to 5" wide.
The butt strap is glued to the inside of the plank - usually the
'bad' side of the plywood. Some people find beveling the edges of
the strap gives it a more aesthetically appealing look.
Support the joint on a flat surface and apply uniform pressure
over the entire butt strap while the glue cures. Remember to lay
down a glue barrier!
Note: Because the butt-strap makes the joint twice
as thick as the rest of the plank, the joint will not bend as easily
as the rest of the plank - resulting in a 'hard spot.' If you are
making a curved boat, you can mitigate the hard spot by putting
your joints where the plank will bend the least or beveling the
face of the butt strap to match the curve.
Method 2: Fiberglass Butt Joint
Another method of joining plywood was popularized by Harold "Dynamite"
Payson - using fiberglass tape and epoxy to make a light, strong,
smooth joint. It is said Payson grinds a hollow area under the fiberglass
so there is no visible bump. Experience has shown that step is an
unnecessary complication for little aesthetic value - the edges
of the fiberglass can be sanded so the join is nearly invisible.
The fiberglass tape should be much wider than the plywood is thick
- 3" to 4" for 1/4" ply. The on-line chandlery Duckworks
Boat Builder's Supply is an excellent source for both fiberglass
tape and epoxy.
This is a 2-stage process - first one side, then the other. You
want to make sure the joint is firmly pressed together - perhaps
holding them in place with temporary screws.
The most difficult part of doing this type of joint is turning
the plank over to apply the second piece of fiberglass tape. If
the first joint cracks, you'll need to start over. Have a friend
help, or attach some temporary battens to hold the joint steady.
Once the epoxy has cured on the second piece of fiberglass
tape, you have a very strong, secure seam. The edges can be feathered
so the seam becomes almost undetectable.
Method 3: Scarfing
Scarfing plywood also makes a very strong, smooth joint and it
does not require fiberglass tape or a two-stage gluing process.
It does, however, require some preparation and since you are overlapping
the ply instead of butting it, your resulting plank is slightly
shorter than either of the previous methods. Make sure you take
this into account before starting.
The trickiest part of a scarf joint is getting the angle for the
joint. Most people recommend a 8:1 or 12:1 scarf angle. 8:1 is usually
sufficient, but different builders have different preferences.
There are many ways to get the scarf angle, including making a
jig for your tablesaw or circular saw. Another common method is
to stack the planks and use a hand plane, power plane, or belt sander
to shave off the angles.
For 1/4" ply, an 8:1 scarf will have a 2" wide joint
(12:1 will have a 3" wide joint.) Stack the planks face to
face (or back to back) so you'll have "mirror images"
for the joints.
Use a very sharp hand plane or fairly sharp power plane so you
don't chew up the softer inner layers of your plywood. Use a very
coarse grit (34 or 40) if using a belt sander or you'll be sanding
all day. Take your time and enjoy the process. Once you've obtained
the angle, the feathered edges will be delicate, so handle the planks
Apply your glue and apply pressure. Remember to lay down a glue
Here's a couple other links to more information on
scarfing - look: It doesn't have to be this hard. Don't let these
And now for even more ways of joining ply - for those
for whom the previous examples were just not complex enough: