Joining Plywood - Butt Joints

The butt joint is the most basic method for joining two pieces of plywood end-to-end or side-to-side to make a sheet large enough to suit a large bulkhead, make a hull side panel or anything else larger than the material that is available. Within the label of "butt joint" there are various ways to do the job. Click on the diagrams to enlarge them.

A butt-strapped joint is a simple end-to-end joint, with the meeting edges glued together and with a piece of plywood on the inside to reinforce the joint.

A basic butt-strapped joint

With traditional adhesives you would have had to clench-nail through the panels and butt-straps to be sure that they would stay in place but modern adhesives are stronger than the wood, so the fasteners become redundant after the glue has cured. I would use short temporary steel screws that I would later remove and re-use for the next joint, then fill the holes with epoxy.

The strap width will vary depending on the thickness of the plywood being joined. About 15x the plywood thickness is good, so:-

135mm wide for 9mm plywood
180mm Wide for 12mm plywood
5 5/8" for 3/8" plywood
7 1/2" for 1/2" plywood

Be sure to chamfer or radius the edges of the strap before fitting it, which neatens the appearance of it on the inside of the boat and removes sharp edges that don't hold paint well. If you are fitting butt-straps between stringers in the bottom of the hull, stop the strap about 10-12mm (3/8" to 1/2") short of the stringers to leave a channel for bilge water, or you will create small traps where water can lie.

This joint is very easy to make but it does have drawbacks. It adds unnecessary weight and it spoils the neat interior surface of a boat that will have the inside of the hull exposed. It also adds a hard spot that affects the smooth curve of the hull surface, particularly if the joint is made on a flat surface before the panel is installed rather than being made in place with the panel already curved. There is a chance of the outer surface of the joint showing a hair-crack over time, so it is worthwhile to glass-tape the outside of the joint, as described below.

This joint is good for bulkheads, where a nice piece of hardwood trim can be used in place of plywood to cover and reinforce the joint on both sides. In that case the hardwood trim could be about double the thickness and half the width of the plywood butt-strap.

Taped joints are the simplest way to join plywood in small boats, with sheet thicknesses of 4-6mm. Easiest is to butt the two pieces end-to-end then laminate a length of 50mm (2") wide glass tape onto the outside. When it has cured, turn the panel over (being careful to support it properly so that the unreinforced side doesn't crack) then laminate a similar strip of glass onto the other side. Sand the edges of the tape to feather them into the plywood surface.

Taped and Flush-Taped Butt Joints

The main problem with this taped joint is that there will be a slight mound at the joint, which can be difficult to fair out so it will show, particularly with a gloss finish. The way to get around this is to recess the glass tape flush into the plywood surface. To do this, slightly bevel the both surfaces of each piece of plywood with a hand plane or a sanding machine, forming a slight slope about 1-1.5mm deep at the edge. See Step 1 in the diagram above.

When you bring the meeting edges together they will form a shallow V into which you can lay your glass tape. build up the glass tape so that it completely fills the V. When both sides have been glassed and have cured, sand the tape down flush with the plywood surface, producing a very neat and almost invisible joint.

For all of these jointing methods you will need to lay waxed paper or a sheet of smooth plastic under the joint before you start gluing or glassing, so that it doesn't bond onto the floor or work surface.

Joining Plywood - Scarph Joints

This is the first of a series of posts about the methods of joining plywood. It is not a course to teach how to do this work but a basic explanation of how it is made and the benefits and drawbacks. Refer to that excellent book "The Gougeon Brothers on Boat Construction" to see various methods for making joints on plywood and solid timber.

I am starting with my preferred method first, only because this has been the subject of discussion between me and a couple of other people recently. This is the scarph (or scarf) joint, more technically known as a feathered scarph because the slope tapers off to a "feathered" edge.

Most amateurs have a fear of making scarph joints that fits anywhere between mild trepidation and total panic. In reality a scarph on plywood is relatively simple work to do. If you can handle a hand plane with reasonable accuracy then you can scarph plywood. A scarph joint is no more than a sloping butt joint between the ends of two sheets or lengths of timber, generally sloped at between 6:1 and 10:1. The 1 is in the thickness of the timber and the other number is in the length of the timber. For scarphing plywood I would generally use about 8:1 slope. So, in 6mm plywood it would be 48mm and 12mm plywood would be 96mm. These numbers are not finite rules, so they can be rounded off to 50mm and 100mm respectively for simplicity as long as both halves are planed to the same angle. In imperial measurements an 8:1 scarph on 1/4 plywood will be 2" wide and 4" on 1/2" plywood.

The Anatomy of an 8:1 Scarph Joint on 12mm Plywood

It needs only a tape measure, straight edge and pencil to mark the scarph, in addition to the plane. Once you have the plywood panel cut to required shape to fit the boat, you will measure in from the edge that will be scarphed by the width of the scarph. Do this at three or more places along the edge, which will show up any errors in your measurement if the straight edge does not touch all points. Draw a pencil line with the straight edge to define the edge of the slope.

If you have a hand power plane, this will make short work of removing the excess material but don't try to get up close to the line or you will risk messing up. Stop at least 3mm above your line then use a sharp jack plane to take it down to the finished surface. The closer you get to the finished surface the finer the plane must be set. Remember that you are working to create a feathered edge right at the edge of the board, so you don't want to remove any material that will destroy that edge. Clamping your board to another sheet of plywood or a firm table, with the edges aligned, will support the feathered edge so that it doesn't move away from the plane when you work.

I can't emphasise "sharp" strongly enough for the plane blade. It must be as sharp as you can get it and you should hone the plane blade again before each scarph is planed. You will be planing across the end grain of alternate layers, which will rip instead of being sliced if the plane is blunt, ruining the accuracy.

Use the edge of your plane or a straight edge to check that your slopes are straight. If you can see light under the straight edge then the surface is not straight. If the surface is concave then the middle of the joint will be hollow. If convex then the feathered edge will not lie against the other sheet so you will have gaps .

If you don't have a hand plane but have a skill saw, you can buy a "SCARFFER" attachment that bolts to the base plate to set the slope. You can see this attachment in use in the Gougeon book mentioned above. I did the same by cutting a block of wood to the required angle and bolting that to the base of my skill saw.

If you have neither a power plane nor a skill saw then you can still do it all by hand. Start with the plane set coarse until you are nearing the line, then sharpen the plane and set it more fine for the final finishing.

Be careful not to make the most common error, which I have done a few times. That is to plane both slopes onto the same side of the plywood instead of onto opposite sides. You don't realise your error until you bring the two pieces together and they don't match.

Is it a problem if your scarphs are not entirely true? That depends on how big the error as well as what glue you are using. Epoxy has better gap-filling properties than resorcinol and also gives a stronger joint if there is a film of epoxy between the two surfaces that are being glued. Resorcinol gives maximum joint strength when there is wood-to-wood contact, so epoxy is more tolerant of bad joints than resorcinol. Still, don't use the fact that you are using epoxy adhesives as an excuse to accept shoddy workmanship from your own hands.

When gluing the joint, remember that end grain is thirsty and will suck glue out of the joint, so don't skimp on the glue. Let the glue lie on the surface for a few minutes so that it can soak in, then apply some more if needed.

Scarph joints on plywood are a lot stronger than the wood that is being glued, so there is a natural safety factor. Also, you will generally be attaching the panels to a wood structure that has stringers and other longitudinal timbers crossing the joint and reinforcing it. You are very unlikely to ever break that scarph once the panels are glued onto the boat.

Making scarph joints is satisfying work once you get the hang of it. It produces a nice smooth surface both inside and outside, much nicer if your boat will not have hull liners covering up all of your handiwork. Scarph joints are neat, strong and can be made by hand. I can't think of any disadvantages to this joint, aside from the unnecessary fear of making them.