Plywood Truck Cap

Plywood and pickup trucks have a longstanding relationship with each other. While the most obvious part of that relationship being plywood riding home from the lumberyard in the back of the trucks, truck owners have used plywood to make a number of modifications to their truck beds, including inserts and caps. Properly done, these can be as good or even better than the commercially available ones and a whole lot cheaper, considering that a commercial cap will cost somewhere between $1,200 and $1,800.

Unfortunately, plywood truck caps have a poor reputation, mostly due to people who do a shoddy job of making a cap. But a plywood cap can be made to look and function as good as a commercial one; so there’s no reason to be embarrassed about the idea of sporting a plywood cap on your truck. Properly done, it can be a project which people stop to see, rather than one they snicker at from a distance.

While the whole idea of a truck cap is to be functional, it’s a good idea to keep the aesthetics of the project in mind as well. Perhaps the most important part of this is making the cap match the contour of the truck itself. Unless the cap is actually going to be a full-blown camper, sticking up above the cab of the truck, this means tapering the sides the same angle that the truck’s cab is tapered.

Another piece of aesthetics, which also serves a useful purpose, is to curve the roof slightly, as well as curving the corners of the roof. While it is possible to make a truck cap with a flat roof, it won’t match the contour of the truck’s roof. But more importantly, it won’t shed water as well. Putting a slight curve in the roof is actually rather easy and will help the cap to shed water, even when sitting still. That will ultimately work out to help make the cap last longer.

There are a large number of design considerations to take into account, when making a plywood camper. I’ll try to cover at least the major ones as we go through this article.

The Cap’s Structure Starts with the Bed

Making any truck cap has to start out with the bed of the truck, specifically with the top rail of the sides to the bed. That’s what’s going to support the weight of the cap, holding it in place. Building the cap in place, starting with those rails, will ensure that it fits properly and that it receives the necessary support from the vehicle.

Cut pieces from 2”x 4”s to fit the rails, leaving ½ of extra space at both ends. This space is for the plywood skin for the front and back walls. The side walls and roof will be made of thinner material. Clamp these foundational pieces in place, so that they can’t shift while fitting other pieces in place.

The outside edge of those rails needs to be cut at an angle to match the sides of the truck cab. The easiest way to measure that angle is to use a digital level or a digital angle block and a straight board. Once calculated, rip both boards to give them that outside angle.

Put Some Ends on Those Rails

The next thing to do is to cut out the ends of the cap. Using the truck’s cab as a template, place a piece of ½” thick plywood in front of the rails that you just attached to the bed and mark out the outline for the front end. It may be necessary to adjust this slightly, providing more of a curve at the top, than what the roof of the truck’s cab has.

Cut the front and back pieces the same, without bothering to cut out for the door or a front window. Those can be cut out later, after the cap is structurally assembled. Try to avoid splintering the material, as that causes problems which have to be dealt with later. I’ve found that using a Rotozip tool or a 1/8” Rotozip bit in a cordless router makes it much easier to cut curves, without splintering, than using a jigsaw. It just takes some time to get used to using the new tool configuration, without getting a crooked line.

Here’s where the first real problem comes in. The front and back ends need to be firmly connected to the side rails. Those are probably the most important joints in the cap, from a structural point of view. But nailing or screw through the plywood and into the end grain of the 2”x 4”s isn’t all that strong. Angle brackets can be added in, but those will only hold one side of the joint. The best way of connecting those ends to the rails is to use some good sized dowels, say 3/4”.

When doweling something like this, I find it easiest to clamp the pieces together, in place, and then drill through the plywood into the structural member behind it. The dowel pins themselves are cut a little bit over-long, with the idea of cutting the extra off, once the glue has dried. Since nobody makes doweling jigs for dowel pins that size, I set the dowels up against a bench hook and use the corner of a chisel to make longitudinal gouges for the glue, the length of the dowels. That way, the glue doesn’t all push down to the end.

With the dowels glued and pounded into place, leave the glue to dry. Then cut off the dowels flush with the surface of the plywood, using a flush cut hand saw or a flush cut blade on a vibratory multi-tool.

In order to clamp the pieces in place, it’s useful to have a square that the pieces can be clamped to, so as to ensure that they end up square. I’ve built a couple such squares out of scrap plywood, as shown in the picture below, actually holding pieces of a different project together for welding. The key, of course, is making sure that the triangular piece in the square is actually square, as checked against a known good framing square.

clamping spindle frame
Clamping spindle frame

Add Some Framing

While it is possible to make a cap with minimal framing, it’s probably not a good idea. That cap is going to be subject to a lot of twisting, vibration and other stresses, especially if the truck is used off-road. While it might hold together for a while with minimal framing, it will eventually come apart. Therefore, it makes more sense to add some framing.

Essentially what is needed are “bows,” something like the wood bows that were used to hold up the canvas covers on the conestoga wagons that were used in the wagon trains moving west. But we need a more robust bow than what they did, as it’s holding up plywood and not just canvas.

Since weight is an important factor, I’d recommend making the bows out of 1” thick material, rather than 2” thick. The thicker material would provide a wider surface to screw the cap’s skin into, but with careful measurement that won’t be an issue.

The contour of the roof bows are shown in the diagram below. Starting with a 1”x 6” or 1”x 8” board, depending on how much of a radius is desired on the corner of the cap, the top and bottom side of the roof bow is cut to curve it. The top side, as mentioned earlier, is done to facilitate water runoff and the bottom side is curved to increase headroom, without sacrificing the structural strength of the cap.

However, I’d recommend using 2”x 2” boards for the vertical elements of the bows. While we’re trying to use 1” thick material to save weight, furring strips are not very strong. Using 2”x 2”s won’t add much weight, but will add a considerable amount of strength to the cap.

At a minimum, three bows should be used, although four is better, especially if windows are going to be installed in the sides of the cap. In that case, the bows should be before and aft of the window openings, providing something to support the windows when they are mounted. If no windows are to be installed, it would be possible to get by with three total bows: at the front, the back and one in the middle, although the roof won’t be quite as strong. While the bows for the front and back are built directly onto the plywood panels, the middle bows need to be free-standing. Even so, they can be formed by clamp them to one of the end bows, then removed and set in place.

Side supports are added in as well. This both provides stiffness to the sides, so that they don’t flap in the wind and additional strength to the roof of the cap, allowing the roof to be used to carry additional items, such as long boards and ladders. Without these supports, the roof would not have the strength to use it for carrying any sort of cargo.

plywood truck cap, front wall, base rails, roof bows, top braces, board outline for bows
Plywood truck cap – framing

The same joinery problems are going to exist in making this frame, as existed in installing the plywood front and back to the base rails. This problem can be solved by judicious choice of the joinery techniques to be used.

For attaching the horizontal and vertical portions of the ribs, the best bet is to use a lap joint. Since the 2”x 2” vertical supports are thicker than the 1” thick material used for the bows, all that’s needed is to cut a dado in the back of the 2”x 2’s to make a secure lap joint. It might be best to make this joint before cutting the curves in the corners, so that both pieces can be cut together.

At the bottom of the bows some people would again turn to angle brackets. However, for strength it would be best to cut a mortise and tenon, fitting it into the base rails. As an alternative, these joints can be doweled, just as the ends were doweled onto the rails.

There are also two longitudinal top ribs shown in that drawing. Depending on the size of the truck bed, one or two of these will be necessary. They must be located so that the joints in the plywood skin are directly over these ribs, allowing for secure attachment of the skin, as well as helping with making it waterproof.

Cutouts for these ribs need to be made in the roof bows, allowing the ribs to pass through the bows, including the end bows. But they should not pass through the front and back plywood walls of the cap. Structurally, these will not weaken the roof bows, as long as they are a snug fit. If a heavy load were to be placed on the roof, the bows would try to sag, with the top part of the bow compressing and the bottom stretching. Since the bottom is not cut through, it will be able to withstand this stretching and with a tight fit between the ribs and the ribs, it will not negatively affect the compression of the top part of the bows.

Attaching the Skin

For the skin of the truck cap, it will be necessary to either find 1/8” plywood sheets or flexible plywood. There are a number of different types of flexible plywood on the market; but they won’t be available from the local home improvement center. It will be necessary to shop at one of the plywood and trim outlets that caters to contractors doing finer work.

The various different types of flexible plywood on the market are pretty much all designed to curve in only one direction, so it is important to buy the right material. The curve we want will be going around the top corner of the cap. That makes the curve direction across the width of the panel, not the length.

Start by attaching the plywood skin at the bottom of the rails, gluing and screw it into place every four inches. From there, work up the ribs, attaching to each of the ribs, every four inches. The ends of the plywood should overlap the edges of the front and rear walls, covering that end grain. Starting from both sides like this, there will be a gap in the middle of the roof, which should come out falling directly on the top ribs. Cut a piece to fit into this area and screw it into place along the ribs and bows.

Some people try to use a full sheet of plywood for the roof, with separate pieces for the sides. However, this eliminates the ability to curve the top corner of the cap. They would have to be angles. The other option, which would eliminate the seams in the top of the roof, would be to run the sheets of plywood crosswise to the bed of the truck, with one of the bows being located at the four foot mark, so that both pieces could screw into it. This is a bit harder to do, and will require strips of plywood on both sides, making those seams visible.

Assuming the seams are going to be on the top, as I originally described, those seams should be coated with fiberglass, using at least a couple of layers of fiberglass cloth and an epoxy resin. That will waterproof the joint and can be painted over to match the rest of the cap. Being located on the top, where people can’t see it, neatness isn’t exactly critical.

Doors and Windows

Up to this point, I’ve been writing this under the assumption that the door in the back of the cap would be made just like most commercial caps, leaving the truck’s tailgate in place and only making a lift-up upper door in the cap. However, some people prefer to remove the tailgate from their pickup truck and make a full height back wall, with double doors in it. It all depends on what the truck owner is looking for.

In such a case, the back wall will need to be cut differently than the front one, extending it downwards to the floor of the bed. At the same time, additional framing would have to be added to support the doors and make them operable. This might be better if the cap is being used as a camper, but it would make it harder to use the truck for hauling more plywood from the lumberyard.

Assuming that we’re going with the standard cap type lift up door, leaving the truck’s tailgate in place, making the door is simplicity itself. All that needs to be done is to outline it and then cut it out with a jigsaw. The panel cut out can become the door, just by adding in a couple of strap hinges at the top and latches. Piano hinge might be a better option, providing continuous hinge all the way across the door. Two latches, one at either bottom corner is recommended, as moisture may cause the plywood to warp to some extent.

Pneumatic prop rods, of the type used for hatchbacks and the tailgates of SUVs are readily available from any auto parts store. Since this door is considerably lighter than any hatchback, not a very strong one will be needed. Try to find something that is designed for a small hatch, as that will be closer to the correct weight.

If windows are going to be added in, it is best to buy windows which are made for RVs, as they will be lightweight and come with the necessary mounting to ensure that they are water-tight. If the windows are sliding, the overlapping glass pane (from the outside) must be mounted towards the front of the vehicle, so that the wind doesn’t drive the rain through the seal.

To provide the necessary structural strength for the windows, if the bows aren’t mounted at the right point, glue a strip of plywood to the inside of the skin, for the window’s mounting screw to go into. All that’s needed is a strip that’s wide enough and thick enough to capture the screws.

Some Final Details

Once the entire cap is built, it needs to be finished. Regardless of whether it is to be painted or a clear finish is to be applied, it is a good idea to go over the entire exterior with a coat of epoxy, sealing the wood. Adding a layer of fiberglass cloth to that will give it additional strength. The single layer will turn more or less transparent when wetted by the epoxy.

If the cap is to be painted, rather than left with a clear finish, I would highly recommend caulking all the joints with a high quality household caulking. Any exposed edges of plywood should also be caulked, wiping the caulking into the end grain with a finger, to ensure a good seal.

With the cap finished, it needs to be removed and reinstalled with a foam gasket underneath the cap. This helps to keep the rain from being driven in through the seam, by the rain. A rubber seal for the door should be added for the same reason.

If the truck bed cap is going to be used for hauling a lot of material, it would be a good idea to add a roof rack. This can either be a commercial unit or cutting pieces of 2”x 4” or 2”x 6” to match the roof contour and then attaching them to the roof. In either case, those should be mounted directly above the existing roof bows for maximum strength.

Tricking Out the Insides

Depending on how the truck bed cap is going to be used, there are many different things that can be done on the inside, making better use of the space. Perhaps the most useful thing to do is to raise the bed and turn the space under the false floor into storage. This can either be left as compartments with access from above or have drawers installed in it, which can be pulled out from the back. On a long bed, using both together, with the drawers in the back and other compartments forward of them might be the best.

Another thing that many people do is add storage in the sides, over the wheel wells and the space forwards and aft of the wheel wells. This provides a considerable amount of space for items that need to be readily accessible and well organized. Even the inside walls and roof can be used, adding straps and brackets to hold emergency equipment in place.

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