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Self Build Guide - 5. Floor, Wall, and Roof Construction.

Article by Steve Smith - Caravans Plus

This is part of a series of articles on building or rebuilding a caravan. You can find the whole series here: Ever Wanted To Build or Rebuild a Caravan?


Time Lapse Caravan Construction

This 5 min time lapse video is a good example of standard construction techniques that you can achieve at home. It is good to see the sequence that the build happens in and how each step adds strength while keeping the weight low.

Time Lapse Caravan Construction

Floor Construction

Tip: Before adding your flooring, ensure your chassis sits level, all the way to the ends. If the chassis is curved while adding the flooring, the joins will be either compressed or separated when the floor is finally flat. And you definitely need it flat when you add the side walls.

The floor is made of 12mm ply. These are laid across the chassis with joins on top of a cross member.

Structural ply is OK, but should be painted with a waterproof membrane on the underside. Marine ply can also be used, but is certainly not necessary.

If the shed floor space in your building area is limited, you may need to build walls and ceiling panels on your caravan floor. If this is the case, you will need a modified mudguard shape that can be added at a later stage. Otherwise construct your mudguard as shown below. This gets riveted and sealed to your chassis.

Standard mudguard
Standard mudguard

I would suggest Sikaflex adhesive and countersunk screws every 300mm on every cross member. The sheets are butt joined at the cross members.

Modified mudguard for adding after the floor is added.
Modified mudguard for adding after the floor is added.

Tip: The front ply sheet should overlap the front metal cross member by about 150mm. This will reduce the weight of the steel used in the chassis, but more importantly it allows a little flex in the front of the caravan body compared to the very rigid steel chassis where the A-frame meets it and transfers shock. A slight overlap would be essential to finish off cladding in any event.

The rear sheet can also overlap the rear chassis by around 100mm. So take these overlaps into consideration if designing your chassis.

Once the floor sheeting has been added, trim to match the sides of the chassis.

Caravan Floor.
Caravan Floor

Give the floor a sand at the joints to ensure your vinyl will sit flat.

Vinyl can be off the roll, but I have found the floorboard style is very durable and is easy to fit. Ensure 100% glue coverage. You will get a more professional look by covering the entire floor.

Caravan Floor.
Adding Vinyl - Caravan Floor

The furniture will sit on top, and be screwed to the floor later.


Standard Wall Construction

The wall thickness will be 18mm frame, with 3mm ply on the inside and the cladding on the outside. This will give an overall thickness of 25mm.

Walls are made in one piece on the garage floor or if space is limited on the caravan floor.

Timber frames have been used as the traditional construction method and still have many advantages. The one disadvantage that may occur is if water ingress is allowed to continue for an extended time. By extended, it would need to occur for a couple of years. If you follow our sealing methods this should not be an issue.

Timber is easy to work with, it is a thermal barrier, it allows very easy fixing of ply and aluminium cladding.

While many prefer aluminum frames, it is more expensive, it requires specialist tools and it transfers the outside heat or cold to the inside ply surface. In colder climates this transfer causes condensation on the inside, and in many cases it stains the paper print on many ply panels.

I have built 12 caravans over 30 years, and except for one using composite panels, all have had timber frames and all are probably still being used today.

What timber should you use? Meranti, also called Pacfic maple, is by far the best timber to use for caravan frames. It is easy to obtain and its weight for strength is exceptional. It is easy to work with and does not have knots. Pine is the last timber I would use. While it is light it has many knots, it can get water logged and rot and if ants get in they love it. Having made this point, I will admit to using straight grain treated pine for the base of wall that will be exposed under the caravan.

You will use only two sizes, 41mm x 18mm or 18mm x 18mm. It is usually a lot cheaper to get the 41x18 and split it down to 18 x 18. That still leaves you with three dressed faces.

Setting Out

Initially set the frame out with the internal face on the floor.

When setting out your frame, use the 41x18 on the top bottom, plus vertically where any sheets join. Remember the ply is integral to the strength of the wall, so resist using offcuts. It is normal to place a 41x18 next to large windows to provide additional strength. Also adjacent to the doorway. 18x18 can be used in all other places, ensuring your largest gap is 300mm. You will have cross frame members above and below windows and perhaps where furniture dictates.

Your caravan should have one or more bulkheads. The purpose is to strengthen and keep the side wall vertical towards the centre of the caravan. A wall going across the caravan, separating the ensuite is a bulkhead. Any furniture that goes from the floor to the ceiling on a side wall is a bulkhead.

When your frame is set out check it is square and glue and staple together.

Tip: For glue I recommend a 'liquid nails' type at the joints. This stays a little flexible during construction. Most manufacturers do not glue these joints, but my hourly rate is low while building and I want strength and strength without weight. The staples I recommend for joints are 10 or 12mm wide by around 16mm long. Place at least two at each joint. It can be handy to turn over sections of the frame, to staple the other side to get some early rigidity.

While placing staples it tends to curve up at the edges and will be floppy to turn over to staple the other side. For this I would recommend temporarily screwing or clamping a long piece of timber to make it more rigid when flipping.

Now you are working on the inside face. Repeat the staples. Allow the glue to dry.

When dry you can sand off any joints and remove any excess glue. Now is the time to router some small recesses into the frame where the wiring will go. This is the frame face that is next to the ply. Any removed frame will be reinforced by the ply that will be glued to the frame.

Standard Wall
Standard Timber Wall Construction

Sheets can have a PVC H-section to join them, however I prefer to just butt join on the side walls. If the join is still visible after all furniture is added, a matching iron on strip can cover it.

Again ensure the frame is square before adding sheets. The sheets can overlap top and bottom, windows and vent holes as it is very easy to remove that excess ply with a router that follows the edge of the frame.

Use a liquid PVA glue on all frame members and staple neatly at least every 300mm. The staples suggested are 4.5mm wide by 16mm long. Where visible they should be vertical and can later have a small matching putty added.

Once the excess ply is removed, the wall should be very strong and rigid. Construct the second wall.

Four stages of the wall build
Four stages of wall build
Tip: If you are making your own shower cubicle, now would be the time to add white laminate to create one waterproof wall.

Prior to standing the walls, add a small angle strip - either aluminum or PVC to the edge of the floor. Also place a generous line of sealer where the wall will be attached. It should provide a dust seal, but not ooze up at the joint.

You will need at least a helper to stand the walls. If you followed the advice on the chassis construction you should have four supports to rest each wall on. The wall can be clamped to the chassis at this stage and a couple of braces added to hold it vertically.

You will secure the wall to chassis with bolts, washers and nylock nuts. I Suggest 1 1/2 inch long x 1/4 bolts. Recess the bolt head into the timber, with a washer and place a washer and nylock nut on the angle iron of the chassis. Attach at least every 400mm.

Front and Rear Walls

These will vary depending on the shape of your design. But by now you know the basics of wall construction.

You have two options here with front and rear walls. Either make them the same width as the floor width, and bolt them between walls or make them 42mm wider and bolt them over the end of the side walls. Attach approximately every 400mm and recess bolt heads as above.

Before adding these walls, you will also need a trim to tidy the edge of the side wall. If you are unable to source a suitable rigid PVC edge you can check our website for the flexible 'Fenderwelt' which is often used by manufacturers on internal joins.

It is also acceptable to use a solid ply sheet on the front as this area can get pushed and pummelled by stones.


Composite Panel Wall Construction

Composite panel walls are a prefabricated material between 18 and 50mm thick. They are a sandwich of a tough outer skin (metal or fiberglass), foam insulation, and an inner skin (metal or wood). The construction is very strong and can be used on walls, floor, and roof.

It might seem like a very simple way to do your build but it's not quite so straightforward. I might be a little biased as I have only made one van with composite panels and found it to be quite limiting. Others might have more luck.

There are issues around how hard it is to make modifications, and physically handling large pieces. Also because there is no wall cavity you need to find creative solutions to running wires and plumbing.

My overall recommendation for a home build is to stick to the traditional wooden framing described earlier.

Composite Panels
Composite Panels and Joint Extrusions from DIY Caravans in Queensland.

Roof Construction

The roof should have a slight camber to the sides. A 41x18mm timber that goes from one side to other side should be shaped. The middle third should be left at 41mm high and each outer third should slope down to 18mm thick where it meets the edge frame.

These cross members will be stapled and glued to the 18mm x 18mm roof frame edge. The width will extend to the outside of the side walls. The timber frame will stop where the metal H-Frame for supporting the Air Conditioner will be added later. There are commercially available H-Frames, but if you've built this much you can probably make your own. The commercial ones come in steel or aluminium and can be padded with timber to suit your frame height.

Warning: If you are purchasing a pre-made H-Frame, the height where it rests on the wall should be equal to the height of the roof frame edge.

Caravan Roof 3D.
Tapered Roof Wooden Frame 3D image

Your frame should accommodate skylights, air conditioner, TV antenna, solar panels, maybe speakers. You will still have time to place backing timber right up to the time you add the cladding - that's a fews days away yet!

The PVC H-sections for joining ceiling ply are recommended, and will stop where the wall meets the ceiling. That should be 21mm in from each end.

Just like the walls, glue and staple the sheets to the ceiling frame while still on the ground. As these are more cumbersome than walls you could make it in two or three sections and join during installation. The roof outer frame can be screwed to the top of the walls. Screw at approx 300mm.

Tip: This clamping the ply between the roof frame and the top of the wall frame is where a lot of the strength comes from. It is also why it is very difficult and time consuming to replace rotted ply on a ceiling. The same is true where the wall ply is clamped between the chassis and the wall frame.

Caravan Roof.
Cross Section of Caravan Roof to Wall Attachment

Tip: Be aware that you will later be screwing the corner mould through the same timber and you will need some way to remind you where those earlier screws and bolts are. I use insulation tape on the side cladding adjacent to bolts and screws to prevent trying to screw into a bolt head.

Was this helpful?

6 Comments

  • comment-avatar

    Chris Wright

    Very informative thankyou. I may renovate our Freedom Express. Will it be possible to acquire these instructions in the future and in what format please?

  • comment-avatar

    Peter - CaravansPlus - CaravansPlus

    Chris These articles will be left on the website for free viewing at any stage. We don't intend to offer a printed version as these will be updated as private builders offer suggestions or alternatives.

  • comment-avatar

    Michael

    Useful information, however, I have a fibreglass caravan with aluminium wall frames and have water damage to the ceiling which makes my repair job a little harder.

  • comment-avatar

    Peter - CaravansPlus - CaravansPlus

    Michael, if the caravan is moulded fibreglass then these help files will be of little use. If however if it is fibreglass sheeting then they should be made in the same order as this guide.

  • comment-avatar

    Michael

    Where would I purchase the internal gloss plywood from

  • comment-avatar

    Peter

    If you are in NSW try Barnes Caravans in Lansvale, they don’t have a website but sell ply and aluminium sheeting. The while gloss has a POLYTEK finish.

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