Suzanne has asked me to write a little bit about the reasoning behind the method of construction we have chosen for Tòti Bleu.
First I would mention that I have absolutely no experience with the design and construction of Gypsy Wagons. On balance, though, I have a little experience in the design and construction of multihull sailboats. It may not seem as though these two things have much in common but it seems to me that they do. A proper multihull sailboat must be light and strong and keep the elements out. A Gypsy Wagon or roulotte has very similar requirements. A horse or horses must provide the motive power for the roulotte even when climbing hills or mountains. For this reason a roulotte must be as light as is practical. The roulotte must be strong enough to withstand the rigors of the roads and fields through which it will travel. The roulotte will live most of its life outside in the weather and it follows that its very survival depends upon it keeping the elements out. It seems logical to me that it is practical that the cabin for the roulotte would be well executed using methods of construction similar to those developed over time for wooden multihull sailboats.
For the above mentioned reasons and because I am basically a one trick pony I advised Suzanne that I thought it would be good to construct the cabin for her roulotte using primarily renewable materials such as high quality plywood made from a wood called Okoume. Okoume plywood is strong and it is light and relatively rot resistant when protected from moisture. This plywood would form the exterior skin of the cabin but would be supported by a frame of another light weight wood. In this case we found a plantation grown wood in Spain called Paulownia. One of my first choices for the framing lumber would have been red cedar but the Paulownia turns out to be significantly lighter than red cedar, nearly as strong and less expensive. Some of the Paulownia is nearly as light as balsa wood but considerably stronger.
Door frame arch epoxy laminated from 15 layers of Paulownia bonded to lower frame. By designing joints with large gluing areas we produce joints that are stronger than the wood.
When building a lightweight multihull or roulotte I believe it is a good strategy to avoid the use of metal fasteners whenever possible. Metal fasteners are easy to use and relatively inexpensive but can create problems as the structure ages. Metal mechanical fasteners such as screws or nails transfer loads in the structure from the skin to the frame. Unfortunately they concentrate these loads around the fasteners. Especially when building a structure of light weight woods that will have some inherent flexibility this “point loading” around the fastener can cause problems over time. As the roulotte travels, particularly over rougher surfaces the entire structure will flex. The repeated cycles of flexing will eventually cause the wood around the fastener to degrade and break down and the structure will become increasingly flexible. At some point expensive and time consuming renovation and repairs are the only solution available to extend the life of the structure. The enlarged holes around the fasteners will also allow the ingress of moisture further accelerating the deterioration of the wood and therefore the structure.
Even without the problems of flexing the fasteners can cause other problems. A wooden structure that lives outside through all of the cycles and seasons of weather can experience great variations of temperature and humidity. Even if the wood is well sealed there is a certain normal ambient humidity level in the wood. When temperatures change repeatedly the moisture in the wood tends to migrate through the wood to condense on the metal fasteners. This moisture will eventually cause the wood to deteriorate around the fasteners exacerbating the mechanical problems described in the previous paragraph. For these reasons I feel it is a better method of construction to avoid metal fasteners whenever practical and instead to bond the structure together using modern epoxy adhesives. We have chosen an epoxy that is made from vegetable sources such as pine oil that is a waste product from industrial wood processing. This epoxy is not only more ecological on its own but by using the waste to produce the epoxy it actually reduces the carbon footprint of the wood processing industry from which the pine oil is derived.
For bonding large structures we must use innovative clamping solutions.
By bonding the wood and skin together with epoxy we not only avoid the potential problems of metal fasteners but we further strengthen the structure by spreading the mechanical loads across the entire surface where wood joins wood or skin joins frame. This for the most part limits or eliminates point loading and in effect makes the entire cabin into a single monocoque structure where every part , every wall, beam, ceiling, or piece of built in furniture becomes a structural element adding to the strength of the structure. Finally once assembled, we will seal the exterior surface of the roof with a composite of flax fiber cloth set in epoxy resin. The flax cloth is as strong as fiberglass but is a renewable natural fiber. We will then protect the epoxy from the UV rays of the sun with pigmented exterior paint. This method of construction is perhaps the strongest method of wood construction for its weight. I believe that with proper planning and execution this construction method should provide a very light cabin for the roulotte that will endure though the years with minimal maintenance.
So far we have constructed all of the major components for the cabin before the weather and temperature changed bringing a halt to our construction for the year. In weighing the individual components we are quite pleased with the light weights. (One side wall weighs 24 kilos) We are on track to produce a strong but exceptionally light cabin for Tòti Bleu.