I lack the experience of having worked in this area of my Morgan, though have worked with the materials mentioned. They originate from the building industry in the UK and no doubt elsewhere as a waterproofing layer to isolate building materials used below a certain level in a building, from those above in order to stop dampness from below ground working it`s way up through the material of the build the problem identified generally as rising damp.
The materials used to create this barrier have varied with time, a thick coating of tar was painted on at whatever level all around the build at whatever level the Damp Proof Course (DPC) was deemed to be effective and unlikely to be bridged at any location around the perimeter of the build which would then allow the damp to rise.. In time the inconvenience of working with liquid tar was by-passed when a tar soaked membrane became available in rolls, generally in widths of the average house brick. of course rolling such material up would end up with it all congealing into a sticky mess thus as Botton mentioned it was normal to find at least one side of the material might have some form of paper attached to reduce the chance of it all sticking together... Today they use plastic DPC and it comes in all sorts of widths to suit the variety of building materials in use.
The question is, why chose to fit DPC on a Morgan betwixt chassis and frame... I am only using logic here... but it seems that any joint in in the location of chassis to frame seems bound to pick up damp and given the flexibility of a Morgan, I suspect there will be movement in that area thus paint on the chassis would seem likely to suffer abrasion and in time expose bare metal, perhaps encouraging corrosion...?
I suspect that the old style DPC had a measure of compliance that modern plastic DPC does not have, that compliance allowing it to fill up small gaps in the contact areas between steel and wood.... Thus as Button suggested some folk found a substitute in roofing felt as following the takeover of plastic in DPC roofing felt still used the same tar/membrane process.... However.. roofing felt has moved on considerably too and modern non tar materials have come into being... A word of warning.. there are in the different types of roofing felts those used as under slate felts and those used as "hut" roofing felts, the latter being thicker but with a rough surface on one side to resist weathering as it is designed as a finished roof waterproofing surface material... You do not want that acting like a file in your chassis frame interface...!!! Under slate felt can be thin so perhaps employ double thickness on that..?
Of course I could be entirely wrong... more than happy to be corrected.
