1. What sort of preparation is needed?
The underlying structure (usually wood) should be in good condition, of course. Make sure it is smooth with no sharp edges, nail pops, or other projections. Run a belt sander over it. It should probably be painted as for any exterior surface, although the railroads generally did not paint the roof boards. If a leak develops in the canvas, you don't want the roof itself to start to rot. It's much easier to patch the canvas than the underlying wood!
Replace all tack molding. Do not try to reuse old tack molding - the biggest problem I've encountered with otherwise good roof jobs is that old tack molding starts to rot out, then the edges of the canvas are flapping loose. It's then very difficult to replace, although it can be done.
Apart from the curved ends of the car, tack molding is usually simple to make. Poplar is a good choice since it is a relatively soft 'hardwood', is straight and even grained and holds tacks well. Other woods, such as basswood are a similar character, and regionally other choices may be available and make sense. The new tack molding should be primed and painted two coats before final installation and covering with canvas to help shed water and prolong its life. The tack molding profile should include a drip edge or drip lip. Generally you will want to reproduce the old molding design. Do not choose a wood like southern yellow pine, plain or treated, since this has coarse grain and will tend to split with a row of tacks applied.
2. What materials do I use?
(L) A professionally sewn seam.
Paint: Special canvas paint is produced. Our local source is APCO Paints in Schaumburg. The product is custom mixed to your color in small batches and is largely linseed oil based. After applying, the free linseed oil polymerizes and forms a sort of waterproof membrane with the canvas weave which still remains somewhat flexible. Rags used to wipe up spills or during cleaning, with this wet product, should be disposed of carefully since heat will be generated and spontaneous combustion is a risk.
Underlayment: Various materials have been suggested for a layer under the canvas to prevent it from rubbing on the wood. Felt roofing paper, butcher paper, and fiberglass mats have all been tried, but our experience has been that it doesn't make any difference. If the wooden surface is in good condition, the canvas should last just fine.
3. How should the canvas be installed?
The first step is to thoroughly soak the canvas, usually by rolling it out on the grass and soaking it with a garden hose. This washes out any sizing and makes it expand a little. Then the wet canvas can be rolled out onto the roof and stretched. The canvas should be clamped every two or three feet and stretched with weights (such as tie plates), bungee cords, or whatever you have on hand. Let it dry for a week. Then wet the canvas again and try to stretch it some more, if possible.
Another tip for canvas roofs which require a lot of stretching over compound curves: If there are wrinkles or puckers that cannot be pulled out with stretching and tacking, the finished canvas before painting can be dampened and a household iron can be used to shrink those wrinkles and flatten them out. On a lot of roof curvature, it may seem that you will never get a flat seam on the tack molding. Tack temporarily every foot or so around the car end. Then split that difference and tack and continue reducing the gap by half, pushing and laying the folds flatter and flatter. By the time it is all tacked and set, use the 'wet and iron' trick to shrink or reduce those folds and eventually they will be less noticeable after painting and the finished job.
Then when dry, start tacking. The canvas should be cut about 2" too long and folded back under itself, so the tacks go through two layers. Usually there are two rows of tacks in a staggered pattern. Standard #8 or #10 carpet tacks are used. Because the canvas paint is very thin, at least three coats will be required.
Installing saddles and other appliances on top of the new canvas is specific to each project. They must all be caulked in place. Anything that sits on the canvas should be carefully shaped so the load is evenly distributed. On the IT, the saddles sit on a thick piece of leather to act as a cushion. That's probably a good idea.
4. What about maintenance?
If the car is stored inside and not operated in the rain, little or no maintenance is required. In any case, the roof should of course be inspected periodically for damage. A good installation should last indefinitely under these conditions.
When the car is stored outside, the roof should be washed down and repainted once every two or three years. Depending on the conditions, a useful life should be ten to fifteen years. Indoor storage should be arranged if at all possible.
You can walk on the canvas, although you should use the running boards whenever possible, and of course only with soft-soled shoes without cleats, etc.
5. How can the roof be repaired?
Small areas damaged by accidents can be patched in the obvious fashion by tacking on small pieces of canvas caulked in place and painted. These need to be inspected periodically, especially when stored outside.
Bob Kutella contributed many valuable suggestions to this article. Most of it was learned by experience at IRM; our knowledge base is due to a number of experts including Bob Bruneau and Frank Sirinek.