Originally Posted by Curious Slug
So we are outsourcing our roof-raising needs, which is going well so far. We've just been quoted for the new steel to skin the sides, two options and I wanted to source some advice -
26g 'Flashing' for $390
20g for $979
Steel isn't my forte. Hence the outsourcing. We are committed to making the bus as strong as possible but wondered if there was a discernible difference between thickness for skinning? The fabricator said that he was concerned about noise levels ('tin canny-ness') with the thinner stuff, but when I said that we'd be spray foaming he said that this wouldn't be an issue.
I worked sheet metal for many years and (not trying to be a smarty) I have seen some of the crappiest work i have ever seen on some of these schoolies. In the interest of sorting this issue out I am going to pass on a few pointers.
Thicker gauges are stronger structurally, but not the end all that some believe it to be and don't always stop the "wrinkly/ripply" (oil canning) that you see on most of these buses. The oil canning happens because the metal is a relatively flat sheet in 1 plane. The window pillar structures that you are fastening it to are not in a perfectly flat plane and when you fasten to them you are causing the metal to conform to these many highs and lows which causes oil canning. Residual stresses in the sheet from manufacturing and uneven clamping pressures of the fasteners also contribute. It is pretty much impossible to stop if you use flat sheet. The better the job of getting the pillars aligned the better the sheet metal will look. I don't think you can stop it, but you can minimize it.
Thicker metals allow relatively larger expanses (widths) of exposure with out noticeable oil canning but come with a significant price and weight penalty. More than 2-8 inches of exposure, depending on thickness, is usually going to oil can in lighter flat sheet.
The main solution used is to combat this is to corrugate or put bends or even textures in the metal. These add strength and drop the flat exposure down. If you look around at professionally manufactured goods and see bends or textures in the metal, many times this is why. Even car panels are never flat but will have some compound curve and bends (modern car panels are also made with high strength, heat treated steels, which is why they can be so thin). The larger the corrugations are the more noticeable and stronger they are. The "rub" rails on the bus are there to help break up and strengthen the wide unbroken sheet metal exposure They are not there so you can rub the bus up against something. The trick is to make the bends/corrugations visually appealing. Using smaller/narrower pieces can help, but can also lead to other problems. Corrugations don't work well with windows.
Installation procedures are also critical. Fasteners should be evenly installed from the center out to prevent trapping a "wave" in the panel and/or under lying structure, kinda like installing wallpaper or decals. Think about it...If you fasten 2 corners and the metal is not perfectly and evenly flat there is a slight curve (air gap) where the metal bows up away from the structure. If you then put a fastener in the middle you now broke the curve into 2 curves. Add more fasteners and create more curves (waves). Professionals use temporary easily removed fasteners called cleco fasteners to help mitigate this, especially when riveting. Fastening from the center out and using temporary clecos to hold the metal in place helps mitigate this.
Don't use excessively long pieces. Besides being hard to handle they can create problems with expansion and contraction from temperature changes (especially aluminum) and may need to have expansion joints provided, depending on design. This is a bigger problem if you are mixing materials with different expansion coefficients (ie. steel structure & aluminum skin).
How the metal is cut can also add ripples. Check out some work put out by the shop and see if it is satisfactory to you before ordering. Also check with the shop making the metal for suggestions, they know their equipment, metals, and capabilities. Most fabricators can also weld 20/22 ga but may have a problem with 24/26 ga. Welding/soldering/brazing may be necessary to put in widows/doors or other penetrations or add ons.
There are many more tricks, to many to post here, but I hope this helps you. I would go for the 20 ga (maybe 22 ga) and price shop it. The 26 ga is not going to contribute much strength. If my pillars seemed flimsy I would go heavier, 16/18 ga and more fasteners. Structurally, if you slightly overbuild you only have to pay for it once. Who knows how many times you will have to pay for it if you under build. Of course, you have a budget to deal with.