shrinkage, aluminum is coming up short

I agree with vikx, 1/2" is more than shrinkage. Do the window(s) and door fit exactly where they should?

Thermal expansion for aluminium is 23.1 µm/(m·K). The roof of my trailer needs to span 84 inches at about 20 degrees C, which converts to 2.1346m (or 2134.6 mm). So how much will it tighten up at freezing? (0 deg. C)

Notes: Change in temperature in C is the same as the change in K. There are 1000 µm in one mm (or 1,000,000 µm in 1 meter).

Change = [23.1 µm/(m·K)] * [2.1346 m] * [-20 degK] = -986.2 µm = 0.9862 mm

in my climate, I run from about the teens in F (so let's say about -10 degC) to over 100 F, but in the sun, surfaces get hot enough to fry an egg. So lets go close to boiling and use 90 deg C as the hypothetical roof of my trailer in the sun of a hot summer day (nope you wouldn't want to touch it!). That gives us a full span of 100 deg C (and 100 deg K).

Change = [23.1 µm/(m·K)] * [2.1346 m] * [100 degK] = 4930.9 µm = 4.9309 mm.

Round that up to 5mm and what is that in inches? [5 mm] * [1 m / 1000 mm] * [39.37 in/m] = about 0.2 inches (or a little less than 1/4 inch).

Thanks for the excuse for me to run these numbers. My roof was awfully tight when I put it back on recently and I was wondering how much it was cooler weather verses perhaps me adding a tiny bit of width when doing my piecemeal replacements including the roof rafters. Apparently my roof rafters are as much as 1/8 inch longer than I'd intended. But the question has also gotten me thinking about the reasoning that trailer manufacturers started adding 'brakes' in the aluminum siding to help with thermal expansion.

If so, I can see the reason for old style 6-inch breaks perhaps. That would help a little in climates like mine where I might see something around 1/4 inch per 8 feet change from the dark of winter to an intense beating sun in summer. But why do the breaks go horizontally? The greatest change in size would be along the longest dimension. For the width of a roof, breaks for expansion would only help lengthwise on the trailer, not for width. But the use of wood for wall construction should give sufficient flexibility to handle the expansion/contraction.

The shift to Santa Fe style texture is ridiculously beyond necessary for thermal expansion. But it does help visually hide the dings on my old trailer. And the horizontal lines help the trailer 'look' longer than it is for the same reason you might think twice before giving a shirt with horizontal stripes to someone with body issues.

The old style 6-inch brakes might not hold dings, but they probably did help make the trailer look longer during a sales pitch. My conclusion is these were all about sales. Engineers designing the trailers probably laughed at the idea, then said 'well it would hurt thermal expansion and I suppose it gives the salesmen an excuse.'

A couple references:
Thermal expansion of Aluminum - www.periodic-table.org/aluminium-thermal-expansion/
Example of calculation with an Al alloy - www.alumeco.com/knowledge-technique/general/thermal-expansion

In another hobby of mine, there's constantly discussion about the expansion of brass vs wood. Yes brass expands with temperature but the application is generally in a temperature stable environment.

What does change quite a bit is humidity, which makes the wood expand, sometimes dramatically.

A camper would be subject to the worst of both factors, but .5 inch is incredible.