Does a joist have to lay “thin side” down?

As a complete novice, I have been reading about outside engineering projects such as decking and patios.

All the information I read tells me that to lay structural joists as a subframe / sleeper joists, the joist should be positioned with the thin side down, the wider side running perpendicular to the ground.

From my understanding, this makes for a stronger frame which can take more weight. My question is does it have to be this way? For example, if the goal was to create a lower deck, is it structurally sound for the joists to be laid "wide side" down?

Beams have a property we call moment of inertia ($I$) which controls how strong they are to resist bending. For rectangular beams, the equation is

$$I = \frac{bh^3}{12}$$

where $b$ is the cross-section's base and $h$, its height. This means that the height of the cross-section is far more important than the base. Indeed, if you need a 2x6" joist "thin-side-down", then you'd need a 2x54" (yeah, that's not a typo) joist "wide-side-down" to withstand the same load. The two sections below have the same strength against bending:

And obviously, this doesn't even take into consideration that joists are usually spaced some 16" apart, which is impossible with the necessary wide-side-down section (since it's 54" wide).

• Thanks @wasabi and also solarmike for your answers. This one has mathematically and visually explained the answer; very useful. – N Bourne Jun 20 '17 at 23:45
• The comparison is even worse if you use the true dimensions of a 2x6 which has an aspect ratio like 3.7 not 3 .. and worse still if you account for the fact the beam has to carry its own weight. – agentp Jun 22 '17 at 22:10

The joist is positioned "thin side down" as that means more material is absorbing the force (mass ) applied to it and that is where the strength comes from. The thickness is necessary to stop the joist buckling or folding with the load.

Before we had the detailed stress and structures calculations, joists were simply trees cut that looked big enough and left round or just flattened a bit. Now, we minimise the amount of material or make more efficient use by using material, the joist, with the strength focused where it needs to be.

• Thanks for the answer @Solar. So with regards to my question of "is it safe", does this mean that it would in fact be safe to lay joists down in the other orientation (i.e. "wide side down") to create a lower subframe, but economically it would just be economically unwise as you'd need to use more joists than necessary? – N Bourne Jun 20 '17 at 13:33
• If you need 8" to support the deck the NO you can't lay the joist on its side and only have 2" supporting the deck. You can use a 8*2 joist or an 8*4 joist or an 8*6 joist as long as the deck has the 8" dimension suporting it ie perpendicular to the ground. – Solar Mike Jun 20 '17 at 17:06
• If the deck is that close to the ground It shouldn't be an issue to put your supports closer together, then you could use lighter lumber. (In principle, I cant speak about building codes) – agentp Jun 22 '17 at 22:15