Armour Layering

Stacking multiple layers of structural blocks (i.e. armour) provides extra protection in the form of bonus AC (Armour Class) for the outer layers, based on the AC behind them. This only applies to explosive damage and kinetic damage (the latter includes frags, but not thump damage), and the effect only extends one layer deep.

Note that only structural blocks benefit from this. E.g. metal, wood, alloy, or stone. Any empty block, non-structural blocks, or the gaps generated by slopes, corners, poles, etc. (as their visual model implies) end the bonus.

General
A beam of heavy armour has 60 AC, in one layer: █ <==[ (my poor illustration of a shell) 100% = 100% = 60AC.

In 2 layers, the layer behind the first adds 20% of its AC (without giving up any of its own) to the first layer. █ █ <==[ 1 * 60 + 0.2 * 60 = 60 + 12 = 72 AC for the outer layer. 60 AC for the inner layer, as no layering applies.

Strategy
If combining several types of armour, it is important to consider both their AC-value, and their hitpoints: A high AC only sets an effective damage multiplier, so having a high AC on blocks with little health may do less than having a medium AC and medium HP.

The damage-multiplier is: $$a = \min \left(\frac{\text{AP}}{\text{AC}}, 1 \right)$$

For example, 2m armour with 1m metal blocks and 1m heavy armour blocks: Metal health = 350 HA   health = 1250 Metal outside: layer |   AC    |    AC * health     |     Sum    | 2m: |  52.00  |       18,200       |    93,200  | HA1m: |  60.00  |       75,000       |    75,000  | Metal inside:        |         |                    |            | HA2m: |  68.00  |       85,000       |    99,000  | 1m: |  40.00  |       14,000       |    14,000  | Due to the damage-multiplier above, AC * health is the AP * kineticDamage a kinetic projectile would need to destroy the block, and "Sum" to pierce the armour in one hit. Note that this only applies if the AP value is below minAC (so only up to AP = 40 in this case).

Note that while wood is effectively paper on its own, backing it with metal can provide you with cost-effective armour due to wood's high health for its cost (although it needs more volume than comparable metal-only armour).

Non-Explosive/Kinetic Damage
As stacking only applies to kinetic and explosive damage, thick layers of armour aren't always the best choice. Notably:
 * HESH and HEAT easily spawn fragments behind even the thickest armour – you'll need spaced armour to counter HEAT and HESH.
 * Thump damage, and Lasers ignore the layering-bonus.
 * Moreover, lasers will almost always have more than 80 AP, so the AC of your armour is ignored (as 80 AP deals full damage to even heavy armour).