Would a roof over a ship's board hinder operation of the ship significantly?

Question

Related to: How would rudder protection against water elementals affect ship's performance and How would sails from a fireproof material affect ship's performance?

Most real-world ships from say 14th to 17th century had no roof over their board. Was it just because it wasn't needed and that extra wood for it would slightly increase the cost of the ship, or were there some better reasons against the roofs?

I assume magical warfare with fireballs and even such spells as acid rain or rain of fire, so wooden roof with some extra protection on its top seems natural. The roof could also protect (at least partially) against shots from ballistae and mangonels.

Also, would it be possible to combine some boarding device (somewhat similar to Roman corvus, but shorter and not solidly fixed to the ship, and used only on bigger ships, so that they wouldn't affect the ship's stability so much) with a roof (or its part) consisting of wooden plates with some protective layer on their top? The roof plates could be lifted from a tower on a mast. Off course, opening part of the roof would decrease its protective value, at least temporarily. But is there any other drawback of this?

Answer 1

A roof over the top of the ship would have a number of negative consequences, the severity of each would vary:

• Restricted visibility - crew members would not be able to see everything else that's happening, particularly people on deck seeing the sails and rigging.

• Rigging would need to be rooted around the roof.

• The roof would make the ship more top heavy, and would also catch a lot of wind. This would combine to make the ship more unstable. In particular if the ship started to tip then sails naturally shed wind and cause the ship to right. A roof though would actually catch more wind at certain angles.

• Increased weight. The weight of the roof would not be insignificant.

There were actually some ships in the real world built along these lines though, for example the Korean Turtle Ship.

Answer 2

In a combat situation as described, there are a few more effects the roof would have than listed in the other answer.

1. Reduced ability to access rigging/sails. If they fire a stone through your asbestos sail you will have more difficultly accessing it unless planned accordingly.
2. Can't run away as easily. (speed of boat)
3. Less flexibility in retaliation. You can't fire spells or arrows through the roof.
4. harder for them to board your ship
5. Lower troop moral, can't hang out on board before battles
6. Higher troop moral, percieved protection from visibly impressive attacks

Basically, I find myself wondering what is the benefit provided by the roof compared to situating the canons and ports on the lower levels of the ship?