Rambam - 1 Chapter a Day

Sefer HaMitzvot (Positive Commandment 184) and Sefer HaChinuch (Mitzvah 546) regard this as one of the Torah’s 613 commandments.

There are commentaries who object to the Rambam’s ruling, noting that the Sifre comments explicitly that the above verse includes these structures as well.
The Kessef Mishneh tries to rationalize the Rambam’s ruling, explaining that the Rambam draws a parallel to the laws of mezuzah, which these structures do not require because they are not dwellings (see Hilchot Mezuzah 6:7). Kin’at Eliyahu, however, questions the comparison, because the reason for the guardrail is that an open roof presents a danger, and that applies regardless of the nature of the structure. Sefer Me’irat Einayim 427:1, however, mentions that it is uncommon to use the roofs of these structures for any purpose. It must be noted that the roof of the Temple in Jerusalem had a guardrail.
Significantly, with regard to a mezuzah, the Shulchan Aruch (Yoreh De’ah 286:2) cites opinions that differ with the Rambam, while with regard to a guardrail (Choshen Mishpat 427:1), the Shulchan Aruch mentions only the Rambam’s view.

Since it is so small, it also is not considered a dwelling.

Which implies that the roof is owned by one individual.

To prevent someone falling from the public domain onto his roof.

Ten handbreadths is approximately 3 feet in contemporary measure. In most instances, this is high enough to prevent a person from falling accidentally.
The Ra’avad differs with the Rambam, and maintains that ten handbreadths are required only in the lower portion of the roof. In most portions, it is sufficient for the guardrail to be three handbreadths high. The Shulchan Aruch (Choshen Mishpat 427:5) rules according to the Rambam.

This indicates that its purpose is not merely to create a distinction so that a person will know where the roof ends, but actually to prevent a person from falling.

Sefer HaMitzvot (Negative Commandment 298) and Sefer HaChinuch (Mitzvah 547) regard this as one of the Torah’s 613 commandments.

And lashes are administered as punishment only when a prohibition involves a deed (Hilchot Sanhedrin 18:1).

Note the comments of the Meiri on Bava Kama 51, which state that the mitzvah of erecting a guardrail applies only when there is a life-threatening situation. A person must endeavor to prevent any harm or injury from occurring, but the scope of this mitzvah includes only situations that involve mortal danger.

I.e., this is a charge to every person not to expose himself to danger.

In this vein, Bava Kama 15b states that anyone who raises a dangerous dog or has a faulty ladder violates this negative commandment.

This is the punishment given for the transgression of a Rabbinic commandment. The basis for the Rambam’s statements - and the laws that follow in this and the subsequent chapter - is that a person’s body is not his own possession, but rather that of God. Therefore, he is not allowed to harm it, and surely not to risk his life.
On this basis, our Rabbis have always advised our people to adopt a lifestyle that conforms to the rules of health, and to avoid life threatening situations. Applying this principle to contemporary experience, one can cite several examples of activities we should adhere to because God commands us to protect our lives, e.g., the Surgeon General’s warning about smoking and the speed limits on our highways.

For a blood-sucking leech could cause mortal danger if ingested.
In places where leeches are uncommon, this prohibition does not apply. Nevertheless, leeches are common in many places, and people should be aware of this prohibition.

And deposited its venom within.
This prohibition does not apply in places where snakes and other poisonous animals are not common. For this reason, the Shulchan Aruch (Yoreh De’ah 116:1) states that today, it is permissible to drink beverages that were left uncovered.
It might, however, be added that a similar prohibition applies today with regard to drinking water that might be polluted.

Once it has become vinegar, however, there is no prohibition.

A legume used as fodder for cattle that can be made fit for human consumption after repeated cooking.

Since the fruit was needed for a sick person, one may assume that it was not left to soak in the water for a sufficient time to change its taste.

For then poisonous snakes will not drink from it.

This ruling is problematic, for in Halachah 14 the Rambam states that sometimes venom will remain concentrated in a certain portion of a container and not mix with contents of the container. Similarly, in this instance one might say that although there is enough water in the container to nullify the effects of the poison, it is possible that the poison will not mix with the water (Kin’at Eliyahu).

For crawling animals will fear drinking such water.

I.e., a depth below that which one would ordinarily expect a crawling animal to reach.

I.e., a height above that which one would ordinarily expect a crawling animal to reach.

Generally, crawling animals are not found in such structures.

For venomous crawling animals, and as the Rambam continues, for holes.

Kin’at Eliyahu questions why this measure is acceptable, noting that there are poisonous snakes that can fit through much smaller holes.

For we fear that the venom will pass through the filter.

Lest the venom have been deposited - or have come to rest - in the portion of the watermelon that remains.

Lest a person walking barefoot step in it and be affected by the venom.

For the same reason.

Lest the person mixing the mortar be affected.

For venom will not kill a cat; venom may adversely affect a cat temporarily, but it will ultimately recover.

Because of the danger.

Generally, it is forbidden to burn terumah that is ritually pure. Nevertheless, because of the danger, an exception was made in this instance.

There are opinions in the Tosefta (Terumah, Chapter 7) that maintain that the heat of the oven will destroy the poison. The Rambam, however, follows the more stringent views, that fear danger even in such a situation.