Put Up A Fence

כִּ֤י תִבְנֶה֙ בַּ֣יִת חָדָ֔שׁ וְעָשִׂ֥יתָ מַעֲקֶ֖ה לְגַגֶּ֑ךָ וְלֹֽא־תָשִׂ֤ים דָּמִים֙ בְּבֵיתֶ֔ךָ כִּֽי־יִפֹּ֥ל הַנֹּפֵ֖ל מִמֶּֽנּוּ.  There is a mitzvah to build a fence around one's flat roof to prevent others from falling off.  The Gemorah is clear that this obligation to put up a fence only applies to what's halachically defined as a house.  Based upon this the Gemorah in Bavva Kammah (51a) says that a house less than 10 טפחים is exempt from putting up a fence.  Similarly, the Gemorah is Sukkah (3a) exempts a house less than 4*4 טפחים and the Gemorah Chullin (136a) exempts synagogues and battei midrash from putting up a fence.

The Gemorah in Bavva Kammah (15b) says that the lav of לא תשים דמים בביתך says that this is a general rule to remove any thing that would be liable to cause harm. רבי נתן אומר מניין שלא יגדל אדם כלב רע בתוך ביתו ואל יעמיד סולם רעוע בתוך ביתו ת"ל (דברים כב, ח) לא תשים דמים בביתך.  Based upon this its hard to understand why something not defined as a house is exempt from putting up a fence, it should be no better than any other thing that may cause damage that there is a lav to leave it unprotected?

We see from here that the mitzvah of putting up the roof isn't a positive mitzvah to remove the damage.  If not for the mitzvah, there would be no obligation to put up a fence because the percentage of damage of a roof is negligible for it isn't commonly used.  The Torah is mechadesh a new mitzvah is regard to a roof to remove even the small percentage of damage that may exist (Chazon Eish likkutim Bavva Kammah siman 18.)

Tosfos in Chullin 105a says there is no blessing on something that comes just to remove danger.  If that's the case how is there a beracha on putting up a fence? Based upon this Chazon Eish its not a difficulty for the mitzvah of putting up the fence isn't just to remove danger.

The possuk says כִּ֤י תִבְנֶה֙ בַּ֣יִת חָדָ֔שׁ, why does it say a new house if the same also applies even if one isn't building a new house?  The possuk is also alluding to one who is building new levels in avodat Hashem.  In order to guarantee that one doesn't fall down from his/her new heights and fall down, one must put up a fence, make extra safeguards to guarantee they don't fall (see Likutay Sichos volume 2 and volume 19.)