The equivalent of the “OU” kashrut certification in England is that of the London Beit Din. There is also a competing kashrut certification called Kedassia. Those who eat Kedassia do not eat Beit Din.
My father, one of the principal dayanim on the bench of the London Beit Din and someone who had invested much of his working life on matters of kashrut, ate Beit Din. His younger brother, my uncle, ate only Kedassia. My father didn’t like that. One Erev Sukkot, shortly before Yom Tov, my uncle invited my father to admire the sukkah he had built with much care and enthusiasm.
“Beautiful,” remarked my father, “but unusable. Look up; you’ve built your sukkah under the branches of an oak tree. But never mind, come and eat in our sukkah. We do, of course, only eat Beit Din.”
The sukkah, which means “shade,” must be built for the purpose of providing shade and must be outside under the open sky. There can be nothing interceding between the sukkah and the sky. Accordingly, the sukkah cannot be built in a house or under a tree, which already provide shade and which, therefore, render the sukkah redundant. But why couldn’t my uncle’s sukkah be fixed by cutting away the overhanging branches? The answer is that the sukkah must be valid when originally constructed. It cannot originally be built invalid and then corrected and made valid.
Thus, for example, a sukkah fashioned by hollowing out an existing haystack is invalid. This disqualification is known as “taaseh velo min haasui,” which means you must create a valid sukkah and not fix an invalid one. This principle is derived from the verse “[Chag] hasukkot taaseh lecha” – “[the festival of] Sukkot you shall make for yourself.”
The principle of taaseh velo min haasui is derived from the fact that the word “sukkot” precedes the word “make.” The only way to save my uncle’s sukkah would have been to cut away the overhanging oak tree branches and then pick up each piece of the existing leaves and lay it down again on the roof of the sukkah. That way, by laying the sechach again, the sukkah would be kosher at inception, because it is the laying of the sechach, which is the essence of the sukkah. But being shortly before Yom Tov, there was no time.
Another remedy would be to bend the branches of the tree over so that they merge with the existing kosher sechach already on the roof of the sukkah. Based on the halachic rule of bitul b’rav, (the majority nullifies the minority), as long as the kosher sechach outnumbers the invalid tree branches, the sukkah is kosher.
In the same way as one may save the sukkah by cutting down the branches and laying the sechach again, one could also remove roof tiles from a roof and sit under the wooden beams. The very act of removing the roof tiles is considered a sufficient act of “taaseh” and renders the remaining construction a valid sukkah. Because of the principle of taaseh velo min haasui, a rain roof on a sukkah should be open when the sechach is laid down and the sechach should not be inserted from the inside with the roof closed.
Other examples of taaseh velo min haasui are tzizit and mezuzah. From the order of the words “gedilim taaseh lecha” the rabbis derive that four separate fringes must be inserted into the garment and then doubled over into eight, which must then be tied and knotted in the prescribed way. One cannot, however, insert one long string, double it over twice, tie and knot it and then snip the ends to make eight. Similarly, one may not fix a mezuzah onto a detached doorpost and then build the doorpost with the mezuzah on it into the house.