The coating that does the structural work before the decoration begins
A wedding cake arrives at a venue already committed to its outer coating. That decision was made weeks earlier, in a workroom, based on a single overriding factor: what the room will do to the surface on the day.
Ganache and buttercream are both used as a crumb coat and finishing layer beneath fondant or other decoration — their job is to seal the sponge, fill imperfections, and provide a smooth, stable base. They are not interchangeable. One behaves reliably under pressure; the other softens the moment conditions shift.
What each coating is, and what it does
Ganache is a mixture of chocolate and warm cream, combined in proportions that determine its final firmness. For cake coating, the ratio leans toward chocolate — typically more chocolate than cream by weight, sometimes significantly more for white chocolate, which contains more cocoa butter and sets softer. Once poured and cooled, ganache hardens. It can be smoothed, scraped, and built up in layers that hold a genuinely sharp corner. Chilled overnight, a ganache-coated cake sets to something close to solid; it can be handled, stacked, and transported with considerably less anxiety than its alternative. When the fondant goes on, it has a firm, non-yielding surface to adhere to, and the finish that results — especially along the edges of tiered cakes — is clean and architectural.
Buttercream is fat and sugar, sometimes with egg white or meringue base, whipped to incorporate air. Swiss meringue buttercream and Italian meringue buttercream are the two most commonly used in professional cake work: both are made by cooking the sugar before whipping it into the fat, which produces a stable emulsion and a less cloying sweetness than the simpler confectioners'-sugar versions. Buttercream is softer, more compliant, and considerably more flavourful as a coating in its own right — which matters if the outer layer is to be eaten rather than removed. It also responds immediately to warmth. Above around 23–24°C, a buttercream-coated cake begins to shift. The surface develops a sheen, edges soften, and any fondant laid over it starts to slide.
The room temperature decides
That threshold is the whole argument. A marquee in June, a conservatory in August, a function room with poor air conditioning — all of these change the calculation. Cake makers assess venue conditions, sometimes asking directly about air conditioning and whether the room gets direct sun. For colder months or reliably climate-controlled venues, buttercream is a reasonable choice for the outer coat; the softness that makes it problematic in heat is also what makes it pleasant to eat and straightforward to work with. For summer, or for any venue where temperature cannot be guaranteed, ganache is the safer base. The structure holds, the corners stay sharp, and the warm room problem becomes an issue for the flowers and the fondant detail, not the cake's fundamental geometry.
The same logic applies to tiered cakes, where the stakes are higher. A three-tier cake with a softened buttercream base is carrying real weight through a compromised structure. Ganache, set hard, distributes that weight more reliably. The internal dowels and boards still do the primary load-bearing, but the firmness of the outer layer contributes to lateral stability — a cake coated in set ganache does not deform at the sides under load the way a soft buttercream-coated one can.
Taste is not irrelevant but it runs second. Most guests eating wedding cake are eating through fondant anyway, which carries its own sweetness. The coating underneath serves the structure more than the palate. Experienced cake makers develop strong opinions about which they prefer to work with — ganache demands precision in the mixing and timing of its set; buttercream demands sensitivity to temperature at every stage of use — and those preferences are real and craft-based. But the preference that carries the most weight is always the venue's, expressed not as a preference at all, but as a fact: the temperature of the room on the day.