"It is a good roof."
Translation:Es un buen techo.
In Spanish, an adjective before a noun merely expresses an opinion, so ‘Es un buen techo.’ implies that the speaker approves of the roof.
An adjective after a noun, though, restricts the set referred to by the noun, so ‘un techo bueno’ implies a contrast with other non-good roofs, as in ‘Esto es un techo bueno’ = “This is a good roof.”, as opposed to, say, that roof. But in isolation, without a determiner like ‘esto’=“this” to specify what's being contrasted, ‘Es un techo bueno.’ doesn't make much sense.
‘techa’ is the 3rd-person singular (él, ella, usted) present indicative form of ‘techar’=“to roof”, so it could be used like ‘Con qué techa él la la casita?’ — “La techa con paja.’ = “He's roofing it with straw.”.
[Note that the Spanish ‘techo’ is cognate with the English “thatch”.]
‘tejado’ literally means “[roof-]tiled”, the past participle of the verb ‘tejar’=“to tile [a roof]”, from the noun ‘teja’=“[terracotta] roofing tile”, so it usually implies a Spanish-style terracotta-tiled roof.
‘techo’ is more general, and can refer either to a roof or a ceiling.
Mary, below is a link to explain the differences. In short if you're explaining a masculine noun: the adjective is "buen" and placed before the noun or the adjective is "bueno" if placed after the noun. But describing a verb requires an adverb which is "bien" ! Ex: Sentirse bien. I feel well. Another tip is bueno=good and bien=well. Hope this helps a little.