objective the back wall 224 of the wall structure 212 is shorter in elevation than the side walls 220 222 so that each of the side walls 220 222 form a corner 223 225 at its proximal end adjacent the interface thereof with the back wall 224 above the elevation of the top edge of the back wall 224 the roof 214 includes means 236 for selectively changing the inclination of the roof 214 relative to the horizontal and also for removably attaching the roof 214 to the side walls 220 222 as shown the means 236 comprises a pair of parallel slits 243 formed through the roof 214 at opposite lateral sides thereof the distance between the slits 243 is somewhat greater than the distance between the corners 223 225 of the side walls 220 222 alternatively the width of the slits 243 can be somewhat smaller than the thickness of the side walls 220 222 each slit 243 receives a different one of the corners 223 225 respectively with a friction fit so that the roof 214 is supported by the side walls 220 222 and the inclination of the roof 214 can be easily changed now with reference to fig1 and 12 there is shown a greenhouse generally denoted by the numeral 310 which has many features in common with the greenhouses 10 110 210 and similar features are denoted by similar numerals as can be best seen in fig1 and 12 the greenhouse 310 includes a wall structure 312.