This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Fig. 165. - Corner Post fixed with Stays.
Fig. 166. - Plumb-board.
This point must now be considered. A greenhouse should have a fall of 6 in. in the 100 ft. This will allow the gutters and pipes to be fixed with the least amount of trouble. Probably the ground will have more natural slope than this. The wall is to be 2 ft 6 in. high, so measure off that height on the post which is in the lowest corner of the site. This can be judged near enough by the eye. At this height nail on a small crosspiece square with the post. About 6 ft. off put up another crosspiece dead level with the top of the first one and in a line with the footings. Now sight along the tops of these two crosspieces, and mark off on the post in the farther corner of the trench where the eye strikes it. This point is level with the 2-ft.-6-in. mark on the first corner post. Mark off on the farther post a height of 2 ft. 6 in., as on the first, and see what the difference is between the measured mark and the sighted mark. A rise of 6 in. per 100 feet is wanted, so that, supposing the house is to be 100 ft. long, the measured mark should come 6 in. or more above the sighted mark. Suppose, for the sake of illustration, it comes 9 in. above; this shows that the ground has a natural rise of 9 in. per 100 feet. Now that the rise of the ground is ascertained, the building may be proceeded with. Take the plumb-board (fig. 166), and against the line as already described put up a row of posts, 5 ft. apart, all the way up the line, and fix them perfectly upright with a batten stay driven into the ground on the inside of the house, and nailed near the top of the post. When these are all upright, stand the plumb-board up against each one, and drive in another post on the opposite side, the plumb-board being thus used to give the right distance between the posts. At the same time join the tops of the posts together with a short piece of slating batten; one screw to each post will hold it well. There is now a double line of posts all up the trench, perfectly upright, the right distance apart to take the boards and the concrete, and tied together at the top with the batten to keep them from spreading when the concrete is put in between the boards. The posts are carried round the ends of the house at the same time, and a glance at fig. 168 will show the arrangement of the posts at the corners.
Fig. 167. - Plumb-level.
Fig. 168. - Showing Posts at corner of Building, and Boards arranged for Concrete Walls.
Prepare a number of little pegs about 15 in. long out of l-in.-square batten, and point their ends; one to every post will be wanted, and two to the corner posts. Get a contractor's level, which is a piece of board about 6 ft. long, with its edges planed parallel and a level set in one edge correctly. The plumb-level shown in fig. 167 is a very simple thing to make, and will do the work quite as well as the level just described. Whichever is used it must be set to the rise of the house - in this case 9 in. to the 100 ft. This is done as follows: Place the level on a perfectly level table or bench. Now raise one end of the level till it has a rise of 9 in. per 100 ft. For a 6-ft. level this will be almost exactly 1/2 in. - 54/100 to be exact. Mark on the level where the bubble comes to, or on the plumb-level where the plumb-line swings to, and use this mark to work to instead of the centre mark. Drive a peg into the ground at the bottom of the lowest inside corner post till it is only about 2 1/2 to 3 in. out of the ground. Go to the next post and drive in another in a similar position, but while it is still too far out of the ground test it with the plumb-level placed on the top of the first peg. Drive the second peg in gently, till, when the level is resting on the tops of the two pegs, the line stops at the mark on the level showing the rise of 9 in. per 100 ft. Proceed in the same way up the line of posts; but the pegs in the end of the house are set dead level. Now take a small level, or a square, and drive in a peg on the inside of each of the outside row of posts, getting it perfectly level with the corresponding peg on the inside post opposite. If there are any lumps on the ground which bring any of the pegs less than 3 in. out of the ground, the bottom of the trench must be shaved out with a shovel till the correct depth is attained. Any hollows are left as they are for the present. The planks are now put into place between the posts, the lowest ones resting on the tops of the pegs; thus, whatever height the wall, as long as the planks used are uniform in width, the top of the wall is bound to come straight and of the correct rise. The planks are kept from falling in by standing pieces of wood the width of the finished wall down between them. The planks being put up as far as they will go, the mould thus formed, starting from the doorway at one end and up the side of the house as far as the planks will reach, is ready for the concrete. Where a doorway is to come a piece of plank is put down between the boards at the correct spot, set perfectly upright with the plumb-line, and nailed in place. Make a platform of boards on a level spot near the mould for mixing concrete on. The concrete may consist of cement mixed with beach and sand, broken brick rubbish and sand, breeze, broken clinkers and sand, or, best of all, ground clinker with brick ends put in as the concrete is put in position. The proportions should be as follows: 5 parts of broken brick, clinker or beach to 2 parts of sand and 1 of cement. The clinker and brick rubbish should have all the large pieces broken up with a hammer till they are no larger than an egg. If on mixing up a small quantity there does not seem to be enough fine stuff in the mixture, a little more sand and a little less of the large stuff should be added, and the same with the beach. The theoretical perfection of concrete is material of some hard nature broken into pieces that will go through a 3/4-in. sieve, enough sand to fill the interstices between these pieces, and enough cement to fill in between the grains of sand. When ground clinker is used no sand is wanted, as the clinker is fine enough; and as this substance forms a very tough concrete, 8 parts of ground clinker to 1 part of cement will be a good mixture. When calculating out the quantities, allowance must be made for shrinkage. Ground clinker will occupy about one-sixth less space when mixed and put in place than it will when dry, while beach may be calculated as beach alone, the spaces between the stones taking up all the sand and cement. The shrinkage of other materials had better be ascertained by trial of a small quantity before ordering the bulk. The best way to measure the materials is to make a box of 1 1/2 cub. ft. capacity, i.e. measuring 1 ft. 6 in. by 1 ft. by 1 ft. Nine of these boxes full makes just 1/2 cub. yd., and this is a handy quantity to mix up at a time. Of course, when the proportions are 1 in 8, as for the beach and broken brick, the quantity is just short of \ yd.