Fig. 221. Details of pergola construction
This weighty question having been decided, prepare to mix concrete. Make a mixing board about five or six feet square of flooring boards fastened to heavy cleats on the under side. It should have two or three sides nailed on, as shown at k. Also prepare eight wooden blocks, as shown in drawing. These may be fifteen to eighteen inches long, sawed from 4 × 4 timber, planed smooth and with a slight taper toward the lower end. The purpose of these blocks will be apparent later on. The concrete is composed of four parts clean gravel to one of Portland cement, and the best is always the cheapest in the long run. Use a pail for mixing and place four pailfuls of gravel on the mixing board. Pour over this one pailful of dry cement and mix thoroughly, turning the whole mass several times. When thoroughly mixed, pour on water, half a pailful at a time, and turn again with the shovel until the whole mass is wet. Shovel this into the first box. It will probably take two such mixings to fill it. Now place in the top of the concrete one of the tapering blocks, allowing it to project about three or four inches above the surface of the concrete. The block should be rubbed with grease or oil before insertion to make it withdraw easily. As soon as the block is placed, pass the line from stakes e to f and make sure that the block is exactly at the centre.
In every case it is to be withdrawn in about an hour, or as soon as the concrete has set, but before it has gripped the block too tightly; otherwise withdrawal will be a difficult matter. The object of this performance is to leave in the centre of the foundation a hole about a foot deep and four inches across. Level the top of concrete with a trowel and test with the level.
All the foundations are made alike and no effort should be spared to see that the boxes are level and in line with each other. Test from end stakes a b c d and from across stakes with the masons' line. This work will be easier if the four end foundations are made first.
To do the work thoroughly, two boys may count on the process, from the staking out to the finishing of foundations, occupying about three days. It is the most important and laborious part of the work and when finished represents about one third of the labour on the pergola.
The concrete should stand at least four days before it is touched again, and during that time the timber may be prepared. Besides the eight columns, there will be needed:
| 6 pcs. 4 × 6 ins.—16 ft. long |
| 15 pcs. 3 × 6 ins.—14 ft. long |
| 144 ft. (running) 1 × 2 ins. |
| 8 pcs. of 5⁄8-in. round iron |
The iron should be 18 inches longer than the total height of the wooden columns, including base and cap. This should be determined by actual measurement. The rods can be obtained from a blacksmith. Have him cut a thread at each end and provide two nuts for each rod. They are to extend clear through each column from the bottom of the hole in the foundation, through the longitudinal timbers that rest on the cap, to bolt the whole structure together securely.
Columns come either built up or solid. The solid ones have a core bored out through the centre, so that in either case the rod can easily pass through from top to bottom. A hole should be bored 3⁄4 inch diameter through the centre of both cap and base to allow the bolt to pass easily.