Quantifying and Costing Your Pond
Before you begin building or installing your water feature, it is sensible to ascertain exactly what the cost of your pond is going to be. While many of the items required may simply be priced at the source of supply, some materials will have to be quantified for the specific project before you can calculate the cost of your pond. Some useful guidelines are detailed in this section.
Remember that you will need to order slightly more of certain of the items to allow for waste and breakage. In the step-by-step projects, quantities have been rounded off where appropriate. An additional percentage has not been included.
Where available, it is usually sold in powder form in 40 kg (88 lb) bags. Bentonite is generally mixed with soil and spread dry over the sloping walls and floor of the pond. The ideal soil type is clay, in which case you will need approximately 5 kg (11 lb) for every square meter (10 sq ft) it must cover. Sandy soil should be mixed with as much as 8 kg (18 lb) to cover the same area.
Estimating the quantity of soil required can be problematic. It is reasonably safe to work on a soil weight of 1.75 kg for every liter (3lb 11oz per 2 pints) or 0.001 m³ (0.035 cu ft). This means that for every square meter to be sealed, you will need approximately 175 kq (386 lb) of soil to mix with the bentonite.
If a pure blanket of bentonite is spread over the pond basin, you will need considerably more of this material. In most instances, 10 kg (22lb) will be sufficient to cover 1 m² (10 sq ft).
Unless your pool is a reasonably regular shape, you will find it necessary to buy very much more liner than you actually need to form a blanket over the excava- tion. Unfortunately, there is very little you can do about this waste.
For minimum waste, it is essential to ascertain the dimensions of various liners during the planning stages. The maximum width of polyethylene varies, depending on the gauge; the thinner the sheeting, the wider it may be. While some types of sheeting can be joined, be cautious, it may not last. EPDM, for instance, is very diffi- cult to bond. PVC can be heat welded in the factory, so although you cannot join it yourself, you may be able to order a cus- tom-made pool liner to fit.
Another solution is to find out what is readily available in your area, and, if ne- cessary, adapt the size of the pond to suit the liner you have chosen.
To ascertain how much material you need, draw the pond roughly to scale, and measure its length and breadth at the longest and widest points. Decide how deep it will be, and add twice the depth, plus a bit extra for around the outside edge, to both the length and the width. Rather buy too much material than find later that you do not have enough.
To work out the quantities of cement, sand and stone required for your concrete mix, it is necessary to estimate the total volume of the foundations and/or the concrete shell, using the simple mathematical calculation: total area x thickness. Since most pool walls are fairly low, a 100 mm- (4 in-) deep foundation trench is usually quite adequate, while a concrete shell will be at least 80 mm (3 in) thick. Remember that the length and width of foundations are always slightly greater than the dimensions of the wall or structure it supports.
Presuming you are mixing cement, sand and crushed stone in the ratio 1.2.2, you will need approximately eight pockets (400 kg/880 lb) of cement and 810 kg, (0.6 m²/21 cu ft) each of sand and aggregate, as well as sufficient water to produce a workable mix, for every 1 m³ ; (35 cu ft) of concrete.
Bricks, Blocks and Tiles
Brick sizes do vary slightly, but most are about 222 mm (8¾ in)long, 106 mm (4 in) wide and from 50mm(2in) to75mm (3in) thick; the thinner bricks being for ordinary domestic paving. Blocks used for building and slabs utilized for paving are less standard; these sizes depend on the individual manufacturer.
When calculating the number of ordinary bricks required for a wall, it is safe to assume you will need 55 for each square meter of half-brick walling (46 bricks per square yard), that is 106 mm (4 in) thick. For every square meter of brick paving, you can count on using as many as 45 pavers (38 per square yard).
Alternatively, simply estimate the surface area of each building unit and the approximate area of your proposed wall or paved surface; divide the first figure into the second and you will immediately know how many to buy.
The easiest way to quantify pre-cast blocks or slabs and tiles, is to calculate the area of a single unit and then divide this into the total area to be paved. If you are using flagstones or pavers of varying sizes, the simplest solution is to add together the areas of each kind and use equal numbers of every sort of paver.
Unlike man-made products which are of standard sizes and therefore reasonably easy to quantify, it is more difficult with rocks and stones, and you may have to enlist the help of professionals.
The availability of natural stone directly affects its price. If you are fortunate enough to have rocks and boulders on site, these will cost you nothing. If there is a quarry in your area, it is advisable to get rocks directly from there. Although transportation can be costly, it is likely to amount to less than if you were to buy from your local garden center or through a professional landscaper.
In general, stone is classified according to its finished surface: it may be rock faced, rough picked, axed, split or sawn.
The quantity of mortar required for brick- laying depends on the cement:sand ratio, and the size of the brick or block you are using. Since a reasonably strong 1:3 mix is recommended for this type of work, you will need approximately one pocket of cement and 155 kg (340 lb) of sand for every 150 bricks laid in a half-brick wall.
The same mix may be used for render, and, provided it is spread evenly to approximately 10-15 mm (about ½ in) thick, the above quantity will enable you to render about 7 m² (75 sq ft) of wall.
Remember that the standard of your bricklaying and plastering skills will have a direct effect on the amount of waste of these materials. If you work neatly and cleanly, the estimates found in the following projects will be fairly accurate.
Standard sizes of timber are reasonably universal, although you will find some variation. In general, metric sizes are simply conversions from the imperial system which was established in Britain many years ago. In Europe these measurements have been rounded off, while in certain other countries, including South Africa, the exact conversions are still used.
When ordering timber, it is not essential to buy the dimensions identical to those specified for projects, provided they are similar. It is more sensible to get what is available at your local sawmill or timber merchant than to have the wood specially planed to size.
Similarly, the lengths you purchase will often be longer than those required, particularly if you can cut two or three pieces to size from one plank or beam. It is always worth doing a little arithmetic in order to minimize waste.