FORMULA SHELF
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The arithmetic behind the work: what to measure, what to type in, and what the answer comes out as. Every formula says what the symbols mean and gives a worked example, so you can check it against a job you have already done.
SHELVES
Construction
23 formulas
Multiply length x width x depth in metres. Add 5-10% for waste and over-excavation. A cubic metre of concrete weighs about 2.4 tonnes, so plan your delivery and your wheelbarrows.
A shed base 4 x 3 x 0.15 m = 1.8 m3 of concrete.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For a round tank: area of the circle (pi x radius squared) times height, then multiply by 1000 to get litres from cubic metres. A 2-metre diameter tank 2 metres high holds about 6,283 litres.
r = 1 m, h = 2 m: 3.14159 x 1 x 2 = 6.28 m3 = 6,283 litres.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Order 5% extra. A half block per row is needed at every corner and opening, and people always run out.
A 10 m2 wall of 440 blocks needs about 100 blocks and 0.2 m3 of mortar.
Condensation on the roof sheets means the warm moist air is meeting a cold surface and there is no way out. Vent the building at both ends. See Venting The Workshop.
Air at 20 C holds about 17 g of water a kg; cooled to 5 C it can only hold 7 g, so 10 g of water comes out of every kg of air.
A drain that does not fall will block, and the smell will find its way into the shed. See Underground Pipe And Trenches.
A 60 m2 roof needs a 93 mm pipe, so 100 mm with a 1 in 60 fall.
A foundation that is too shallow gets lifted by a hard frost and pushes the building out of square. See Building A Step.
A frost line at 450 mm means the footing must go to at least 1,050 mm.
An undersized gutter overflows in a storm and wets the foundation. See Draining Water Pipes.
A 120 m2 roof needs a 132 mm pipe, so 150 mm.
For a yard or a track. Compact in layers of 100-150 mm or the bottom will go soft. See Building A Step.
A 500 m2 yard at 200 mm of hard core = 100 m3, which is 7 tipper loads.
For comparing a shed, a workshop and a house, and for working out what a stove has to make up. Bigger U-value means more heat going out.
A 10 m2 stone wall with a 12 K difference: 10 x 1.7 x 12 = 204 W.
A suspended timber floor over a cold ventilated void loses a lot of heat. See Insulating A Workshop.
A 12 m2 room with 0.6 m2 lost to the joists needs about 11.4 m2 of 100 mm board.
For working out how much insulation to buy. Rule of thumb: 100 mm of mineral wool is about R2.5, and a well insulated roof wants R4 or better.
150 mm of PIR: 0.150 / 0.023 = 6.5, so R6.5.
Before the trucks arrive, work out how much hard core or soil you need. The average of the high and low spots is the honest way to do it.
A 400 m2 yard that needs an average of 75 mm of stone = 30 m3.
The strength comes from the sand being sharp and clean and the joints being filled. Lime in the mix makes it work better.
A 0.1 m3 batch of 1:4 mortar needs about 1 bag of cement and 0.2 m3 of sand.
Rushing a render is how it cracks. Two coats, and let the first one set before the second.
A 10 m2 wall at 15 mm needs 0.15 m3 of render, about 3 bags of cement and 0.6 m3 of sand.
Digging a hole the right width makes the job ten times quicker, and the post is stronger. See Setting Fence Posts.
A 150 mm radius hole 600 mm deep = 0.042 m3.
The sand bed is what makes the blocks sit flat. A slabbed area that wobbles has not enough compacted sand under it.
A 20 m2 path needs 1 m3 of sand, which is about 14 wheelbarrows.
A blocked sump floods the yard and the smell reaches the house. Keep the grate clear and the outlet clear.
A 2 x 2 x 1.5 m pit with 20% free space holds 3.6 m3.
Mould on the same corner every winter is not bad luck. Look for the cold spot with a thermal camera or the back of your hand.
Damp at the base of a cold wall is condensation, not a leak, unless it moves when it rains.
The number that decides whether a workshop, a stable or a grain store is workable. See Venting The Workshop.
A 3 x 3 x 2.5 m workshop is 22.5 m3; a 300 mm fan at 0.64 m3/s gives 102 air changes an hour, far too much cold air in winter.
The first number for any insulation job. Measure every wall and subtract the openings, and buy 10% extra for the cutting.
15.7 - 3 m2 of window and door = 12.7 m2 to insulate.
For a header tank and a trough system, and for a yard where the mains cannot keep up. See Water Troughs And Algae.
2 people and 10 cows need 1,300 L a day; a two day buffer is a 2,600 L tank.
Windows are the weak point of any old building. A secondary glazing frame on the inside of a single glazed window is cheap and effective.
A 2 m2 single glazed window at 12 K: 2 x 5.0 x 12 = 120 W, which is a small heater running all winter.
The way to sell an idea to whoever holds the purse. Half the cost of an insulation job often comes back in fuel in the first winter.
Insulation saving 900 euro a year for 1,800 euro of work pays back in two years.
SOURCES & PROVENANCE
1 listed
This page is an AI-organised index. The label means the grouping and cross-links were compiled with AI; the information itself comes from the public-domain books, reports and manuals listed below. Open any source to read the local copy or follow the link to the original publication.