A 1m2 of masonary walling, assuming a 200mm/9inch stone thickness, will be as below.
Surface area =0.2m x 1m=0.2m2.
5 Surface levesl in 1m2 =5 x 0.2m2=1m2 For horizontal mortar joining the stones.
Surface area=0.2 x 0.2=0.04m2.
15 vertical surfaces in 1 m2 of wall=15x 0.04=0.6m2.
Total surface area =0.6 + 1=1.6m2.
Volume = assuming a 1 inch/0.03m thickness =0.03 x 1.6=0.048 m3 of cement sand mortar per 1 m2.
Ratio of cement to Sand for walling mortar.
Assuming a cement/sand ratio of 1 to 8, the volumes will be as below.
4 sand 1 cement.
The usual mix by fundis in Kenya for cement sand mortar for joining and plaster is 4 wheel barrows of sand to 1 bag of cement.
2 bags of cement are equal to 1 wheelbarrow.
This translates to 2 bags cement to 8 wheel barrows of sand or a ratio of 1 to 8.
Sand volume will be 8/9 x 0.048m3=0.042m3
Cement volume will be 1/9 x 0.048m3=0.0053m3.
Sand density is 1.6 tonnes per m3.
Therefore, 0.042m3 of sand will weigh 0.07 tonnes=70kg.
1 tonne of sand costs kes 2,000.
0.07 tonnes of sand will cost kes 137 .
Cement density is 1.6 tonnes per m3.
Therefore, 0.0053m3 of sand will weigh 0.009 tonnes=9kg.
1 tonne of cement costs kes 14,000, Equivalent to 20 bags cement @kes 700 per bag.
9kg of cement will cost 9/50 x kes 700=kes 126 for 1 m2 of walling.
Total cost of cement/sand mortar will be 126 + 137=kes 263 per m2.
A fundi plus one mtu wa mkono will complete 150 stones per day.
1 stone is 0.2 x 0.4=0.08m2. 150 Stones are 12m2 of stone wall surface per day.
Labour cost is kes 800 for a fundi plus kes 400 for a mtu wa mkono =kes 1200 per day.
Cost of labour for 1 m2=kes 1200/12m2=kes 100.
Total cost of cement, sand plus labour =kes 363 per m2.
In 1 m2 of masonry stone walling,
costs are as below
stone 13 pieces@kes 50=kes 650
Total =kes 1013
Sand has increasingly become more expensive than cement.This is because of environmental concerns on river sand harvesting hence heavy taxes levied on sand harvesters. Labour is also threatening to surpass the estimates given due to the increased higher costs of living around Nairobi.
Cement prices have even gone lower, with Simba cement reducing their prices to around kes 600 per bag recently.
Stone prices will also increase due to rules limiting the tonnage of lorries travelling along Thika super highway. Juja is the largest supplier of machine cut stones in Nairobi hence affected by Thika road tonnage limit where the lorries are now carrying less tonnage for the same transport cost.
Francis Gichuhi Kamau, Architect.