Richard wrote:
Let's imagine that I want to buld a wall 5 bricks long and 12 courses high.

http://www.ibstock.com/highlights-brick-calculator.asp

You see, on the brick calculator, that if I did that the area is exactly
1 square metre. You also see that it would take 60 bricks. That tallies
with the brick quantities on page 13 of Ibstock's Guide to Good Practice
cataligue.

What I understand is, that if I built the same wall, 5 bricks long and
12 courses high, but English bond, I'd use 86 bricks. Or, Flemish bond,
77 bricks.

But, if I use less bricks, because I choose Flemish over English bond,
say, I must use more mortar. That's what I think is the case. If mortar
is more expensive than brick, it would be more expensive to build with
less bricks.

Or, am I making a mistake somewhere?

Possibly. We always used to order the same number of bricks for a 215mm
thick wall, no matter what the bond, ignoring minor stuff like chopping
bricks in half for odd sized holes and corner bonding. The calculator on
the page is for 102mm thick walls assuming that for each header, you use
a complete brick, throwing away the other half brick. There may be an
allowance in their calculations for spoilage when the bricks are cut.

For 215mm thick walls, it's about 120 bricks per square metre when you
order them. For 102mm walls, it's about two thirds of that, due to
wastage when cutting to make short headers. The bits that get cut off
normally end up filling the bottoms of trenches as hardcore.

The other thing is, you don't build brick walls to exact metres, you
design and build them to multiples of 112.5mm horizontally, with 10mm
allowances for mortar joints being missing at the ends, and 75mm
multiples vertically, with 10mm allowance for the missing mortar joint
on top.

--
Tciao for Now!

John.