"FROG-UP OR frog-down?" This hardy perennial of building site discussions has nothing to do with the most comfortable way to wear overalls, or with the frog tunnels discussed in last week's column.

The frog-up / frog-down debate concerns the best way to lay bricks, the frog in question being the name given to the hollow in the top of a brick - or the bottom, if you are in the frog down camp. Why is it called a frog? Nobody knows. The most plausible explanation is that the hollow underneath a horse's hoof is also called a frog.

And why do bricks have frogs? Again, opinions differ. A frog makes a brick lighter, which is a boon for the manufacturers, but this makes little difference to hod carriers and brick-layers, because the lesser volume of frogged bricks has to be made up with a greater volume of mortar - to fill them up.

Frogged bricks also supposedly make for better burning in the kiln - most bricks are made of fired clay, just like pottery, and it is said that the hollow of the frog helps the heat to reach the centre. But these arguments are not conclusive; some types of bricks have no frogs, and still seem to be cooked right through to the middle. And, as with many aspects of the building game, the science is inexact, and brick shapes probably owe as much to tradition as to efficiency.

But the burning question on site is which way up to lay the things. Common sense would seem to dictate that bricks should be laid frog-up, so the frogs get filled with mortar, and the brickwork achieves maximum density. But bricklayers can work faster laying the frogs downwards - there is less mortar to be picked up and spread, and the bricks can be easily pushed into position without having to tap them with the edge of the trowel.

Unfortunately, the structural results are not good - the weight bearing down on the bricks, instead of being spread over their whole area, is concentrated on the four edges, and this can result in gradual cracking and surface damage, known as "spalling". The funny thing is, some surveyors don't seem to know about this - I have seen several cases where spall- ing was diagnosed as frost damage, salt attack or even sub-sidence, when the real problem was that the bricks had been laid upside down. It doesn't help that some books perpetuate the myth - even the otherwise admirable Penguin Dictionary Of Building mistakenly states that walls built frog down are stronger.

Some hand-made bricks come out of the kiln with a slight upward curl, and this is the only situation when it is better to work frog-down - for aesthetic reasons bricks in a wall should always be frowning, rather than smiling. And then the brickie is supposed to fill the downward-facing frogs as the bricks are laid - no easy task.

So, if you're having that new garage or extension built, surprise your builders by popping your head round the corner on your way to work and saying, "Frog-up, boys, OK?" That should keep them on their toes.

'Struck Off - The First Year of Doctor on the House', by Jeff Howell, is available from Nosecone Publications, PO Box 24650, London E9 7XQ. Price pounds 9, inc postage.

Jeff@doctoronthehouse.demon. co.uk