Lap Siding – Revealing how it works

When we discuss Siding we are talking about a huge category.  Fortunately most of them are pretty straight forward and rely on a straight square foot to board size conversion.   When we get to Lap Siding, however, things are sometimes more complex than they seem at first glance. 

Just as a starting point, if we have a home that has 10 foot exterior walls and has a total wall lineal footage of 500, we would know that you need to cover 5000 square feet.  If we deduct 1000 square feet of doors and windows, our final siding coverage is 4000 square feet.  Easy stuff.

Basic Calculations

In the following example we will use a Hardie product and see how it affects our totals.  If we decide on 8-1/4” boards in 12’ lengths that’s 8.25 square feet per board. 485 boards will cover our 4000 square feet with a little to spare.  Add 5% waste and order 520 boards.  Perfect.

The Lap Factor

Except for the fact that it is LAP siding.  we actually lose the lap factor – in this case we will lap 1-1/4 inches – from each board.  The exposure then is 7”, not 8-1/4”.  Our true square foot of coverage per board is 7, and so to cover 4000 square feet we need 572 boards.  Add our 5% waste and we need 601 boards. If we don’t properly account for the lap factor that’s money left on the table. Right now, on average, that’s about $950 plus time lost.

Lap Factor

If you have less exposure (5” isn’t uncommon in some areas) the error magnifies.

Starter Strip

Once we know the lap it isn’t hard to calculate what we need.  As for what the lap should be, that is determined by the type of material, the manufacturer’s recommendations, and the desired visual effect on the house.  One last thing – don’t forget the material for a starter strip.  There needs to be a strip at the base of the wall to kick the first plank out.  This can be ripped from the siding being used or purchased as starter strips.  If the starters are to be ripped from the siding material be sure to order enough material to cover it.

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