Estimate Composite Decking


let's use the above drawing as our example for figuring deck boards. We will have to do our figuring in a few different sections.

Let's start with the main rectangular section of deck, assuming we will figure the angled corner area and stair later. That way, we will calculate what we'll need for the width of the deck first (board lengths and mullions/borders) and then figure how many boards we will need to cover the depth of the deck.

As far as the width of the deck goes, estimating width is easier, because of the limited lengths that decking is available in.

Because the main rectangular area of the deck is 26 feet wide, and the longest length most deck planks are available in is 20 feet, this deck would either have to be "random-seamed", or have the decking areas split up. Random-seaming is not ideal because decking butt seams are unattractive and can move because of expansion/contraction. We try to use "mullions" or divider boards where decking is divided up into multiple sections.

If money was no object, we could split this deck area in half with a single mullion to make 16' boards cover both sides, trimmed down to approximately 12' 9". This would leave you with over 3 feet of scrap off every board used, so instead, we will try to make up some extra width by adding borders or mullion boards.

splitting a 26 foot (312") wide deck in half makes two sections that are 156" wide. If we can make up 12 inches on the width of each half, we can bring the width of each main decking area down to 12'. Since deck boards are commonly available in 12 foot lengths, this would leave us with very little scrap material.

Keeping the mullion and a double borders on both ends and a mullion board down the middle could be added to make up an extra 6" or so on either section.

The final result here would be a 26 foot wide rectangle with double borders on the ends, and a single mullion down the middle.

Measuring across the width of our hypothetical deck, we would see:

5-1/2" border +

1/4" gap between border boards +

5-1/2" inner border board +

1/4" gap between border and ends of boards +

12' boards trimmed down to 141-1/2" +

1/4" gap between ends of the first section and the mullion +

5-1/2" wide mullion board +

1/4" gap between mullion board and start of second main decking area +

12' boards trimmed down to 141-1/2" +

1/4" between ends of boards and the inner border board +

5-1/2" wide inner border board +

1/4" gap between border boards +

5-1/2" wide border board


This measures out to make a rectangular area that is 26 feet wide. If a project is a simple 26' wide rectangle, this is the majority of the figuring you would need to do.

For the depth of the deck: Distance from the house to the front of the deck / (width of deck board+desired gap between deck boards)

The gap between boards is usually between 1/8" and 1/4", we'll use 1/8" for this project.

Total depth of the deck is 10 feet or 120", and the width of a deck board plus our desired gap is 5-1/2" + 1/8"

120" divided by 5-5/8" is just over 21, so we would need 22 boards to cover the depth of the deck. Generally, the fraction of the board that we calculated is installed as a board ripped down to size, and tucked under the flashing at the house (this hides the cut edge).

Since our main deck area has two sections, we would need two sets of 22 boards, to make 44 boards.

If we include our 10 foot long mullion and border boards (5 total) that are cut out of 20 foot deck boards, we would need 2-1/2 20 foot boards, so we will round up to 3 of the 20 footers.

Add this all up, and we get 44x 12 foot boards and 3x 20 foot boards (for this area)