How To Calculate the Cost Of Your Decking

If you’re planning to build a new deck, there are two things you absolutely need to remember. The first is to check with your local building department to find out if you need to apply for a permit (you probably do), and the second is to draw up a materials list so you can budget for the project. Lumber prices may not be experiencing the hefty rise they were a few years ago, but they’re still high, and no one appreciates the shock of an unexpectedly big bill after a project has been completed.

Every large decking project I’ve ever done has had its share of glitches and spontaneous design changes that make the initial cost estimate just that — an estimate. Irregular soil conditions, sloped terrain and other factors tend to make labor costs variable, but those costs don’t matter if you’re building the deck yourself. As long as you know the deck dimensions and you’re clear on the type of decking you’re going to use, materials costs should be fairly predictable.

Here’s the strategy I use for drawing up a materials list for a decking project.

Calculating the Cost of Decking

The choice of decking material greatly affects the cost of the deck. If you’re on a budget, you’ll probably want to go with pressure-treated pine or fir ($2 – $5 psf), which is the least expensive choice, but if money is no object, you may be attracted to premium materials like ipé, tigerwood or other exotic wood varieties ($7 – $20 psf). Composite and pure PVC decking from such suppliers as Trex, AXEK/TimberTech and Fiberon are mid-range options ($5 – $22 psf). Rot-resistant softwoods like redwood and cedar, which are perennial favorites, are a little cheaper than composites.

Decking materials are generally priced by the square foot, but if you opt for redwood or cedar, you’ll probably find them priced by the linear foot at the lumberyard. Assuming they are standard 6-inch decking boards, it’s easy to convert the price per linear foot to the price per square foot. Just double it. This works because it takes two 6-inch boards (which are actually 5 1/2 inches wide) to make a width of one foot (counting the gaps between the boards). Similarly, if you come across 4-inch boards priced by the linear foot, just triple the price to determine the price per square foot.

Having determined the price per-square-foot of your chosen decking material, all you have to do now to estimate the total decking cost is measure the deck area in square feet and multiply that number by the cost per-square-foot of your decking material. I always tack on 10 percent to the total square footage to account for cutting waste.

• Example: Suppose I’m planning to build a 10X10-foot redwood deck. Redwood currently costs $2.24 per linear foot at my local lumberyard, so the cost per square foot is $4.48. Considering the deck area is 100 square feet, and adding a 10 percent overage, the decking will cost $492.80.

Calculating the Cost of Foundation Lumber

There’s a lot less choice when it comes to the posts, beams and joists for a deck; in fact, the only realistic options are pressure-treated pine or fir. You could build the foundation with natural redwood or cedar, but the cost would be higher, and the deck wouldn’t last as long. The parts in contact with the ground would be especially prone to rot, but you won’t have that problem with pressure-treated wood that is rated for ground contact.

Dimensional pressure-treated lumber is always sold by the linear foot, and there’s no special trick involved with calculating the total cost. Simply add up the lengths of each dimension you need (4X4s or 4X6s for posts and beams and 2X8s for joists) and multiply that by the price per-linear-foot charged by your supplier. I have a contractor’s account at the lumberyard, so I get a 10 percent discount, which saves a considerable amount for a large project. If you don’t qualify for a discount, perhaps you have a contractor friend who will buy the lumber for you.

Stairs, Railings and Trim

Code requires any deck more than 30 inches above the ground to have a railing, and if your deck is high enough to need a railing, it usually also needs stairs. Fascia trim isn’t a necessity, but you may want it for aesthetic reasons. The cost of lumber for these features isn’t insignificant, so don’t forget to include it in your materials list.

You need pressure-treated wood for the stair stringers (usually 2X12s), but not for the treads, railings or trim. Natural redwood or cedar, composites and even vinyl are options. To calculate the cost, determine the total length of each dimension you need in feet (4X4s for posts, 2X4s and 2X6s for railings, 2X2s for balusters and 1X4s or 1X6s for trim), and multiply by the price per-linear-foot. Again, I always add a 10 percent overage for waste.

Hardware and Fasteners

When assembling the foundation, you’ll need post holders, joist hangers and bolts. They aren’t cheap, but you can save money by buying some of them in bulk. One joist hanger, for example, costs around $2, but a lot of 100 costs less than $100. Don’t forget to include these structural necessities in your materials list.

You’ll also need screws for assembling the foundation and attaching the decking boards. If you’re installing composite decking, you may prefer hidden fasteners. Both types of fasteners are cheaper when you buy them in bulk, so it’s usually more economical to buy more than you think you’ll need. It won’t hurt to have a few extra fasteners on hand when you need to make repairs.

Don’t Forget These Incidentals

Besides the cost of lumber and fasteners, you’ll have a number of predictable extra costs. These could include:

• Building permit – A building permit can cost $600 or more, depending on where you live,
• Drainage – If you’re building on a slope, you may have to divert water away from the deck foundation to avoid erosion damage. This may or may not be a significant expense, depending on the amount of work you need to do.
• Ready-mix concrete or pier blocks – Concrete to support the posts rarely adds more than $200 to the total cost of a deck project.
• Finishing materials – Are you planning to stain or seal the deck? Add the finishing materials you need, based on the size of the deck, to your list.