Home Repair and Paint Guide

Choosing the right lumber is a critical part of building a deck. Get this wrong and you’ll be disappointed with the final result. Maintenance is a big consideration, and using the right materials will make this task easier. Sun and moisture are the primary enemies of deck lumber, but mold and mildew also cause problems. Spills (especially grease from foods) will cause stains that are difficult to remove if sealer is not applied. Using inferior screws during construction will give you headaches because they tend to break when heat from the sun causes the lumber to flex.

Some projects require you to calculate the amount of lumber needed in board feet. For instance, a lumber yard might ask for this figure to provide a price quote, although I haven’t personally had to do this. I usually create a materials list and hand it to the person at the projects counter for a quote. If you need to calculate the board foot requirement when ordering materials, see our post at Lumber Size and Calculating Board Feet.

Pressure treated deck lumber should be used for structural parts such as posts, joists, and beams. It’s generally stronger and supports more weight than cedar, redwood, or other woods and will last for many years.

Pressure treated lumber is rated by pounds of preservative retained per cubic foot of wood. Higher numbers mean greater protection against fungi and insect attack. Matching the rating to intended use will give adequate performance while helping to reduce overall costs. Pressure treated lumber is tagged with the treating solution and concentration that was used during manufacturing. The three common ratings are as follows:

• Above ground (.15 or .25). Used for the floor, railing, and other topside trim. 
• Ground-contact (.40). Posts, beams, or other items that will come into contact with the ground, but will never be buried. 
• Below-grade (.60). Posts and support items that are buried below grade. 

When buying cedar or redwood lumber, select boards cut from heartwood. Heartwood contains the natural oils that give these woods their resistance to decay and insect attack. Sapwood (cut from the outer edges of the tree) lacks these oils. If you’re going to use redwood, look for lumber labeled “heartwood common”. This contains more heartwood than “construction common”.

Buy wood that’s dry. Lumber that’s still saturated with the water can be twice as heavy as dry wood. It will also shrink when drying. This can cause your deck to have uneven gaps between the boards, and could also shear the screws or loosen nails that hold mounting brackets to joists. If the wood you purchased is wet, let it dry for a few weeks in an area that’s covered and away from direct sunlight. Drying too quickly can lead to warping.

Tall decks should be built using 6×6 posts instead of 4×4. They will look better and are more solid. Make sure the posts are straight with no twists, bows, or deep cracks. Remember that the posts have to support the weight of the deck plus people and furniture. That’s another reason to go with the 6×6 size if the deck is tall.

There are three main categories of deck boards:

• Natural woods - The rich color of redwood and cedar looks great when the deck is new, but will start to discolor within the first year if a good stain or sealer is not applied. Natural woods are stable and tend to lie flat with minimal cracking. On the downside, they’re soft and more likely to get scratched and gouged when items are dragged across them. If you have dogs that will be using the deck, their claws will leave scratches on the surface.
   
• Treated lumber - This type of deck lumber is strong and will last for many years if maintained on a regular schedule. The boards are harder than redwood or cedar, and more resistant to scratching. The trick to a satisfying installation is to use dry lumber. Pressure treated deck lumber must have a good, high quality water repellent applied once per year to help prevent cracking. This is especially true in areas of low humidity.
   
• Composite deck boards - Composite boards are made from wood and plastic. They are advertised as low maintenance (I have seen ad material at a local lumber yard that claims “maintenance free”) and many are available in a variety of colors. But there have been problems in the past. Not all brands are code compliant in every zoning jurisdiction. There have been recalls due to defective manufacturing processes. And contrary to advertised maintenance requirements, they can become dirty and stained. Composite decking can experience color fading after about 7 years.

Six inch wide boards are ideal for the average deck. Four inch is too narrow and will have a lot of gaps. Don’t use eight inch wide lumber because it has a tendency to crack and cup due to the width. For thickness, go with a 5/4 material that is 1 or 1-1/4 inch thick. Avoid lumber that’s only 3/4 inch thick. It moves too much underfoot and is not as sturdy. If you plan to put furniture on your deck, 3/4 inch thick boards will most likely flex under the weight. I personally like the 1-1/4 inch thick lumber. It can support a lot of weight without excessive flexing and works great on joists mounted at 16 inch centers.

Try to purchase decking that can run the full length of your deck. Full length material eliminates end-to-end gaps that are more absorbent, slower to dry, and susceptible to rotting, swelling and splitting. It’s very difficult to flow sealer or stain between the board ends. Fasteners close to the ends tend to split the lumber.