06/08/2026
You can’t judge a deck the way you do interior trim.
When I say material “moves” it can be kinda hard to explain. Deck boards are an extreme example though.
This is a normal composite deck. We installed these two boards yesterday (Thurs) around 1:30, it was almost 90 degrees, full sun, and humid. The splice was closed perfectly, we screwed both boards down heavily.
We didn’t touch these boards again. It was down in the low 50s overnight, and still cool the next morning. At 9am there was about a 1/4” gap, and around 2pm that afternoon the sun was at full strength and the splice was perfectly closed again.
Wood swells and contracts like this over longer periods of time, think entire seasons (your doors might stick in the summer and have no friction at all in the winter.) This is entirely due to moisture.
This is a composite deck. It’s significantly more expensive, but it will last longer, and the boards will never bow, cup, twist, splinter, or rot like wood. Composite swells and contracts due to temperature, entirely independent of moisture.
The material will move. Unless it’s getting installed in 85 degree temps, the material should always be installed with a gap. If you look at how much this board moved from 9-2, imagine if it was installed tight at 9am. The boards would push over top of each other, or buckle, damaging the surface.
Cellular PVC Decking is the only option that won’t move, and because of this, is significantly more expensive than the already expensive composite. If you want truly seamless or intricate deck details, PVC is worth the money.
With that being said, unless you use PVC, a good deck builder’s product won’t have tight miters at 9am, it’s not an apples to apples comparison to the rest of the house. You can judge their work by how clean and consistent those gaps are. These dark shadow lines should look like a thought out detail, adding to the finished product instead of taking away from it.
Side note, the boards visible below the splice are obviously not finished, and have yet to be cut and trimmed.
