# Will 1x4 or 1x6 boards span 3 feet between beams for a walking bridge?

I have a foot bridge I wish to plank with 1x4 or 1x6 material. The bridge is 3' wide. The planks will be simply supported at each end by 4" beams. Will a 1x4 or a 1x6 3' long support a 200 pound load?

• You say "plank" but the plank is generally the top - ie walking - surface of the bridge. I think you mean cross-beam? Commented Jul 2, 2018 at 17:38

The boards will flex something awful, which makes for an unpleasant walking experience. Tying them together with an unsupported stringer in the middle will make things quite a bit less wobbly. If the bridge is short then an X-shaped brace between the beams will give you much better support with the same look.

I personally would opt for the 1x4 if all other things were equal. Three feet is a really wide span for rotten or insect-damaged boards. Falling through a floor all the way to your hip is hilarious for everyone who sees it, but not an experience I'd be keen to repeat. With a 4" board any way you stand you're spanning two of them.

This all seems like “opinion-based” answers.

Okay, I’ll give one more “opinion-based” answer. The op’s question is, “...will 1x4 or 1x6 that is 3’ long support 200 lbs.?”

Structural calculations are based on “ultimate design” or “working stress design”.

When using ultimate design (or failure) we then calculate a “safety factor” into the calculations. Depending on the structural element we’ll use a higher or lower factor, (say:1.5 for common area floors and 2.2 for critical beams.)

Working stress design has a safety factor already calculated into the equations.

Unfortunately, I don’t know of a safety factor for “bridges”. (FIU bridge designers evidently don’t either.)

Hmmm...so, based on experience, I wouldn’t use anything less than a 2x material. If the material is cedar or redwood, which is good for exterior use, (but weaker than other species,) or if it’s pressure treated material (which looses its strength when pumped full of pressure treatment liquid), only 2x material will support 200 lbs. (BTW, the op didn’t say 200 lbs. per square foot...just 200 lbs. for a 1x4 or 1x6. )

Also, all grading rules allow some “non-compliant” material to be included. So, if you choose one of those boards, they will fail.

• I agree with your grading comment I see #2 stamped wood all the time that I would not trust for use in the bridge. In years past the wood had many more "rings" the minimum today is 4 per inch if memory serves. Commented Jul 2, 2018 at 19:11

1x4's will flex more than 1x6 material. As to whether it can support a 200 pound load is up in the air because we have no idea what type of material you are using. Oak boards would hold more than fir boards which in turn will be able to support more than soft pine boards.

Another thing to consider is that often time a bridge structure is built with the support beams closer together than the full width of the bridge tread boards. This has the decided advantage of letting the boards support more because the span between the beams is less. The boards overhang some but can still support the load near the ends. For say a 3 foot wide tread let them overhang the beams by 3.5 inches on each end. If you have 4x4 posts bolted to the side of the beam that extend up for a safety/hand rail just notch the boards around the posts.

• Although I kind of agree I would never use less than a. Clear 2×4 depending on the type of support and conditions I might even want more substantial planking. I have repaired an old steel cable foot bridge long ago we had a tough time because of limited \$ we used rough fir that was soaked in cresode, that lasted many years close to 20 before some planks needed replacing. It was easy to see the boards that had to be replaced because they were treated with copper as cresode was not legal in California any longer. They did use a rolled roofing on the planks and sealed the nails with black Jack. Commented Jul 1, 2018 at 21:56
• Where does one find clear 2x4s? Commented Jul 2, 2018 at 15:09
• They can be had at mills that cut larger trees, my mill cuts small trees so not many boards we cut are clear hull oaks lumber? Spelling?, still cuts large logs and they do have a larger price tag but the tighter grain and clear wood is worth the extra \$ for projects that have flex, we had a cable bridge at our last farm that could support a horse and rider we used it as a schooling device for mountain trail competition this was built from for. Mary's lumber has cedar but it is even more expensive. So it is out there I just happen to know a few since I am close and use their products. Commented Jul 2, 2018 at 19:04

Five-quarter decking is much more appropriate for that span, and even that is still a bit light. Two-by decking is much better.

Regardless, run a floating rail down the center and screw each deck board to it with two screws. This will spread the load to adjacent deck boards and prevent most of your flex.

The bottom line is that pressure-treated one-by boards are probably more expensive than PT 5/4 decking anyway. I would certainly not use untreated one-by boards. They'll be rotting and (more) dangerous within a few years.