Using Oak Logs for a Wooden Bridge

Thoughts on converting site trees to a foot bridge for a landscaping project. January 17, 2011

I want to build a bridge of about 20 feet. We will be removing some oak trees with a trunk diameter of 12 to 14 inches. I was wondering how these would work for my beams for the bridge. This would be a people bridge to walk across. Would I need to remove the bark? How long would they last? Also would I need to treat them with a water preservative?

Forum Responses
(Sawing and Drying Forum)
From contributor A:
It would need to be white oak, and yes, remove the bark. Also make sure the ends do not sit on the ground. If you can, flatten the top and nail your deck down. Get some 30 lb felt on top of the logs under the decking so the water will shed off or put roof coating on the top third of the log. The logs would do better if they are not sealed with any type of waterproofing, so they can dry out.

If you need a load rating, use the square timber the log would saw out. So a 12" top log would make an 8x8 timber, and that would be your load rating for that log.

From contributor S:
The roof felt is a good idea. You could also get a roll of metal flashing and use that instead of the roof felt. White oak is best and if given the flashing/roof felt treatment, you may get 20 years or more if you can build some concrete or rock abutments to put the ends of the bridge on. The longevity is going to depend on the climate. In warmer areas you'll rot out faster. Red oak will also work, but won't last as long.

For a foot bridge, you can purchase 20' pressure treated 2x10 for about $35. I haven't priced them in a while but that should be close. 3 2x10's should do the job (plus deck boards) and they'd be easier to handle than 20' 14" logs. My two cents: get the bridge done the easy way (with 2x10's) and then go fishing or take the Mrs. out to dinner.

From contributor A:

But just think of the chemicals leaching into that pretty little stream? (I am just messing with you, sorta.)

From contributor M:
If people can walk on it, then build it for the max load expected. 20' long, so room for 15 people x 200 lbs = 3000 lbs load. Add safety margin for deterioration over the years? Build to support 10,000 lbs.

You need to alternate butts and tops so one end is not weaker than the other. Build nice level abutments and lay out your logs on them. Take a chalk line across the ends leveling the larger butts with the smallest top. Then rotate the logs 180 degrees and all you will have to level up for the decking is the odd knot. The logs also need to be tied together one third of the way across, so two tie points. This is usually done to ensure that no one stringer bears the entire load.

From the original questioner:
I live near Houston, TX. Mostly what I have on my property are live oaks. The property is heavily wooded. We are going to dig a pond with an island in the center. I was hoping to use a couple or three of the live oaks to build a bridge. I want to stay away from the treated lumber because of the chemicals. We plan on stocking the lake with catfish and bass. I appreciate all the responses - you've given me a lot of valuable information.