# Boardwalks / Ladder Bridges



## dburatti (Feb 14, 2004)

I created some drawings to show the basic construction of boardwalks / ladder bridges using lumber to pass on to my assistants and posted them here *for reference only.* These drawings are not engineered and are only meant to give a visual description of how *I* build these technical trail features. This is not an exhaustive and complete description.

You take full responsibility for any feature you build and/or ride; I assume no responsibility for your actions. This is only one way to do this; there are many ways to construct these features. I am not addressing those here. Furthermore, I am speaking as myself and not representing any person or business entity.

Ground level boardwalk/ladder bridge








Boardwalk/ladder bridge using posts








Rising/descending boardwalk/ladder bridge








One method of joining two sections of boardwalk/ladder bridges








Another method of joining two sections of boardwalk/ladder bridges








Posts: 4" X 4" minimum
Sills: 2" X 6" minimum
Stringers: 2" X 6" minimum
Cross braces: 2" X 4" minimum

*Types of Wood*
For framing I use treated pine with a retention level of .35, which is suitable for ground contact. I do not recommend ever using pallet lumber to build a feature. Use only hard woods and treated lumber.

For decking I generally use rough cut red cedar b/c of its feel, look, and rot resistance. Treated lumber will outlast red cedar but not provide the same traction. Use whatever decking you desire.

*Stringer Info*
I usually install stringers of no more than five feet in length, i.e. my posts are about five feet apart from one end of a stringer to the next. Longer lengths will allow bouncing and flexing of the stringers and decking.

*Post Info*
The spacing of the posts across the tread and thus the width of the sills depends on the desired deck width.

The depth of the posts depends on the height of the posts. I dig post holes a minimum of 18" for structures 12" to 18" off the ground and deeper for taller structure. Personally, i do not use a quick setting cement b/c I am a full time trail builder, inspect the features regularly, and am able to address issues promptly.

*Decking Info*
Decking is spaced no less than 1.5" apart. If I know a trail dog is accompanying a rider over a boardwalk, I'll tighten the spacing.

If you are fanning the boardwalk, i.e. incorporating a turn, ensure that the decking at the outside of the turn is spaced no more than 1.5", which means the deck spacing on the inside of the turn will be tighter.

In regard to decking overlapping the stringers, I determine the overlap based on the width of the boardwalk. For example, a boardwalk that is two feet wide will only overlap about three or four inches, while a boardwalk that is six feet wide will overlap up to eight inches. The more overlap you have, the greater the chance of your boards pulling up on one side, which is also known as "piano keying."

*Fasteners*
For fastening the lumber together, I use 3" galvanized Paslode-driven ring shank nails. In my environment in southern WI, they work well for at least five years, which is about the amount of time a TTF stays up before I rework or replace it. I have hand driven spiral nails before; they work but can back out more more easily and need more frequent inspection. Galvanized or stainless steel ($$$!) screws also work well.

*Miscellany*
For deck widths more than five feet, I use three posts and three stringers spaced evenly apart.

I inspect my structures frequently, as I am around them full time during riding season.

As I stated, this is how I do it and one of a few ways boardwalks/ladder bridges can be built.

Please post up if you can contribute to the thread in a positive manner.

D


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## TFitz (Jun 21, 2008)

In the Puget Sound area, in both wet and dry soils, we've found that concrete pier blocks (esp. the ones with brackets that hold a 4X4) work well for low boardwalks. In wet areas you may have to dig out muck and make a bed of crushed rock to support the pier. 

If we double up on the stringers (i.e. make them 4X6 or 4X8) we can go up to 12 feet o/c between pier blocks, with at least 2X6 or 2X8 blocking at each end or one in the middle.

Before we put the decking on we usually roll out a strip of tar paper on top of the stringers.


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## leeboh (Aug 5, 2011)

Why space the decking that far apart? 1/4 to 1/2 in" should be good enough for dirt and water passage. 1.5 inches seems to invite problems for dogs.


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## TFitz (Jun 21, 2008)

Speaking only for myself, I used to use 1.5 inches out of lazyness: it was so quick to just put a tread board temporarily on edge for even spacing. In our local park with lots of walkers, etc, I soon heard about it. Now, I'd say take the time to make a couple jigs out of 1/2" plywood. A ratty old piece of OSB nailed to a short piece of 2X4 is fast and easy. With two of these, it's easy to get consistent spacing for tread boards, even if they're 5 feet long.


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## Trail Ninja (Sep 25, 2008)

I agree that 1.5" is a little wide for spacing on a MUT but 1/2" would be way too narrow here. It would plug up with debris in no time. It's definitely a location thing. I usually use a spacer jig like TFitz.


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## dburatti (Feb 14, 2004)

Wow. You guys sure zoned in on the 1.5" If you look closely, it says NO LESS THAN 1.5". I actually use an old post hole digger handle that comes close to 1". Sure, you can go as little as 1/4", but it's not necessary. Find a balance that takes into account the users, the need to keep litter off the boardwalk, and how much lumber you want to use for boardwalk.

D


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## slowridemo (Jan 17, 2006)

dburatti said:


> Wow. You guys sure zoned in on the 1.5" If you look closely, it says NO LESS THAN 1.5". I actually use an old post hole digger handle that comes close to 1". Sure, you can go as little as 1/4", but it's not necessary. Find a balance that takes into account the users, the need to keep litter off the boardwalk, and how much lumber you want to use for boardwalk.
> 
> D


Right idea just wrong phrasing. "No less than 1.5" indicates the narrowest gap you recommend is 1.5" and wider gaps would be acceptable. No more than 1.5" indicates the maximum gap is 1.5"


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## dburatti (Feb 14, 2004)

slowridemo said:


> Right idea just wrong phrasing. "No less than 1.5" indicates the narrowest gap you recommend is 1.5" and wider gaps would be acceptable. No more than 1.5" indicates the maximum gap is 1.5"


Damn. Thanks for pointing that out. The drawing show less than or equal to 1.5"; I misspoke in my next post.

Sooo, what I meant to say was 1.5" or narrower between boards. 

D


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## Trail Ninja (Sep 25, 2008)

We picked that because we couldn't find anything else to pick apart D.


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## Loren_ (Dec 3, 2006)

Nice. Only real comment is that I'm not fond of using sills that rely on the shear strength of mechanical fasteners like this when they are bolted or nailed to the sides of the posts. By placing sills on top of the posts, you can have wood decking bearing on wood beams bearing on wood sills bearing on posts and none of the fasteners are structural. Like this:









It's just as fast and easy to build, and I think makes for a more robust worry free bridge.


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## thickfog (Oct 29, 2010)

Hmm. I thought that was new Loren. I only rode there once early June and did not remember it, but it surprised me last weekend when I rode there again. Nice work.


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## Loren_ (Dec 3, 2006)

Thanks! That bridge has been there for a year now, but it's so easy to zone out on a ride...


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## dburatti (Feb 14, 2004)

Loren_ said:


> Nice. Only real comment is that I'm not fond of using sills that rely on the shear strength of mechanical fasteners like this when they are bolted or nailed to the sides of the posts. By placing sills on top of the posts, you can have wood decking bearing on wood beams bearing on wood sills bearing on posts and none of the fasteners are structural. It's just as fast and easy to build, and I think makes for a more robust worry free bridge.


Thanks for contributing. I have not built a bridge in that fashion before. It's definitely a viable option.

By design, we're not having to build anything that needs to take such a load that the sills have to sit on the posts. I can see having to do that if one's structures have to support ATVs and horses, though.

Thanks again.

D


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## dburatti (Feb 14, 2004)

Loren_ said:


> View attachment 840809


I forgot to ask a question: How do you fasten the sills to the posts?

D


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## Loren_ (Dec 3, 2006)

We drill and use a lag screw as a pin to keep the sills from moving off the posts. I did a quick notch with a chainsaw, but don't think that's necessary really.


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## dburatti (Feb 14, 2004)

Loren_ said:


> We drill and use a lag screw as a pin to keep the sills from moving off the posts. I did a quick notch with a chainsaw, but don't think that's necessary really.
> 
> View attachment 840903


That's what I thought but coudn't tell by the pictures. Thanks!

D


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## leeboh (Aug 5, 2011)

Trail Ninja said:


> I agree that 1.5" is a little wide for spacing on a MUT but 1/2" would be way too narrow here. It would plug up with debris in no time. It's definitely a location thing. I usually use a spacer jig like TFitz.


 I'm in the New England area so we have small rocks, big rocks and boulders with a little dirt in between. And lots of downed trees too. My left index finger is a custom built in spacer. I could send you the app


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## Trail Ninja (Sep 25, 2008)

leeboh said:


> I'm in the New England area so we have small rocks, big rocks and boulders with a little dirt in between. And lots of downed trees too. My left index finger is a custom built in spacer. I could send you the app


OK but how are you going to brake?

But seriously, most of my boards are split cedar. A finger (actually two would be needed here) wouldn't work. I've done it, but a board to slip in the space gives you a nice average when your decking is uneven. I also have no depth perception so eyeballing doesn't work well for me.


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## TFitz (Jun 21, 2008)

Got a structural question: how wide can a boardwalk/bridge tread be and still be supported by 2 4X8 stringers for a span of 11 or 12 feet? Looking at a situation where the land mgr wants a 5-ft wide tread, would like to keep the underlying structure as simple as possible. They will supply cedar decking 3" thick. I looked at span tables etc in various sources and found nothing en pointe. Thanks!


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## mtbikerTi (Jan 15, 2004)

IMBA used to have a pdf on their website that included some good drawings and info about bridges. It appears they no longer have it listed, but I found it on another site: http://www.midsouthtrails.com/docs/trailbridgedetails.pdf

According to that if you have 4 2x8 stringers (the closest design to what you're asking about TFitz) the maximum span for a 5' wide bridge is 9' when using southern pine.


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## Loren_ (Dec 3, 2006)

So much depends on the design load. With a 5' wide bridge, people might be tempted to drive ATVs and Gators over it, which would require a much more robust structure.


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## TFitz (Jun 21, 2008)

mtbikerTi, that is a fantastic resource; bookmarked. Thanks! Loren, we will be accommodating only walkers and bikers; no motor vehicles, no horses. And thanks for posting those photos.


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## Trail Ninja (Sep 25, 2008)

TFitz said:


> Got a structural question: how wide can a boardwalk/bridge tread be and still be supported by 2 4X8 stringers for a span of 11 or 12 feet? Looking at a situation where the land mgr wants a 5-ft wide tread, would like to keep the underlying structure as simple as possible. They will supply cedar decking 3" thick. I looked at span tables etc in various sources and found nothing en pointe. Thanks!


I wasn't able to find any specs for 3" thick decking but if you had a 10" overhang and 4X8 stringers, your gap is only 36" with 5' deck boards. A little wide for 2X decking but I wouldn't think it too wide for 3X. Especially if you only have a maximum load of maybe 300lbs. I'd build that with confidence.


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