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Ask the Builder: A worried dad and his son’s tree house

Joe had noble intentions to build a dream playhouse for his son, but he made lots of critical errors. (Tim Carter)
Joe had noble intentions to build a dream playhouse for his son, but he made lots of critical errors. (Tim Carter)
By Tim Carter Tribune Content Agency

Q. I’ve built a free-standing treehouse for my 9-year-old son, but I’m a novice in carpentry. I recorded a video showing my nearly finished project. I’d like you to watch. I’m wondering if using 3/8-inch galvanized bolts in 4-by-4 posts was sufficient. I had a professional carpenter visit yesterday, and he said he would not worry about it, that the structure should last 20 years. But my son means the world to me, and I just want to be sure that I haven’t created a death trap. Thanks for your help. – Joe H., Travelers Rest, S.C.

A. The instant I read Joe’s overnight email, I watched his video. I’ve got it archived on my askthebuilder.com website. You should watch it, too, because it’s filled with love and concern. But it’s also filled with serious building errors. I don’t want you to make the same mistakes Joe made when you build something for your child or grandchild. Just type “free standing treehouse” into the search engine at my site to get to the video.

Seconds after watching the video, I emailed Joe. It was so important, I would have dialed him up if he had included his phone number. I told Joe to stop working on the treehouse and wait for a recording I would make to help him fix the errors. I also told Joe I would draw up a few simple sketches illustrating what he needed to do. I used to draw these years ago sitting at a dining room or kitchen table for customers as I explained how I was going to build something for them.

Let’s run down the list of errors Joe made. The first one, in my opinion, was a lack of research before he got out his tools. I say this not knowing exactly what Joe did, but if he did do any, he either got bad advice or, if he found great advice, he failed to follow it. The internet is overflowing with conflicting building advice, and if you don’t know what’s right and what’s wrong, it’s easy for you to go down the wrong path.

Allow me to describe what Joe built. For simplicity, he built a 12-by-12-foot deck in the air. The top of the platform is about 6 feet in the air, and the entire structure is supported by eight 4-by-4 posts.

This is where all the problems started. It’s important to realize I see professional carpenters make this first mistake all the time. Joe bolted the outer 2-by-6s to the 4-by-4 posts. He had two 3/8-inch diameter bolts at each post. Can you imagine how much weight is bearing on these bolts? The entire surface area of the top half of the two bolts at each post only adds up to 1.17 inches times the thickness of the 2-by-6. Realize the actual point loading is much more severe because the surface area of bolt contact through the 2-by-6 is a half-circle, not a flat spot like you might find on top of a post or column.

Joe should have used a notched 6-by-6 post so the outer 2-by-6 beam rests on solid wood. The weight would be directly transferred to the buried concrete piers in the ground. I suggested he fix this error by attaching a treated 2-by-4 to each 4-by-4 post. This new piece of lumber would extend from under the 2-by-6 down to the top of the concrete pier in the soil.

As I watched more of the video, I saw Joe had used what appeared to be galvanized drywall screws to attach the joist hangers to the outer beams. Drywall screws are not structural screws. You can buy special structural bolts made to fasten joist hangers to beams.

Later in the video, Joe brought up how when you stand up on the deck it sways back and forth. No doubt it would. With little effort, a group of boys celebrating a victory in a mudball war with friends on the ground could get the entire structure to collapse, as the legs would fold under with little effort.

This defect can be solved with ease by installing full-length diagonal bracing on both sides of two opposing corners of the structure. A treated 2-by-6 could be fastened to the top of each corner 4-by-4 and angle down to the bottom of the center 4-by-4. This is how builders of structural steel buildings prevent racking. They’ll use solid steel or tensioned steel cables for the diagonal brace.

Another serious defect was Joe’s railing at the top of the deck. He took 4-by-4 posts, notched them and just bolted these to the sides of the 2-by-6 outer beam. When you notch a piece of wood, it’s only as strong as the remaining piece of wood. In his case, he no longer had 4-by-4s, but a 2-by-4 railing. This is completely unacceptable. There are special metal connectors that are made that will exceed the minimum code requirements for deck railing safety. I showed these to Joe, and you can see a video about these on the same page where I have Joe’s video.

There are other issues that you should know about. First, the treated lumber you purchase today has a high concentration of copper in it. This copper, when it gets wet, starts to corrode steel. All bolts, nails, screws and metal framing connectors must be coated with the best galvanizing to prevent corrosion.

Realize that treated lumber dries out and shrinks. While building a deck or treehouse, you might feel things are nice and snug, but six months later, there’s play in connections. Shrinkage also can cause cracking at stress points within the structure. Lastly, don’t even think of using lag bolts. These are the worst fastener. You can overtighten them and reduce holding power. Only use through bolts that have washers and nuts.

Subscribe to Tim Carter’s free newsletter and listen to his new podcasts at askthebuilder.com.

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