How NOT to build a curved deck. (revised 4.22.2017)
Curved decks have been a trend of late–however, there is little information about building well on the curve. The advice available at the book store may not get you there!
Sorry, I don’t know who built this death trap, but I did let my builder friends that are guilty of using kerf cuts know about it.
This is a perfect example of how NOT to build a curved deck. I know this is how they do it on TV, however, I visited this one when it was 5 years old, and it wiggled like a cobbled together scaffold. It had nice strong looking support posts and the deck was flimsy. Unfortunately, by the time you fix everything that is wrong with this deck there is nothing to save on the existing structure.
My God, what a beautiful deck it was! Brilliant design, just badly, badly, terribly orchestrated. I love the concept, I love the look, but when you see the mistakes you will shake your head in disbelief.
A Curved Deck with Structural Concerns Everywhere!
Where’s the Meat?
Kerf Cuts, Don’t do it Outdoors!
- No lateral bracing
- The steel stairs project the weight outward causing them to spring.
- The deck has no rim joist to speak of which means the joists all move independently. The joists are held in place by the fasteners in the decking like a pallet.
- Some of the footings moved, which means they were poured on disturbed soil or were too small for the deck.
- There was evidence of water infiltration into the stucco finish of the home as well.
- Patios are a disaster
- No flashing/counterflashing to stucco wall of house
The owner of the home asked me if he could add a 3 season room with pergola on top of the deck. He didn’t like what I had to say and might have been upset when I laughed. I let him know that I wouldn’t erect a big box umbrella on the existing structure and that it should be removed and rebuilt properly. Yeah… I need to work on my “Tact”.
A Kerf Cuts Rant
As you can see, the rim joist has released and failed completely. Even if you clad it in composite decking it will fail in a short time using this method.
Kerf Cuts are often used in construction, however, these should NEVER be exposed to the elements. When they are used in interior work they are usually filled with glue and then bonded to something. Anything you build outdoors needs to have the end grains sealed to prevent rot and should be built twice as strong. Building heavier for the outdoors just makes sense for safety after the structure starts to rot. Code is a MINIMUM!
Here is a note for all you building inspectors out there– “Kerfed Rims do not meet CODE!”. These should be an instant fail!
When nails are driven through this kerfed rim into joists, only about 3/8″ of solid materials is secured–which doesn’t take long to rot. The nails will pull through and if stressed all the fasteners will release.
The load is also supported by a nail that has 1″ of space to flex, which means it will carry even less load. Can you imagine what would happen to this deck during an earthquake?
Who started this Kerf Cuts Outside thing?
Though hard to say for sure, kerf cuts were originally used in pattern making. Form work for making forms for casting steel parts. If it worked well enough for that, maybe for framing it should work?
I did find these details for building curved decks published in a book by Black and Decker, “The Complete Guide to Decks“. The information inside is deeply flawed. The book is so bad that a carpenter likely wasn’t even consulted during production.
I just glossed through it on the shelf, but among the numerous bad details, they instruct you to countersink all lag bolts into beams and framing. That has the effect of making the beams break away under load.
It weakens the connection by half. Your 2×8 becomes a 1×8. Rot sets in quickly to weaken the lag or carriage bolt connection. Please do not build any deck from this book! 400,000 copies have been sold, so if 1 in 4 actually built decks there are at least 100,000 dangerous decks out there if they followed instructions by Black and Decker!
Alternate bad ideas– My Pinto to your Vega!
I jumped online to see what information was available on curved decks. There is actually some guy making rim joists out of exterior plywood–another temporary method.
Exterior plywood / pressure treated plywood is not made to be exposed to the elements. It is made of thin layers or spruce, pine and fir, and a water resistant adhesive. The voids in the adhesive allow moisture to infiltrate. I wouldn’t expect those to last longer than a few years.
Try Cre-zone or Marine Ply. Crezone is fiberglass clad plywood, (you have to seal the edges), but this is the material that highway signs are made of. It can be laminated together in layers to make a good srong and durable curve.
The best material for curved structural parts is marine ply. Exterior suitable wood in many layers using waterproof epoxy. It may outlast the entire deck.
Steel framing may be a better answer, though I haven’t tried it yet– I will let you know how that goes!
This article was originally published in 2011, but the information is relevant so it has been revised and re-edited.
What kinds of disaster decks have you run across? Send me photos or a story by email at email@example.com
By Lawrence Winterburn