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Community and Q&A

Deck frame: cedar vs. steel

Charlie Sullivan | Posted in Green Products and Materials on

I am interested in opinions on whether northern white cedar is a good material to use for a deck frame. I’ve heard opinions that it actually lasts longer than pressure treated pine, but also doubts about using it structurally. I would protect the tops of the joists with some kind of flashing, as that seems to be where wood decks rot, and upsize the joists as called for in structural tables for the lower strength.

The case in point: we are planning a simple 12×12 deck and had been planning on avoiding PT by using a steel frame from Trex and cedar decking. The idea was that the decking might decay before the occupants of the house do, but that frame would last longer, and it would be reasonably easy to replace the decking without replacing the frame. But, partly motivated by the slow process of getting the steel frame parts properly specified (working with a distributor who apparently doesn’t deal with them much), I am wondering whether I should consider doing the frame in cedar too. We can buy cedar from a local mill in Vermont for very reasonable prices, so it would certainly be cheaper than the steel frame.

It probably comes down to the steel frame being more expensive and likely to last longer, and we’ll simply have to weigh durability vs. cost, but if someone has a strong opinion or relevant experience they are willing to share I’d appreciate it.

Fortunately, we don’t need to tie the deck to the house at all–we have four piers to put it on and plan to have a ~1 inch gap between it and the house, so we can avoid the challenges of thermal bridging and water intrusion associated with a ledger.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    This article is a little old but may be helpful: http://www.nadra.org/industry_news/dont_build_decks_that_rot.pdf.

    For the metal frame, are you looking a red iron or something else. Metal is usually very expensive compared to wood.

  2. Charlie Sullivan | | #2

    Thanks Steve. That describes similar things to what I was thinking, but has some helpful specifics.

    For metal we are looking at the Trex Elevations system, which is galvanized steel with a baked-on primer + paint. It is expensive compared to wood. They try to sell it based on it being a pre-engineered system that is easy to assemble and saves time in the field because you are not trying to deal with warped lumber, etc., but I'm a little skeptical of that argument. Their other argument is longevity, which seems a little more credible, for example with a 25 year warranty.

  3. Steve Knapp CZ 3A Georgia | | #3

    Thanks for clarifying, Charlie. It looks like you have to be real careful about painting any scratched or cut surfaces. Could you assembly an Elevations frame so it is self-supporting? Attaching a deck to a house seems to create a lot of potential issues.

    BTW. Here is an article that compares LGS to Elevations and wood. http://www.deckmagazine.com/framing/framing-decks-with-steel.aspx

  4. Malcolm Taylor | | #4

    Charlie,
    If I was designing a deck with longevity in mind I would:
    - Use ground contact PT dimensional lumber for the beams and joists.
    - Run a bead of Sicoflex over the top joints between built-up beams.
    - Run 2"x4" purlins over the joists, separate the purlins from the joists with pieces of I&W, and attach the decking to them.
    - Consider using a synthetic decking or patio stones as a finished surface.

  5. Malcolm Taylor | | #5

    Steve,
    I'm not as familiar with your codes as ours, but I think the IRC, while allowing "freestanding" decks to be supported independently from the structure of the house, still mandates that they be attached for lateral support.
    Flashing and securing ledgers aren't a problem if done correctly. Invariably the failures have turned out to be the result of obvious errors and poor detailing.

  6. Charlie Sullivan | | #6

    Malcolm, interesting idea about using purlins. Sort of like having a rainscreen gap between the decking and the joists? I am not sure how much resilience it would gain--do you think it would really last longer before needing any attention, or is the idea more that the purlins would be easier and cheaper to replace than the joists would be?

    I'm trying to avoid PT on general principals of avoiding toxic chemicals, even though I know the modern stuff is not as bad as the traditional arsenic brew. It's not only avoiding whatever harm might result, but also avoiding the need to attempt to contain sawdust and make sure no scraps end up mixed in with mulch and kindling. I'd rather have a cedar frame that lasts 20 years and makes nice compost at the end of that than a PT frame that lasts 30 years and has to go in a landfill at that point. But I don't whether those numbers of 20 and 30, 30 and 20, or 5 and 50.

    We haven't succeeded in warming to the idea of synthetic decking, but we might want to consider patio stones.

    Thanks for bringing some additional ideas in.

  7. Malcolm Taylor | | #7

    Charlie,
    Fair enough. Cedar doesn't last long enough here in the PNW to make it worth using for other people's decks, but like you I'd be tempted to use it over PT for my own.

    My experience has been that deck rot starts either where boards are laminated, as they are in built-up beams, or through the connections between the decking and the joists. Purlins reduce the number of connections, and as you say can be seen as sacrificial.

    To me wood decks just have too many negatives.. Without really regular maintenance they go downhill and look like hell pretty quickly. They have a short life and are expensive. I don't like synthetic decking either, but I weight that against losing one precious summer weekend to re-finishing a wood one.

    I do a lot of work for a resort, where high occupancy rates make summer work very difficult. I have been renovating the hot tub decks by replacing the PT surface with patio stones. Most haven't needed any additional structure, but that can be a concern. However, when you are done, you have an almost maintenance free, long-lasting structure. There are a few proprietary systems that do a similar thing which might be worth considering:
    http://elevatedecksystems.com/silcasystem.html

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Charlie,
    I think you can expect to get 20 years of use from a deck framed with northern white cedar. Strips of peel-and-stick membrane along the tops of the joists are a good idea.

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