Hey all,
I’m about to buy a new house and the home, a ranch with a walkout basement, has a deck on the back of it. The home inspector pointed out some concerns with the deck. He felt it was “fine” but we “shouldn’t have a block party on it”. It’s a fairly simple deck as far as they go. Rectangular in nature (17′ x 12′, long side runs with the house), with 2×10 joists. The deck rests on a beam (of sorts) which consists of a 2 2×10’s, each is nailed to the side of the 4×4 posts. There are 4 total posts, and the span between the posts varies from 5’6″ to 5’9″.
Upon visual inspection his primary concern was with the beam. The inspector’s “test” was to stand on the deck and “wobble it”, and it wobbled like a surfboarder in Malibu… It was more than a little disconcerting . We went and then completed the inspection. Short of needing a new water heater, this is the only major issue found with the house.
Later that evening I discovered the “Prescriptive Residential Wood Deck Construction Guide”, and wow was that helpful! I realize that the version I’m looking at (09) is newer than when this deck was built (Home built in 01, I can only assume the deck build date was around then), but it certainly seems like this deck is a bit underbuilt. (No lag screws on the ledger either. Also no lateral load devices).
I also had a deck building firm come out and look at it today. They felt the beam could be repaired by adding a proper beam on 3 new 6×6 posts (properly notched), where each part of the beam (2-2×10’s nailed together) would span 8 feet, and the ends would have 2′ of cantilever to accomodate the current 17′ span. They also could fix the ledger attachment and the railings. The estimator was very honest with me and said it was really “all labor”, to the tune of $3750.
SO, now on to my question – the deck construction guide advises (in table C4), for an 8′ beam and 12′ joists (this deck) a 19″ round footing diameter. This deck is in Michigan, with a 42″ frost line. Is the guide saying that I should really have a 19″ sonotube to the depth of 42″ (And an additional cone-shaped footer, ideally). The way I read the guide, that seems like what they recommend, but I wanted to make sure I was reading the guide properly. It feels like it might be too much of a concrete column (especially when you see pictures online of posts being on 4″ or 6″ diameter columns..). I would greatly appreciate all your expertise and “gut feel” on this. Thanks!
Replies
The 19" diameter is the size
The 19" diameter is the size of the footing, not the column. There are forms for columns that have a belled out area at the base to accomplish this. Otherwise a pad could be poured, with vertical rebar embedded in it to extend up into a smaller diameter sonotube, which would be filled with concrete after the pad sets up. The column has to be strong enough to support the load, and should have a minimum specified diameter. I'm guessing 8" is large enough, but I could very well be wrong.
Aha! Thanks very much.