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Question DetailsAsked on 5/17/2011

Please comment on this deck foundation

I recently had a deck installed by a local contractor. The deck is 14 ft wide from the edge of the house and 24 ft long. The contractor used two 4x8 pressure treated (PT) beams to support the joists opposite the ledger board. There is a wide gap between piers (5 ft) and the PT beam is noticeably bending downward under the unsupported stress. This has caused the ends of the PT board to bend upward to the point of detaching from the pier on the right. Tension cracks have also formed in the middle of the PT board. The PT beam on the right is also bending downward in the middle, but not as severely as the one on the left. However, the end has lifted upward just enough for the support to lift upward off of the pier block by about 1/8 inch (I can wiggle it with my hand). That means one of the pier blocks is not even supporting the weight of the deck! It appears to me that the weight of the deck would apply a torque to the pier block when placed this way, which on certain soils or under certain conditions, could cause the block to rotate. Shouldn't the supports be centered on the pier block so that the weight of the deck and the ground resistance force directly offset each other?. The contractor ordered 6 pier blocks from the supplier, which if placed evenly, would have resulted in a pier spacing of 4 ft. The contractor only used 5 pier blocks and now only 4 are actually supporting the weight of the deck.I am not an expert on decks, but I am very concerned about the integrity of this foundation. What if anything should have been done differently? Is this an acceptable design? Wouldn't a single PT beam spanning the length of the deck be more appropriate than two short PT beams? Shouldn't piers be placed evenly along the beam? I've heard that the spacing should be 3.5 ft and definitely not greater than 4 ft. Is this true?I need to know whether this contractor constructed a poor foundation and especially if there are any fatal flaws.

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2 Answers


First off, the PT should not be stained or sealed for at least 6 months, just a rule of thumb,[:$] The piers do not have to be placed evenly long the beam, although they should have some sort of even distribution. (for example I usually put one about a foot in from the side and 5 feet after that and I measure the same distand from the left side as the right side, so the two center piers may be slightly closer to each other. The major problem I can see here, is that a 4"x8" beam is used to support the weight. My guess that the lumber is warping for different reasons, one the weight, secondly, not drying evenly due to stain on the PT, thirdly, it is a single piece of wood, more apt to warping. The spacing is fine, the supports are fine for this deck, more than enough. Considering the foundation of the piers is stable you should have no problem with this design. [:O]What needs to be done: the 4x8 needs to be replaced with 4 2"x12"x10' and one 2"x12"x8' PT beams these are joined together grain facing opposite directions Cut the 8 foot 2x12 in half nail the 4 foot section onto one end of the 2x12 by 10 foot. Easiet way lay two of the 10 footers in line on the ground lay a four foot piece down after them, now you have 24 foot piece not nailed. Now lay a for foot piece on top of the 10 footer, and then the two 10 footers, Every 12 inches put 3 faming nails across the boards, flip them over and nail again every 12 inches from the other side. [:O]Now you have a beam that will not warp. and is plenty strong to support your deck.[<:o)] (in other words you are nailing the boards together, staggered, you essentially have 2 24 foot beams nailed to each other, so you have a 4x12 x 24foot.[:D]

The beam should not be split like that, the suporting beam should be a manufactured, (joined together PT with grain facing opposite directions) and should span the entire length without a break.

Answered 9 years ago by SaintContracting*com


Hi me again, just a quick note, Something looks wrong to the left of the picture. The First floor joist looks like it is sunk into the beam,? It should sit on the beam.

Answered 9 years ago by SaintContracting*com

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