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Unread 06-26-2009, 03:21 PM   #1
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Stone on deck -- structural issue

Hello. I was wondering if you fine folks could give me your thoughts on a structural issue. When one goes out the front door of our house, one steps down and walks out onto a deck of about 133 square feet that's the roof of a storage room below. Steps off the side of the deck lead down to street level. The deck was originally paved with brick, with two radial or concentric brick steps leading up from the deck to the front door landing. We're in the process of having the brick replaced with natural stone.

We were told the weight of the new stone and mortar wouldn't be more than the old brick and mortar, and maybe even less. The mason has been laying the stone over a thick mortar joint of about two inches, though, and I'm concerned about the weight. The contractor told us that based on dump receipts, the old brick and mortar amounted to about 2.5 tons. Based on the number of bags of mortar and amount of stone going into the current installation, and based on extrapolations from the square footage and the thickness of the new mortar layer, I believe the weight of the new stone and mortar will be significantly higher, maybe on the order of 3.5 or even 4 tons.

My questions are (1) can the deck structure support such a load and (2) what is an approximate maximum safe load for the structure, in pounds per square foot? I understand that in these situations, there's a "dead" load and a "live" load. I assume the stone and mortar are considered part of the dead load. Of course, answering my questions requires knowing the structure, so here it is.

For the sake of discussion, assume the back of the deck, i.e., the part next to the house, is facing directly south. The deck, i.e., the ceiling of the storage room below, is supported by 2 x 9.5 joists, 16 inches on-center, running east to west. (When I give measurements for structural members, by the way, I'm giving actual figures. The joists are indeed a full 2 inches.) The joists are supported at their ends by the framing of the east and west walls of the storage room. In the middle of their span, the joists are supported by a 6 x 7.5 beam, running north to south. The distance from the west wall to the face of the beam is 10 feet 3 inches. From the east wall to the face of the beam it's 10 feet. The beam in turn is supported by a 6 x 6 post at the south end and a 6 x 6 post placed 74 inches north of the south post (center to center). From the north post, the beam continues for about 33 more inches, with its north end terminating in, and being supported by, the framing of the north wall of the storage room.

(If you try to calculate the area of the deck using joist and beam measurements, you'll come out with around 178 square feet, not 133. That's because the northwest and northeast corners of the deck/storage room are "beveled" off, i.e., the east and west walls join the north wall not at 90-degree angles, but through diagonal sections that cut the corners, as it were. I didn't think the measurements of these diagonal sections were necessary for the pounds-per-square-foot calculations, given the joist and span information I've listed above, but if people need to know those measurements, I'd be happy to take them.)

If you're still awake at this point, thanks in advance for your opinions. If necessary, I'll have a structural engineer come and take a look, but I thought I'd first see what the community here had to say. After all, the people here know everything about everything.

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Unread 06-26-2009, 05:17 PM   #2
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Hi Tony,

I will alert our chief engineer.
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Unread 06-29-2009, 04:16 AM   #3
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There are a lot of variables that your structural engineer will have to consider. Were the old bricks full-size solid bricks, brick pavers, or face bricks used as pavers? Is your mason using 2" of mortar to build up the stone to the same level? These questions are in regards to the total load. Also, what species and grade are the joists and support beams? If they were Douglas Fir, and your stone was a nominal thickness (1" or so), then they would be adequate for the new stone. The 74" long support beam is a little undersized, though.

What kind of subfloor do you have?
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Unread 06-29-2009, 12:31 PM   #4
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Hello, Chief Engineer Bob. Thank you very much for your reply. Here are my answers.

The old bricks were full-size solid bricks. The mason is indeed using about 2 inches of mortar, over lath, to build up to the same level of the old bricks. By the way, does mortar (in this case, Quikrete Mason Mix) weigh less than brick? When I pick up a piece of mortar in one hand and about the same volume of brick in the other, the mortar seems lighter.

I think the joists and beam may be Douglas fir. The house is in San Francisco and was built in 1927, and I have a vague impression that Douglas fir was commonly used in construction around here back then. I don't know the grade, but to my layman's eye, the wood seems quite sound and solid, with no splits, loose knots, etc. As for the stone, it averages around 3/4 inch, ranging from around 1/2 inch to a full 1 inch, with a few pieces here and there exceeding an inch. For grout, the mason is using mortar thinned out a bit with water.

The subfloor seems to be tongue-and-groove planking of some sort. I'm ready for a barrage of warnings that it's inadequate for between-joist deflection, and I confess I should have thought of the issue before.

Given what information I have, is there a way to estimate a safe upper load limit for this structure? I'd just like some peace of mind that the deck/storage area isn't in any danger of collapsing from normal use. Also, something else occurred to me: When it rains, isn't the load going to ramp up from the water soaking into the mortar? The water has a way out over the underlying hot mopping (three layers of paper with tar over each layer) and down the paper of the supporting stucco'd walls (with weep screeds at the bottom), but while it's raining -- especially when it rains continuously for several days -- isn't there going to be significantly more weight on the structure?

I keep kicking myself for not having thought about these things before. Here was my entire thought process at the time: Take the old brick and mortar off; put on stone and mortar that should weigh about the same -- what could go wrong?

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