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.
Tony
