I'm planning to put a French door in place of an existing floor to ceiling window in our bedroom. The door will open onto a small 8x10 ft concrete deck on which our 2 person hot tub(350 gal) will sit. The 10 ft dimension will run parallel to the house. The deck needs to be about 6ft above grade in order to be at the door level. Our home is a ranch and built on a sloping grade, which slopes ( 1/2 ft in 10 ft) along the long axis of the home. I would like some advice as to how to approach getting a design for this project. The hot tub, including equipment,water, people and tub itself, weighs 4200 lb ( I'll use 6000 lb for the design) plus whatever steel and concrete is required for the deck. The home is brick veneer and I can remove some of the brick to get to the top of the block foundation to set a steel lentil on the block. I would like the deck to be supported by two front corner columns and attachment to the lentil which can lie fully on top of the block foundation. Can you provide some guidance on steel required, spacing, concrete type, and thickness, reinforcing steel, expected deflection etc. I am an electrical engineer and pretty handy in building things - I will consult with PE or structural engineer for further details. I'm looking for some guidance on what to expect. I would like to do a stamped finish. I've tried the wood deck thing and am tired of all the upkeep. Thanks for any help and guidance. Sorry for the long post.

 

I am having little trouble visualizing where the foundation is in relationship to the slab.

Are you thinking of setting a steel lintel directly on top of the foundation and then pouring a slab on top of the lintel? If you are building directly on top of the foundation, it would be better to use masonry and a concrete block bond beam.

Your engineer will be giving you your steel requirements and concrete information.

Make sure you flash the siding, etc at the top of the slab.

You should be able to use any type of concrete you wish for the top of the slab.

You can have the slab designed to be supported a lintel between the columns or you can have a completely flat slab with no lintel. The latter will be a cleaner appearance, use a little more concrete, no lintel and the labor may be less because the finishing area is still the same.

Since you are only spanning 8' with concrete, the deflection will be very low. After you chose which way you are going, it can be calculated, but either way it will be a small number.

Dick

 

I am curious as to why you want this to be open underneath? Since the top of it is only 6' above grade, the space under it isn't very useful, considering the trouble and expense you will go through to form up and build this raised slab and columns. You could lay more block for walls around the slab, fill it up with gravel, and pour your slab on top - less forming, less engineering, and every inch of the slab completely supported.

I understand not wanting the maintenance associated with a wood deck, but perhaps switching decking materials would solve that problem. I don't know where you live, but the winters here in the midwest require that we re-seal stamped concrete patios/driveways/walks pretty much every year, to keep them from flaking apart during freeze/thaw cycles. That might be more of a problem since your slab will be elevated, and like a highway overpass, freeze more readily.

You will also want (and codes probably require) a railing around this raised platform.

 

Joe -

I am assuming you may have windows either below or on the lower level, so you want an open area and more light. To maximize the open appearance you could coat the under side of the slab with something like Thoroseal (white) to it give a finished look. - Sure beats the look of the bottom of a wood deck.

If you are in cold climate, I would recommend air entrained concrete for durability.

Dick

 

Thankyou all for the responses. I will try to clear up the confusions. The slab needs to be elevated on columns so I can still have access to the crawl space entrance which is below where the slab needs to go. Also my home is small and I don't think that building block support walls and filling it in would "look" right, I think it would look to "chunky". I guess that what I'm looking for is to basically build a steel I beam grid upon which to support the slab. By removing some brick veneer I can easily get access to the top of the existing block foudation and place an I-beam there where I can now attach my other I-beams ( similiar to joists) - almost like forming a regular flooring system with rim boards and joists. I live in central Tennessee. The winters are mild - summer humidity is high and mildew forms on almost everything quickly, I think it would be easier to clean concrete. I'm just tired of the excessive amount of work in upkeeping a wood deck. My aggregate driveway has held up very well with a sealing every couple of years. I've tried all the mildewcides and specialty coatings for wood and really haven't been satisfied with any. Perhaps I am over thinking this project!!. I absolutely plan on railing - I was thinking of a cast aluminium (maybe powder coated) simulation of wrought iron. I am looking for any advice or possibly contractors who can do this in my area. Dick - I really liked what you had to say about No lintel design - I agree it would look a lot cleaner - I'm just not sure what is involved in the design of a slab like that - that can support the hot tub weight - I don't have a "feel" for what is involved. I always wondered how concrete floors were designed in high rise buildings. Before I go talk to a structural eng/ architect I'd like to be informed to some degree about what is involved.

 

If you do not have some experience with concrete this is not a DIY project.

Forming the job is just like building an ugly, strong removable deck built out of reusable or rented materials.

Pouring the concrete will take some experience, some help and equipment rental unless the R/M supplier has a pump or conveyor.

Finishing the concrete is just like finishing a patio or driveway. It is possible to have the same exposed aggregate finish you referred to. I have it on my patio, but I don't know if I would use it around a tub, but it does look great.

Eliminating or minimizing the lintels or beams can be done because it is not necessary to support one structural material with another structural material.

Have a local structural engineer determine the feasibility of your project based on an on-site visit. He may turn you off or he may find it very doable. Concrete design is not a "cookie cutter" design system like wood where you use tables, connectors and standard details. Concrete has strengths in all directions and does not just work in one direction unless it is acting like a diaphragm. I have done both and recognize there is a difference.

It will cost more than a wood deck, last much longer and it definitely will feel different.

Dick

 

Joe, If you haven't already finished your project, check into the product Epicore. Your engineer will be familiar. I just added a 26' x 30' elevated slab and used it with a structural steel frame. If you need access to the crawl space use steel studs and enclose the space for extra storage and access as well. David