Expert: Dan Griffin - 1/21/2010

Question
QUESTION: Hello Dan and thanks for your time.

I have a patio about 7'x7' and 2' high. Two continuous sides are against house (balloon frame) and the other two are brick and mortar on a concrete footing. Built about 60 years ago and needs to be replaced. I would like to do myself and plan on learning/planning as much as I can before the spring. Here are some basic ideas I have come across and would appreciate any feedback or best practice suggestions.

if footer is in good condition after demoing old patio, drill holes for rebar on 2 sides and build up patio on these two sides with cinder blocks. fill with mortar. Where blocks come against house...use concrete expansion material?

within the blocks and on other tow sides...anchor 2x6 pressure treated wood to house framing

top of the patio is where i get lost. i was thinking of attaching 2x6 lumber around the inside of the blcoks..if good how do i attach? and then installing joists with either hangars or blocking them all in. on top of that...plywood and cement board...plywood sub floor would be built flush with top of blocks so that cement baord would be flush with top of blocks and extend to edge?  finish sides and top with brick and mortar? good bad?....alternative is to build form within at appropriate level to pour a slab in such a way that the outside edges along blocks are contained by a temporary form.......but whereform is agsint house i cant imagine how i would remove it and if i did would be left with a gap?....

like i said..i am only attempting to learn and plan as of now...any ideas feedback or comments would be great...btw...project is in california...minimal winter rainfall..no freezing temps.

THANKS

ANSWER: Barn, I'm not seeing the problem real well.  Patios are outdoors.  Most patios are slabs on grade, meaning they are concrete about 4" thick that reat directly on the dirt called the subgrade.  It is possible to pour a suspended slab a bit like the upper floors of a multistory building.

It sounds as if your patio is much higher than the grass at the outside edge.  I would still imagine that the slab is sitting directly on dirt/sand/some type of fill.  Now the problem becomes holding the dirt under the slab with some type of wall so the dirt can't wash out.  This is done by building a wall around the outside of stone, masonry, brick, or concrete.  The wall can stay under the slab or rise above the slab to become a wall or seat.  If the dirt will be against the structure of the house, you must make provisions to flash water away from the structure and ideally make provision to drain any water away that does get there.  The dirt needs to be fully compacted or it will settle and allow the slab to break.

If I've not understood your question, please let me know.  If I have and you need any more advice about creating the "walls" ask me back.

---------- FOLLOW-UP ----------

QUESTION: I apologize Dan. I suppose the correct term is 'porch'. It is a raised porch about 2' high to the door of a house that is part of a corner (2 sides against wood framed house, 2 sides are exposed brick and mortar. The inside/underside of porch is not filled as it is raised. A more specific question I suppose is with respect to the top. After re building 2 sides with cinder blocks, I think I could build new top one of two ways

1. build frame within 4 sides (2 anchored to house...2 to cinder blocks) to accept plywood sub-floor flush with inside edge of blocks, Then attach cement board which will fit and lay across top of blocks. Cover with stone and mortar.

2. Build frame on 2 house sides by placing blocks within wall studs where bottom is level with top of cinder blocks. Install joists between house and blocks to accept plywood. This will serve as a form for poured slab that would cap or sit on top of the cinder blocks and will be left in place....and temporary form on outside edge of cinder blocks.  

hope this is more clear...what do you think?

Thanks

Answer
Barn, I'm sorry it's taken so long to get back to you.  I would not put a stone and mortar exterior slab surface on any type of wood structure.  I would still raise the dirt to make the porch a slab on grade with proper pitch to shed water.  If yuou decide to cap the slab with flagstone or similar, you have a great surface with which to work.  The slab should be about 4" thick with 1/4 in 12 slope.  Assume the stone work will be about 3" thick would indicate theat you need a bit more than a 1' of fill to hold the slab.

To clear span 7x7, the concrete would be 5-6" thick and should have the formwork removed.  The stone will be another 3-5".which will not leave enough space under the porch to use for much of anything.