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Pour concrete porch against house?

karlkwa | Posted in Construction Techniques on November 20, 2002 06:21am

We are adding a small concrete porch/landing for our kitchen entrance. We prefer masonry to wood for durability–the landing will be exposed to weather, though covered by a wide eave. The entrance has outswing french doors, so the surface of the landing needs to be at the bottom of the door sill (code). The landing is to be finished with stone mortared on the concrete; this puts the concrete surface above the top of the foundation stemwall, probably 5″ up the sheathing. Contractors have advised me to waterproof the foundation and sheathing with bituthene membrane or even liquid-applied membrane, perhaps cover this with a metal flashing, then just pour the concrete against the flashing (no spacer or provision for drainage)–the belief is not much water will get to this joint if the landing is properly sloped away from the house. The stone top would also be pushed right up to the wall. Access to the sheathing is no problem right now as the siding is removed. The concrete at the edges of the slab needs to be deep enough to provide proper footing, and it is planned that the center of the slab be poured directly on fill (compacted dirt and/or rubble) placed on grade. My questions:

–is it reasonable (though obviously not ideal) to pour against the house, above the foundation, like this? Concerns are, of course, rot and termites/ants/insects. If so, what’s the best waterproofing/flashing method? Do I need some sort of drainage provision in the joint? If so, what material? Any better ideas?

–the landing/porch is small, approximately 4′ x 6′, with house on two sides and one step to grade on the other two sides. How deep should my thickened edge for the slab go to provide proper stability for the slab? Does it need to go as deep as my house footings? We live in the Seattle/Puget Sound area so it’s wet but not generally very cold in winter. Does the edge just need to go deep enough to rest on well-compacted subsoil? OK for the middle of the slab to be directly on rubble fill?

Thanks in advance,

Karl

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Replies

  1. Jamie_Buxton | Nov 20, 2002 11:15pm | #1

    With construction like you propose, in California you'd likely get termites.  The stoop will eventually separate from the foundation, and provide a sheltered path for termites to get from dirt to wood.

  2. karlkwa | Nov 20, 2002 11:53pm | #2

    Thanks for your response, Jamie. I'm sure you're right. Maybe my original post is so stupid/horrendous/wrong that nobody wants to touch it. It doesn't seem like the right solution to me, either--it's just what I've been advised by apparently excellent builders.

    So, please, I want to know what all of you would do in this situation. Is concrete/masonry possible here? Do I have to suspend it in the air, cantilevered to the house with some space? Is a wood deck and a conventional porch ledger the only solution? What is the real world solution you would use when faced with this problem? I am interested in all opinions and solutions.

    Karl

  3. FrankB89 | Nov 21, 2002 04:00am | #3

    I live on the South Oregon coast, a climate similar to yours, and what we usually do in your situation is replace the rim joist along and slightly beyond the slab contact area with .40 pressure treated wood, flash it somewhat as you've described, making sure the flashing runs up behind the siding 4 to 6 inches or so (except at the doorway, of course) and down past the top of the stemwall.  Cut your siding back so that it stands proud of the slab an inch and a half or two.

    As far as the thickened edge of the slab and what it's sitting on and all that:  It's always best to pour over a well-compacted base of crushed rock or sand and that base should be pretty smooth in that you want your concrete to be at a fairly consistent thickeness in the field of the slab (usually 4" for what you're doing) with the edges thickened to make a perimeter footing probably of 8'' or so all the way around including along the house.

    Since you're finishing the top with a stone surface, if I was doing the work, I would probably drill in some exposed  anchors near the top of the stemwall that the rear of the poured slab footing can grab on to so that if any settling does occur, the slab will remain fixed to the house. 

     

  4. broadmeadow | Nov 24, 2002 07:16pm | #4

    Funny you should ask. I am in the process of completing a terrazza on my house and will also be placing masonry against the rim joist. As with your project, this location will be exposed to those driving rain and wind systems so prevalent in the Pacific Northwest (I live just north of you in Canada).

    Leaving an air gap with some kind of ventilation is the most obvious answer but will leave room for insect habitation. Most of the wood-boring insects you will be concerned with (carpenter ants, termites?) require a certain level of moisture in the wood in order to grow a fungus which they use symbiotically to break down those hard to crack cellulose molecules - keeping it dry is imperative.

    My main concern was not necessarily with moisture from precipitation but actually with warm moist air from my crawlspace moving through the rim joist area and condensating on the adjacent cold masonry and creating ideal conditions for wood munchers and rot fungi. I have not yet finalized my design but my intent is to isolate the rim joist thermally with extruded polystyrene to eliminate the dew point problem and to use two part flashing which extends up the sidewall. If you could somehow create a cross-flow of air with a side to side vent pipe you would be way ahead of the game but screening for bugs and water would be a must. I am also going to use coarse fill against the house wall area again to reduce any possible moisture problem. Treat the masonry slab like a site-built shower pan.

    And I wouldn't tie the slab statically with the stems wall as someone suggested, just be very vigilant with prep and compaction - it's going to move a little no matter what you do. Better to settle a little and have a flexible weather-tight joint than to create a tension crack in the middle of the slab.

    You could pre-treat the rim area with borax or some toxic fungicide but reliance on these as a solution to moisture problems is a band-aid which doesn't address the real problem.

    Hope it helps, good luck.

    1. karlkwa | Nov 24, 2002 11:44pm | #5

      Thanks for your thoughts. I've thought myself that it would be good to put in some sort of drainage material/mat. Problem is, I haven't thought of anything that drains well (without absorbing and holding water) yet has small enough pores to keep out insects. Most drainage mats seem to have large channels. Maybe something like Enkamat or a fine gravel layer could work. I like the idea of some sort of drainage layer--water will inevitably get in there so it needs to be able to drain.

      Otherwise, are you planning to put the polystyrene on the crawlspace side of the joist? If you don't mind, would you tell me what you mean by the two-part flashing? I agree with you about the chemicals.

      1. broadmeadow | Nov 25, 2002 12:58am | #6

        The polystyrene would go on the otside or cold side of the rim joist - you dont want to isolate the wood between concrete and the insulation. Think of how a glass of ice cold (fill in name of your favorite beverage here) draws moisture in a warm room. You want to thermally isolate that rim joist from the cold damp concrete.

        As for the flashing - it's just in the idea stage right now, but it essentially would be a flashing extending from the slab side up the side wall while the top flashing would come down over top from behind your siding. This would allow some movement without compromising the ability to shed water. I try not to tie dissimilar elements together structurally - wood will move seasonlly with changes in humidity but the concrete won't. I wouldn't worry too much about insect entry - in our area of the continent it's rot fungi we have to guard against. If you can keep the area dry you will be way ahead. Make sure to put a decent slope on the surface to shed water before it can go where you don't want it - the same applies to grading the surface.

        If you could tie a capillary breaking mat in with the flashing you'll have a great setup.

        As with all things in life - use Occam's razor.

        1. karlkwa | Nov 25, 2002 08:24am | #8

          Now I understand the insulation and two-part flashing--makes perfect sense. Thanks to all for their answers. I think I have enough now to choose a plan to get this done.

          1. calvin | Nov 25, 2002 02:21pm | #9

            Karlk7, just so you realize to pay good attention to the flashing, all (4) of the doors I've replaced as of late, included reworking the framing under door due rot caused by moisture and water entry.  All had the porch slab poured against the box.  This one, trim and kick cover, band joist and sheeting all needed to be replaced.  Most of the time they don't even rely on caulk to hold back the water.  Take the time now, the alternative is difficult to repair.  best of luck.__________________________________________

            Remodeling Contractor just outside the Glass City.

            http://www.quittintime.com/

          2. karlkwa | Nov 25, 2002 07:01pm | #10

            Hi Calvin: I appreciate your advice. I suppose no matter how carefully I do this there is likely to be rot over the long term, which really bothers me. That's why I've sought advice from many quarters. Unfortunately, the concrete will make it difficult to inspect and repair. If I can find a totally "right" way to do this, I will do it.

            I have to choose some solution, because I need to build an entry, so I can finish shingling the house, so I can start working on the inside of the house, so we can eventually move back into it, etc... There's lots of work waiting to get done after I do this.

          3. noone51 | Nov 25, 2002 07:48pm | #11

            Karl, not being able to see your situation makes it difficult to understand what you are really dealing with. I have done something similar but took a totally different approach. I first when under the house and supported the floor joists in the area with girders sitting on pier blocks and hydraulic jacks. I raised the jacks just enough to take the presure of the sill plate and then demo'd out the area where the new raised foundation would be. I dowled in rebar to the existing foundation then formed it up with new plate bolts and horizontal bolts every 12" so that I could hang a replacement pressure treated rim joist on the inside of the foundation extension using nylon spacers. I then replaced sill plate on top. There is more to do but I won't bore you with the basic carpentry stuff. It will be obvious should you attempt this method.

            To facilitate hanging the new rim joist piece, I cut the existing floor joists back far enough to give me the room I needed then sister'd them and attached them to the rim joist using strong-tie joist hangers. I also added in blocking for added strength.

            I admit, this was probably a lot of overkill but at the time it seemed like the best way to do the job. This type of solution depends entirely upon the way your home is constructed and it won't work in every situation but it may in your case.

    2. Piffin | Nov 25, 2002 03:07am | #7

      The borates are water soluble so in a situation like this where a regular supply of water travels through, it wouldn't be good for long..

      Excellence is its own reward!

      "The first rule is to keep an untroubled spirit.

      The second is to look things in the face and know them for what they are."

      --Marcus Aurelius

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