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Old 05-15-2013, 10:45 AM   #1
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How to attach new enclosed porch to existing house

I have a two-story colonial with a full masonry basement. The exterior of the house is 3/4 T+G sheathing, covered by wood clapboard, covered by aluminum siding.

The 'L' shaped, existing screened-in-porch with shallow foundation was demolished and replaced with a 48" deep 8" CMU block wall, built over a 20" wide by 10" deep poured footing. On either end of the 'L' shape (where it meets the rest of the house), haunches were poured all the way down to the basement footing, and the haunch was pinned/rebared to the house basement footing.

Here is detail from the foundation plan:

The roofline of the main house extends over the screened-in-porch and was supported during all of this.

I've put down a double sill plate on top over a foam sill sealer, and I've overlapped the sill plates at the corner (as shown below):

The sill plates have been bolted down to the new foundation wall below. However, I've just butted the ends of the new sill plates against the existing sill plates on the existing house foundation.

The plans are vague with, 'typical construction', and I am unsure as to the proper way to tie the new structure to the old structure.

1. At the sill plate level? I can strap it at one end of the 'L', but the other end is a 'T'.

2. At the double-bottom plate level with an overlap of the new onto the old?

3. At the rim joist level?

3. Via blocking at the vertical stud level?

5. At the double top plate level with an overlap of the new onto the old? Or put a third top plate to tie both double-top plates together?

At some point in the future, we will likely be building a master bathroom above this structure (just can't afford it right now)--thus the massive, overkill footing work done.

Any advice is appreciated.


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Old 05-15-2013, 08:32 PM   #2
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there are many different ways of connecting an addition to an existing dwelling. In my area we have to design structures for 110 mph wind loads, so I'll describe the methods we use locally. This may differ from the way things are done in your area, or even in other parts of the country.

I typically would specify that the finishes be removed from the exterior wall down to sheathing where the addition was going. We want to leave as much of the original sheathing here as possible as sheathing provides wall bracing to resist wind loads.

Open the existing wall where the two side walls are going to be attached. Confirm that studs are present for the wall-to-wall connection, or install studs/columns sized for your proposed wall. We would use steel framing angles at the top connection to the top plate and at the bottom connection to the sole plate to secure the new studs/column against lateral loads. You want something solid to attach your new walls to.

you sill plate(s) should butt up against the existing sill plate(s). the anchor bolts will hold the sills in place.

I'm guessing your new floor joists are the same size as the existing? If so, check the condition of your existing rim joist and if in good condition you could use joist hangers to attach your floor joists to the existing rim. once all joists are installed install your sheathing and butt against existing wall. Here we install what is known as diaghragm bracing for floor joists. this goes perpendicular to the floor joists and goes from the exterior walls inward for 48". We typically start at the new rim joist and go every 48" back towards the existing dwelling. This provides wood to nail the sheathing down (48" x 48") along the panel edges to strength the floor where the joists run parallel to the wall. keeps the wind from wanting to bow in the rim joist under that wall. This diaphragm bracing may not apply to your area. An advantage to this blocking is that it helps to brace the top of foundation against lateral loads.

The rest is pretty much standard constructed. butt your side wall stud against your studs/column in the existing wall and nail with 2-16d commons at about 12" on centers to stitch your new walls to the old wall.

do not know what you are planning on for the roof. don't know if the addition has a cathedral ceiling or flat ceiling. If flat you may want to size your attic floor joists not for the current attic loading but for the future bathroom loads. this way in the future you don't have to rip out the attic joists and replace them. Maybe frame with the same size joist as your second floor. I'll leave that up to you ..... LOL but food for thought

with all that said depending on your wind/seismic loading you may be able to strip the outside side of the exterior wall down to studs, certainly makes things a lot easier. Like I said in my area we want all the sheathing we can get.

Good luck! Hope this helps.



"You get what you pay for, and sometimes free costs more!"

Last edited by GBrackins; 05-15-2013 at 08:40 PM.
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addition , attach , existing , porch , sill plate

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