Block retaining wall becoming a foundation wall?
We’re planning to enlarge our patio which involves building a 6 foot retaining wall. In the future we may build an addition which encloses the patio and adds a few bedrooms on top. We were advised to build an Ivany block retaining wall, steel reinforced and filled with concrete. This wall could then be built higher in a few years and serve as the foundation for the addition.
The retaining wall will be 25′ long and terraced from 6 feet down to about 3 feet. For the addition, it would have to be built up to about 9 feet to meet the first floor. (The build up could be wood framed since the block top is already above grade?)
I believe the footing will be 3′ below grade (4 feet wide x 1 foot deep concrete) with stone and filter-covered drain pipe. No drainage holes will come through the wall and there won’t be any tar-like waterproofer applied behind it. The Ivany blocks are 12″ thick with rebar going up and across.
Are there any special considerations we should be aware of with a plan like this? The hope is that we could get a nice patio now and not throw away money if we decide to expand later.
Thanks!
Replies
You need an engineer.
Really.
Not just saying that because it's reflex here.
See, a retaining wall and a foundation wall are similar, but are not alike very much at all. Part of that is that the forces on a retaining wall are subtle, and far larger than many imagine (this is why far too many are seriously underbuilt). The structural "shape" of a retaining is often replaced with mass or mechanical fastenings, all of which winds up not being very usefull for a foundation wall.
Now, secondly, I'll steal a quote from an old dear engineering professor and "foundation wals ought to be desinged like the pyramids or the panama canal, with an eye to effortless longevity." Buildings no one imagined lasting much more than a family's generation or two can wind up still standing hundreds of years later. More to the point, in this case, we are talking about holding up rooms where your present/future family will be sleeping. Half-donkey is not the way to approach this.
But, we are talking a wager here. Are you willing to bet that vague rule-of-thumb design will hold the hill up now, and double-down that bet later to hold up rooms where your family will sleep?
Engineering is cheap. Not simple, true; sometimes requires great patience (and thorough checking of references & recommendations), but, given the wager, cheap.
I tend to agree w/ Capn .... get an engineer. Your situation is simple, yet a bit unusual. He'll give you peace of mind. You might want to put drainage behind it and a coating on the wall as if it was a foundation wall. You could do that later, but may as well do it now. This depends on your climate, site, and groundwater conditions.
Many basements are similar to retaining walls ... however, they rely on part to the transfer of horizontal forces through the upper floor and to the end walls. Otherwise, it shouldn't be a big deal and your concept is sound.
In the future we may build an addition which encloses the patio and adds a few bedrooms on top.
With that said, it's not a retaining wall. It has to be built as a foundation because that's what it will be when you put a room above it. Have it designed as a foundation and present it to the town.
Joe has the point here - if you ever want to be able to build structure on it later you need to take it through the permitting and inspection process. And most retaining walls need to be engineered anyway if over 4-6 feet tall.
Joe took the words right out of my mouth.... if it will be a foundation in the future then it should be built as one now. In some ways this simplifies things as whatever it is backfilled with will need to be retained by the foundation wall.
I too would get an engineer involved. You can save a buck by drawing the plans yourself. Add length, width and height dimensions, but leave sizing and schedule details up to the engineer.
Thanks for the advice...
This project will definitely require an engineer's seal and we will be doing it fully permitted.
I jumped the gun by including the details on the wall. I'm mainly looking for thoughts on whether the idea "makes sense" from a construction standpoint and if it really saves money down the road. I get a strange look from some contractors when I tell them the plan. Sometimes it's more expensive to retrofit something than just to build new, and I don't want to be in that position.
For example, would we need to know exaclty how we build it up before being able to do a footing? (i.e. wide openings for doors and windows creating "load points" for the headers on top of them).
To me, it seems like good planning-- get the box with the option to build on top. To a contractor, it may be two entirely different beasts and I shouldn't even try to build one into another.
I agree with all the comments posted. Since you know in the end you'll have a new addition, you should be designing this wall accordingly. I think you need to get an architect or an engineer involved in this right from the start.
First of all, make sure your intended final addition can be built per whatever zoning bulk regulations you may have for your site. There would be no sense to spend money to construct a wall designed for an addition down the line if you can't add so much volume to your home. Maybe you've already checked that out.
Curiously though, why would you not waterproof this wall? If you do wind up and build the addition, wouldn't you want the wall to be waterproofed against any below grade moisture infiltration. Draintiles work great, when installed properly, but you still have to waterproof the walls.