Anchoring or not in existing wall during extension.

[MoggeS]

Hi!

I have a question that I already know will get different answers..
I'm working on an extension, the slab is cast and not in contact with the existing foundation so it can move as it pleases. The outer walls of the extension are built in isoleca, with a sliding layer against the slab. In places where the new outer wall meets the existing house, it is anchored with rebar every third layer according to the constructor's advice. I can mention that the existing house is from the 1940s and made of brick (canalmured). What do you think in this case, anchoring or not against the existing facade?

 

[justusandersson]

Whether it matters or not entirely depends on the nature of the subsoil. However, it is a good principle in all major extensions to have an expansion joint between old and new and to avoid anchors between these parts.

 

[Martin_B]

And then comes the follow-up question that no one seems to react to/think about. How on earth should you make the seal between the existing building and the extension tight if the extension is not anchored to the main building?

1.
If there is a movement joint, which seems completely logical to me in the case you want to avoid future problems with settling, then the extension can also move freely from the existing building/main building. This, in turn, should cause the seal where the extension connects to the main building to "wear out" slowly but surely—again in the case the extension moves (which it should be able to do according to the principle of "free-standing extension with a movement joint against the main building").

So how do you solve that problem?

2.
The next problem would be the stability of the part of the extension that is closest to (adjacent to) the main building. Since there is no wall built for the extension (in the case the extension is built with only 3 sides), there is nothing to stabilize that side in case of wind or the extension being loaded with snow or anything else. You usually make use of the exterior wall of the existing building and use it as an interior wall for the extension, right?

One solution to that problem is to simply build the extension with 4 sides, not just 3, which means building an additional wall where the extension meets the main building, resulting in double walls (the main building's exterior wall + the extension's 4th wall, back to back, directly against each other). In that case, the extension is stable in that direction as well, provided you attach panels to the wall or alternatively have diagonal braces in the framed wall.

But I seem to see a lot of extensions that simply use the main building's exterior wall and use it as an interior wall in the new extension. So how on earth the stability is solved in those cases, I cannot understand if the extension is not even anchored to the main building.

 

[KnockOnWood]

I think it would feel reassuring with some kind of anchoring.
For example, long bolts (threaded rod + nuts + washers) that sit in oversized holes through standing studs in the old house and the extension.

And maybe one can check on the whole thing now and then, and see if the parts start moving out of sync.

 

[hagv]

I also have this question regarding my extension. My KA said that usually buildings are joined together; my original thought was to have them as detached as possible. I also have a bathroom with foil and tiles & tiles against the outer wall that faces the extension. You don't want to risk the waterproofing being affected by settling. My house is from the late 1800s, and the foundation under the bathroom consists of various stone blocks stacked on each other. Joining it with the much more modernly constructed extension might be a good idea considering that? Part of the footing is also cast in rock, so there might not be much settling...

 

[justusandersson]

I want to emphasize that when building an extension on rock or very solid ground, this is not a major issue. In other cases, however, one should consider it.

Joints between the existing house and the extension must be tight, which is not the same as the parts being anchored to each other. Small nails do not count as anchoring. The sealing can consist of raised paper, caulking, silicone joints, etc., depending on where it is done. In a movement joint between hard parts, different types of panel materials can be advantageously used.

An extension must be stable in itself. It is not a problem to achieve stability without anchoring to the old house. The stability (not to be confused with load-bearing capacity) is solved using panel materials, diagonals, and similar aids.

Buildings from different eras and with different construction methods move in different ways. This must be taken into account. Movement joints (in technical terms: expansion joints) exist in all larger modern buildings, although they are normally not visible.

 

[Martin_B]

My KA said that usually buildings are joined together, my original idea was to have them as freestanding as possible.

Yes, it's strange that you get such different advice. Almost everywhere you read it sounds almost hysterical that you shouldn't connect. "It must be freestanding!" etc. etc. A while ago I was looking for information on this, and principle solutions for different constructions, how to connect to an existing building, how the connection at the roof should look so it doesn't leak, but I couldn't find anything concrete that solves the problem of it needing to be freestanding, yet still stable and watertight against the existing building without water seeping in when it rains, etc.

I also have a bathroom with membrane and tiles & tiles against the outer wall that faces the extension.

But then you built the extension with 3 walls so to speak? And thereby utilize the existing building's outer wall as an inner wall in the extension? How do you achieve stability in the extension when the wind blows from the side then? It should fold like a house of cards if it's not anchored in the main building and if there's no 4th wall built with panel material?

 

[Martin_B]

Joints between the existing house and the extension must be tight, which is not the same as the parts being anchored to each other.

At the same time, you read "everywhere" that an extension should be detached so it can move freely. But how on earth do you get tight joints between the extension and the existing house if they can move freely from each other? The connection at the roof will eventually come apart, assuming the buildings move individually, and then it will start to leak?

An extension must be stable in itself. There's no problem achieving stability without anchoring it to the old house. Stability (not to be confused with load-bearing capacity) is solved using sheet material, diagonals, and similar aids.

Yes, but this assumes that you build the extension with 4 walls instead of 3 (assuming it is square or rectangular), i.e., so that the extension has its own wall directly against the main building's outer wall. You then frame up that wall and add diagonal braces or sheet material so it becomes stable.

I cannot for the life of me understand how one could make that side stable without building a 4th wall for the extension, which is stabilized with the help of diagonal braces or sheet material, especially if you're also not allowed to attach the extension to the existing building (the main building).

 

[bossespecial]

If you have a square building, at least 3 "stable" sides are needed. For this to work, it is required (for example) to have a roof that functions through diaphragm action, in regular houses, the tongue and groove boards in the roof.

 

[Plupp 122206]

I see no problems at all with building a three-sided structure stable, look at a carport.

 

[Martin_B]

If you have a square building, at least 3 "stable" sides are needed. To make this work, it requires (for example) having a roof that functions through shear action, in ordinary houses the tongue and groove boards in the roof.

However, it doesn't help the side that is connected to the existing building. It would likely collapse like a house of cards in strong winds if it isn't anchored and doesn't have its "own" wall? Or am I thinking completely wrong now?

 

[Martin_B]

I see no problem at all with building a stable three-sided structure, look at a carport.

Also considered the comparison with a carport. But how stable is a carport that doesn't have a storage room at one end?

 

[Martin_B]

Here is an example. However, they have placed a couple of diagonal supports at the "entrance end," which are missing in many. So they might help a little. But such supports are not usually found in an extension.

 

[bossespecial]

The wall against the existing building does not need to be "stable" as long as the other three are. When the wind blows against the side that only has a "stable" wall, the other two help absorb the twisting of the building that occurs. Provided that you have a roof that works through diaphragm action or that you cross-brace the roof. In the attached image, the braces and columns function somewhat like a frame that stabilizes the gable. Without the braces, the carport would not be considered stable.

 

[Martin_B]

Without the braces, the carport would not be seen as stable..

But extensions are generally not built with such diagonal braces, yet they can still be seen as stable, right?