External Cracks Between Original House & Single Story Extension

JKS$(

I’m hoping to buy a home but have noticed some large cracks at the top of where a single story extension (kitchen & bathroom) joins the original Victorian/Edwardian terraced house.  I’m having a survey in 6 weeks, but thought I’d ask for comments here - thanks in advance. It’s a flat roof & I couldn’t see the other side of the extension.

grumbler

Typically for the majority of extensions, foundations aren't good enough.

If the extension is new, the gap can get even bigger and became really bad. If it's old and the gap is unlikely to grow, it can be filled.

At a closer look it seems to be repaired already at least once and, obviously, unsuccessfully.

FreeBear

grumbler said: Typically for the majority of extensions, foundations aren't good enough.

It is the lintel over the doorway - Wonder how it has been tied in to the original wall and if it is supported by a big enough area. There shouldn't be much loading in that area as it is a (relatively) lightweight flat roof. So if foundations the other side of the door are insufficient, it may well be settlement (or even subsidence) pulling the whole extension away from the main building. Either way, it needs an expert opinion to identify the cause and what remedial action is required.

The joy of cement render - You can't see what is really happening with the wall, and it hides all sorts of surprises (including damp).

stuart45

Most extensions will settle a certain amount, depending on the type of subsoil. For example ones built on chalk will move a lot less than those built on clay. 
Years ago extensions were generally toothed in to the existing and any movement could cause a crack at this point. On a good job the brickwork was built a fraction higher to allow for the settlement and the joints filled in 6 months later.
Nowadays the use of wall starters has helped stop this problem.
On your house a wall starter would not be possible as the frame is tight against the wall so the masonry is only tied to the existing by the frame. I would open up the crack to see how much of a bearing the lintel has on the house wall. It is also a place were water can get into the wall.
Usually you find that settlement stops after a while, although with some clay soils it can take a few years.

grumbler

stuart45 said:

Nowadays the use of wall starters has helped stop this problem.
On your house a wall starter would not be possible as the frame is tight against the wall so the masonry is only tied to the existing by the frame. I would open up the crack to see how much of a bearing the lintel has on the house wall. It is also a place were water can get into the wall.

A wall starter kit stops the new wall from falling along the old wall, not from moving away from it. The latter is impossible - the force is far too high. If you create a stronger link the new wall will just pull the old one and crash it.

stuart45

A wall starter stops a wall moving away from the wall, not downward settlement, that's what their job is.

grumbler

No, I think it's a common confusion. In the first place it stops the new wall from falling along the old. It's the support that any tall walls need and provide each other at the corners.

As I said, the the old wall is generally incapable of supporting the new wall from moving AWAY, e.g. if a new wall is linked to the middle of an old one.

ETA: and even if it's a corner, not  middle, long walls have movement gaps for a reason - with SLIP ties linking them.

stuart45

If you butt joint a wall to an old wall there will normally be an opening up of the joint as the new wall settles and moves. A wall starter helps stop this. If using a wall starter damaged the original wall they would not be used. They are designed to allow for a certain amount of movement. 

weeg

stuart45 said:

A wall starter stops a wall moving away from the wall, not downward settlement, that's what their job is.

I'm with him on this. Obviously if the rest oft he design is wrong it won't stop a major issue, but we specify wall starters specifically to keep the new and old walls hard against each other.

If it's a common confusion it's one shared by every structural engineer I've ever worked with. It's a tension system, not a shear system.

grumbler

stuart45 said:

If you butt joint a wall to an old wall there will normally be an opening up of the joint as the new wall settles and moves. A wall starter helps stop this.

It cannot help THIS. At least because the long part that you screw to the wall with just 3-4 screws is far to flexible in THIS direction.

ETA: and the link between small horizontal parts and the vertical plank is extremely week in THIS direction too to provide any significant force.

If using a wall starter damaged the original wall they would not be used. They are designed to allow for a certain amount of movement.

That's exactly my point. It cannot damage the old wall by PULLING it because it's not strong enough to PULL it. However, it's strong enough to stop the new wall from falling along the old one and by providing this support it cannot damage the wall.

stuart45

Well you are wrong about this. It does help lateral movement as well, but without them the walls tend to separate away from the wall. You need to listen to the post from an SE.
You are also wrong about most extension foundations, as they are usually deeper than house foundations, and have been for around 40 years now.