Do old walls have DPC

[padstar]

I would have thought the answer to this was yes but i am trying to locate mine to set out my footing depth and cant locate it inside or out.

The house is early 30's i think and solid block construction with pebble dash finish externally. Will there definitely be one somewhere just i haven't

found it yet?

 

[^woody^]

The floor level is the criteria, not the DPC level.

 

[padstar]

Agreed Woody. I am trying to sketch out for my own piece of mind from bottom of footing to DPC so i have clear in my head where each element will sit. The DPC location was part of that exercise and has confused me. So forgetting the footings for a moment should the DPC be visible? I know that you shouldn't render over it now days but has that always been the case. The architect has detailed the new DPC tieing in with the existing.

 

[^woody^]

It does not matter where the DPC is you can forget it until you get up to DPC level on the new work.

Guage down from the floor level and work to that. You'll need to knock that render off to butt the new wall to the exiting wall, so when you get up to that level with the brickwork, you will find the house DPC.

Then whether you chase the new DPC into the wall/existing or just run it up the wall 200mm or so, is up to you.

 

[padstar]

Thanks again. Render will be removed probably before i dig footings.

Does this make sense for guageing the footing depth? This is my understanding of what is required from reading various documents.

- Door threshold/internal FFL must be 150mm above external ground level
- 223mm 1 layer of blocks with clear cavity
- 223mm 1 layer of trench blocks or filled cavity blockwork
- 750mm concrete footing to architects specification

Total just over 1m dig below external ground level

 

[^woody^]

It's a 1m dig below external ground level. Do that.

Then the important thing is to gauge down in multiples of brick courses (75mm) or block courses (225mm not 223) to the top of the concrete - as you want it to work courses not have to mess about with split courses or cuts.

The concrete thickness is largely irrelevant as the things to concentrate on are

• the depth of the trench
• the level for the start of the wall (which is also the top of the concrete)
• the finished floor level (ie DPC/top of the wall)

BTW, that sketch shows just one course of bricks below ground level externally. Has that been approved?

 

[padstar]

Thanks again woody.

So the 750 deep footing on the architects drawings is an indication only i am guessing. I plan to start digging this weekend so i will take it 1m from external floor level, then gauge down from the internal floor level/door threshold (150mm above external floor level) to a full block size to give my top of concrete level.

How many courses of blocks would you allow from DPC to top of concrete and how many of these courses would you use trench blocks for to avoid filling the cavity?

I am meeting the BCO next week and have a list of questions for her.

 

[noseall]

So the 750 deep footing on the architects drawings is an indication only

When you consider that 225mm is adequate thickness for most extensions then any more is a bonus and less (expensive) masonry to build.

so i will take it 1m from external floor level

Ideal. Once you are through all the dark fertile stuff, look for consistent virgin earth. If you come across any old coke cans - keep digging.

gauge down from the internal floor level/door threshold (150mm above external floor level) to a full block size to give my top of concrete level.

How many courses of blocks would you allow from DPC to top of concrete and how many of these courses would you use trench blocks for to avoid filling the cavity?

DPC and FFL are pretty much the same and are at a set level. Your foundation concrete should be set at defined increments down from that level. Those defined increments are 75mm and 225mm.

The make-up of the masonry should reflect your ground levels and dictates the amount of brick courses you need to show so that you don't see any block-work no matter if the ground wanders up or down.

An ideal make-up of masonry would be for the concrete to finish say 525mm below DPC and consist of:
Two courses of dense concrete block and a course of engineering bricks - inner leaf.
A course of blocks and four courses of engineering- outer leaf.


​

 

[padstar]

Thanks for the info. It is starting to become clearer now. I had it in my head that both skins would be blockwork all the way up as it was being rendered but reading further i now see that the render must stop above the DPC and so blockwork below is no good. It will mean that there is a change in finish to the bottom 150-200mm as the original house has render all the way down but if that has to be then there is nothing i can do about it.

Can the 1st row of blocks be 300mm trench blocks to give the solid cavity and then open cavity for the next 225mm to dpc?

What is the reason for the course of bricks internally rather than just 2 courses of blocks with the build up being 450mm (2 blocks inside and a block and 3 bricks outside + more concrete fill)

Thanks for all the help i really want to do as much of the smaller extension as i can. I am confident in my skills to do the work, i just don't have the knowledge of the detail.

 

[^woody^]

One thing, don't mix up floor and ground levels - it's "ground level" not "external floor level". It can cause problems if you refer to the ground as floor.

 

[padstar]

One thing, don't mix up floor and ground levels - it's "ground level" not "external floor level". It can cause problems if you refer to the ground as floor.

Noted - Is Ground level deemed as the top of the paving to the drive or the underside of the finish?

 

[padstar]

Can you render below the DPC as well as above as long as you don't cover the DPC so that you don't see the engineering bricks and it matches the existing walls (minor detail i know)

 

[noseall]

Noted - Is Ground level deemed as the top of the paving to the drive or the underside of the finish?

Top of the paving or (as is often the case) 150 - 225mm below DPC

 

[padstar]

Thanks guys. Just need to get the shovel out now and prepare for the pain. Appreciate the help as always.