Ground Levels and Air Vents

[Chemical Cat]

We recently bought this 1910 brick house. Cavity walls. Walls are approx 20 inches thick. Exterior brick formation is one header course alternating with five stretcher courses.

The problem I'm looking to solve is damp and ventilation in the corners of the sub floor void. Inside the rooms the relative humidity, temperature, and ventilation are all ok. RH is always around 40%, temp 20-23c, and it's an easy house to ventilate.

Two most obvious sources are the blocked air vents and the high external ground level. Looking at Google maps archives I can see the levels have been high for at least 15 years with soil and gravel. It was made worse with monoblock around 5 years ago.

I have one reasonably large cast iron air vent open on the side of the building but it's also one of the points water is probably trickling in.

There is one joist that looks covered in what I think is salts. It's the one nearest the gas meter. The slabbed areas are quite uneven. They've been laid on soil only and have moved over time.

The end of it that I can see is sitting on slate. The other section of wood that moisture is getting to is the underside of the subfloor next to the open vent.

I'm looking at digging a trench round the three sides of the house to below the level of the front air brick to begin with. I'm not sure how far down I can dig safely.

I want to open the air brick below the glass door. What's the best way to check if I can drill the vent holes open?

We would eventually like to remove the monoblock drive and restore a front garden.

Do I need to consult with a builder or structural engineer before making this kind of change, or can I crack on and lower the ground level as a DIY task?

I'd appreciate any thoughts or advice on the most practical approach to take.

Thank you.

 

[Cubist]

As a general rule of thumb I understand that you should not excavate to a depth greater than two courses of masonry - roughly 150-200mm below the bottom edge of the air-bricks. Beyond that you risk disturbing the foundation/footing of the wall.

Built in 1910 your house most likely included a DPC, in your case, and as you've found some examples in the sub-floor void, this is most likely slate. In your excavations outdoors you should find a continuous line of these above a particular course of the masonry. Ideally you should excavate the earth to leave at least one or possibly two, courses of masonry exposed to air movement in the future.

Noting that the house has been extensively 'pebble dashed' it's likely this was applied using a cement mortar. This probability needs to be confirmed but a lime based mortar would have been far healthier and less damaging. Regardless of what mortar was used, some clown has 'bridged' the DPC and therefore damp will be rising from the ground and masonry to penetrate above the level of the DPC. This source of damp will also be exacerbated by the houses limited ability to 'breathe' effectively due to internally absorbed moisture being trapped by the pebble-dash render. You should consider removing the pebble-dash/render to a height circa one to two courses of masonry above the top edges of the air-bricks.

As to 'drilling out' the material blocking that air-brick. This could be done with a sacrificial HSS or small diameter masonry drill bit BUT NOT using the Hammer Function. There is considerable risk of damage/destruction of the vent itself. Before doing so I would want to know what the 'Drain Cock' is used for, I'm guessing but suspect your wet CH system. Also, what is that cable? I ask as you may be planning to drill through the vent from outside and you must first check to ensure the drill bit does not damage the pipe-work or the cable for obvious reasons.

 

[Flyfisher]

Just picking up on a question about "gas smart meter power?" in one of the above photos, I'm pretty sure these meters don't require any external power and are powered by batteries that should last for around 10 years. My gas smart meter certainly has no external power source, though I appreciate that's only a sample of one and I suppose there might be other variations out there.

 

[Chemical Cat]

Just picking up on a question about "gas smart meter power?" in one of the above photos, I'm pretty sure these meters don't require any external power and are powered by batteries that should last for around 10 years. My gas smart meter certainly has no external power source, though I appreciate that's only a sample of one and I suppose there might be other variations out there.

Thank you for this. I'll take a closer look at the meter. I didn't consider they could be battery operated. That would keep this cable on the mystery list!

 

[Chemical Cat]

As a general rule of thumb I understand that you should not excavate to a depth greater than two courses of masonry - roughly 150-200mm below the bottom edge of the air-bricks. Beyond that you risk disturbing the foundation/footing of the wall.

Built in 1910 your house most likely included a DPC, in your case, and as you've found some examples in the sub-floor void, this is most likely slate. In your excavations outdoors you should find a continuous line of these above a particular course of the masonry. Ideally you should excavate the earth to leave at least one or possibly two, courses of masonry exposed to air movement in the future.

Noting that the house has been extensively 'pebble dashed' it's likely this was applied using a cement mortar. This probability needs to be confirmed but a lime based mortar would have been far healthier and less damaging. Regardless of what mortar was used, some clown has 'bridged' the DPC and therefore damp will be rising from the ground and masonry to penetrate above the level of the DPC. This source of damp will also be exacerbated by the houses limited ability to 'breathe' effectively due to internally absorbed moisture being trapped by the pebble-dash render. You should consider removing the pebble-dash/render to a height circa one to two courses of masonry above the top edges of the air-bricks.

As to 'drilling out' the material blocking that air-brick. This could be done with a sacrificial HSS or small diameter masonry drill bit BUT NOT using the Hammer Function. There is considerable risk of damage/destruction of the vent itself. Before doing so I would want to know what the 'Drain Cock' is used for, I'm guessing but suspect your wet CH system. Also, what is that cable? I ask as you may be planning to drill through the vent from outside and you must first check to ensure the drill bit does not damage the pipe-work or the cable for obvious reasons.

Thank you, Cubist.

I'll aim for remove the lower portion of the render to start with.

Do you think I'd be safe to remove the first row of monoblocks from the front (200mm) and dig to below vent level and put in some large pebbles or gravel? Similar on the side path, I'd need to remove the concrete haunching that slopes diagonally into the wall. The gravel you can see is only surface deep and embedded in the concrete in places.

I've attached a few more photos. The cavity on the side of the building appears dry. I've taken these photos from where an old TV cable was, approx level with the open vent.

You're spot on. The pipe is for the heating system. The cable is one of many we're tracing through process pf elimination. This one is a redundant TV ariel cable. It has since been cut and is stuck solid in the air brick.

I'm tempted to either cautiously drill where the cable is, or lift the carpet and underlay to get under the floor. I've attached a photo from before we flooring down.

Without lowering the ground level, I'm at a loss for where to locate a new vent.

Thank you for taking the time to read this.