DPC questions

tony3619

Hello, 

Just after some general knowledge really...relating to air bricks and dpc.

1. Why does the dpc have to be 150mm above ground? 
2. Can air bricks be level or below ground level if there is a channel infront? 
3. How can I tell if the floor is suspended and if timbers have been used? (The house looks like brick block construction) 

Basically it looks like the patio Has be made too high  and a channel has been placed Infront of the house but this can't be filled because it would cover the air bricks. The dpc is on top of the air brick so is in line with ground level but is about 100mm above it if you count the ditch.  The wall doesn't seem to have signs of rising damp and no signs Inside either. The property is about 6 years old. 

I have looked at all the other houses on this site and it's the same on all of them. 

Any explanation for this other than they screwed up on the entire site? (50 plus houses) 

I definitely know it's cavity wall construction with breeze block so don't know if that gives a clue on the ground floor construction and whether air bricks are needed 

FreeBear

In answer to your questions..

1. NHBC & Building Regulations specify 150mm to limit or prevent rain splashing up and soaking the wall above the DPC.

2. Yes, the air bricks can be level or below the ground level (with some coping round it). It is not ideal as water could drain into the void under the floor and is best avoided.

3. Air bricks would suggest that you have a suspended floor. It may be block & beam, or it might be timber - The only way to find out is to lift the floor covering and take a look. If you have carpet, it is easy enough to peel back a small corner. If laminate or tiles, pop the kickstrip off a base unit in the kitchen or pull out one of the appliances if they are free standing.

If you are concerned about ground levels being too high, dig out a trench 200-300mm wide and fill with gravel. Cutting slabs can be tricky, especially if they are thick heavy duty ones. It may pay to get someone in with a petrol driven slab saw - You could hire one for the day, but in the wrong hands, they are very dangerous tools (and you also need appropriate PPE).

tony3619

FreeBear said:

In answer to your questions..

1. NHBC & Building Regulations specify 150mm to limit or prevent rain splashing up and soaking the wall above the DPC.

2. Yes, the air bricks can be level or below the ground level (with some coping round it). It is not ideal as water could drain into the void under the floor and is best avoided.

3. Air bricks would suggest that you have a suspended floor. It may be block & beam, or it might be timber - The only way to find out is to lift the floor covering and take a look. If you have carpet, it is easy enough to peel back a small corner. If laminate or tiles, pop the kickstrip off a base unit in the kitchen or pull out one of the appliances if they are free standing.

If you are concerned about ground levels being too high, dig out a trench 200-300mm wide and fill with gravel. Cutting slabs can be tricky, especially if they are thick heavy duty ones. It may pay to get someone in with a petrol driven slab saw - You could hire one for the day, but in the wrong hands, they are very dangerous tools (and you also need appropriate PPE).

Thanks for the reply. 

The floor under the existing laminate is concrete? There is already a trench roughly 8mm -10 mm wide. 

The only reason I can see why this could be like this is that there are patio doors that take up most of the wall and are in line with patio for wheelchair access.  Maybe the dpc needed to be just below the doors hence why they installed the trench. Doesn't explain the air brick being below the patio. 

tony3619

This may sound like a stupid question but I'm a total novice in this area....does a concrete subfloor mean it's on concrete joist? Would it be floorboards if it was suspended timber joist?

stuart45

The chances are for a 6 year old house with air bricks you have a block and beam floor.
As modern houses need wheelchair access, a cavity tray is installed at ground level and drainage next to the wall. Occasionally a double DPC is installed.

Doozergirl

I can't quote you for some reason but yes, the concrete subfloor would mean block and concrete beam.    It would be incredibly unusual for a new house to have a suspended timber floor these days.  

Are you sure that trench is 8-10mm and not 8-10cm? 

A photo would help if you want to know if what you have is okay.  

tony3619

Doozergirl said:

I can't quote you for some reason but yes, the concrete subfloor would mean block and concrete beam.    It would be incredibly unusual for a new house to have a suspended timber floor these days.  

Are you sure that trench is 8-10mm and not 8-10cm? 

A photo would help if you want to know if what you have is okay.  

Sorry I meant 8-10 cm! Lol, I'll try and get a photo later. It's definitely got cavity trays and weep vents just above the air brick.  The problem is the top of the air brick is in line with ground level or patio so most of the air brick is below the patio in the existing trench. 

The only solution would be to dig down 200-300mm clearance in the trench and cover with pebbles  up to 150mm. this will mean a big drop in the trench without lowering the patio which would be one heck of a challenge. 

stuart45

You shouldn't need to do any digging if there's a tray fitted in the wall and there's a drainage channel by the wall. This is standard on new builds.

FreeBear

To be honest, if it has cavity trays fitted along with weep vents, the patio level shouldn't cause any real problem.