Using Trench Blocks

[realtek]

Hi,

I am looking at using Trench/Foundation blocks but I have never used them before and I had a couple of questions:

1. Do you only use them to get up to ground level and then build the cavity wall up to dpc?

2. If I were using standard blocks and had a cavity below ground then you would normally fill with a loose concrete mix and angle the top so any water runs to the external brick leaf. Does a smaller amount of this need doing from the trench blocks near to dpc is it not really required?


Thanks

 

[tony1851]

It doesn't really matter, particularly if you have full-fill insulation.

 

[^woody^]

BCO's won't normally accept these blocks exposed due to frost damage, so you would normally stop these 225mm below ground and use engineers up to DPC.

Depending on your design, cavity insulation would stop at DPC level or go down to the underside of the floor insulation.

Any cavity below ground is filled with lean concrete as you described and with weep holes at ground level.

 

[realtek]

Thank you Woody,

I will be using blocks on the inside and outside as I will be rendering it so it will be quicker and easier to use blocks.

It is a conservatory with a dwarf wall (except one wall which is roughly 2m high) so exempt from BC and Planning Permission but I would like to do it right.

I was looking at using H+H foundation blocks. I called the technical line and they said they could be used to just go above ground level, and according to the product description is says they are frost proof so I am assuming I can use these right up to one block below DPC?

Due to the height of the foundation and the whether on concreting day we decided to stop at 75mm below where we intended and I will just use 1 course of engineering bricks to make up the difference.

So I intended on using the following:

1. 2 rows of foundation blocks which brings me above ground level
2. Followed by 1 course of concrete blocks
3. Following by 1 course of engineering bricks
4. I will infill the cavity with a loose mix and angle to the outer leaf with weep holes

I will then be at DPC.

Does that sound about right to you?

Thanks!

 

[angelboy]

BCO's won't normally accept these blocks exposed due to frost damage, so you would normally stop these 225mm below ground and use engineers up to DPC.

I used them up to ground level and they were signed off buy the BCO.

 

[^woody^]

BCO's won't normally accept these blocks exposed due to frost damage, so you would normally stop these 225mm below ground and use engineers up to DPC.

I used them up to ground level and they were signed off buy the BCO.

Leave one of these blocks out and exposed in the garden for a few years and see what it looks like. Especially the bits of grey powder around it.

I would never use these where they will be exposed to frost/thaw conditions

 

[^woody^]

Does that sound about right to you?

Thanks!

It just depends if you want to see a block and brick - structurally its OK, but visually, I don't know.
If you don't mind seeing blockwork, then it may be better to put the course of bricks in the ground rather than seeing a random course of bricks?

Personally, I'd do it all in engineers up to DPC.

 

[realtek]

ok thanks Woody.

As I said, everything will be rendered so visually i'm not concerned.

I was looking at speed mainly. I understand what you mean about the foundation blocks. I may rethink that but I was trying to speed things up and also avoid having to infill too much.

Worst case I could use one course of them below ground to help things along.

I thought it would be easier getting the first few blocks plumb up to DPC rather than using bricks as its easier to plumb something that bigger and I'm not super experienced.

 

[^woody^]

They are great for getting started and much easier than messing about building two skins in a trench.

Use a straight bit of timber fixed to the corners as a profile, and put your line on these to run the brickwork through. This saves plumbing and guaging the corners.

 

[realtek]

Yes I will probably use at least one course of them. Do you think there is much risk in getting frost damage if any above ground level are rendered?

If you still don't recommend, apart from engineering bricks I assume the only other alternative is dense blocks.

Good tip on the timber thanks, I was seriously considering buying a builders profile this week (Mustang) it would be very accurate and helpful. It also means I can double check its all square with the profiles sticking up above ground.

If you think timber will be sufficient I will probably save some money and go down that route, however if I am successful at this then I will be onto building a garden office next year

 

[^woody^]

Did you post recently about render below DPC?

It's never recommended. Yes render would prevent rain penetrating the blocks from the front and thus stop any associated frost damage, but the wall below DPC will get damp from rising ground water and this will blow the render off.

Timber is OK as a profile for a few course, but as one edge is up against the return of the wall, you need to be very careful as you go higher not to push the timber out. Proper profiles are kept off the wall so don't have this problem. Only problem is the expense for one or two jobs - you could spend it on engineers instead. Lol