Engineering Bricks, DPC, and double DPC?

[MAJ786]

I'm doing a small Kitchen extension.

Front of house. Site is slightly sloped going down to house from the road. Most of the area is crazy paved driveway on a hard concrete base of 2-3 inches.

Trenches were dug to 1.7m to keep inspector happy WRT youngish oak Tree 11m away.

Concrete footings are down, but I do not like the look / aesthetics of engineering bricks proposed for below DPC (3 courses). The slope does not help - end up with 4/5 lines of engineering on side of house.

It’s been suggested that facing bricks (yet to be matched, but labeled as Flettons 22 - Heather?) will be too soft and not get Inspector approval.
Current thinking is one course of engineering, then DPC then two more courses, this time facing, with the "proper" DPC to join existing.

Any thoughts?

Must we use Engineering bricks?

Sensible to use two DPC’s - won't that be a bit of a moisture trap?

Sensible opportunity to install a gulley at front of new extension for rainwater?

 

[noseall]

I think your trade off of aesthetics over practicality is clouding your judgement and common sense.

Whilst i agree that the difference in sub dpc masonry over the facing bricks above can be a bit stark, i don't think it is wise to compromise such an important and irreversible part of the build.

Why not look at Staffy blues or brown engineers?

I agree, London heathers are really not suitable below dpc.

 

[tony1851]

Why not look at Staffy blues .

No doubt because you're from Staffs! What's wrong with Accrington Noris!

 

[^woody^]

Do salt stained, algae coated, efflorescence ridden, damp facing bricks look more aesthetically pleasing than plain old engineering bricks?

You don't need engineering bricks, you only need ones that have a suitable frost rating. Having a suitable frost rating does not mean that they will look good though

 

[MAJ786]

http://www.brick.org.uk/about-the-brick-development-association/ask-the-expert/ says....

Generally all bricks are acceptable for work below or within 150mm above ground but F2 designated frost resistant bricks should be used where there is a high risk of saturation with a poorly drained site and freezing may occur.
I want to specify engineering bricks on a project because I believe they will be more durable. They are very dense and will not get so wet, thus preventing them from spalling. Why are there not more buildings built of engineering bricks if they are so durable?

For clay bricks strength and water absorption are not properties that give a dependable indication of frost resistance. More importantly there are many bricks of only modest strength (7-20N/mmî) and high water absorption (20%-30%), which have excellent resistance to damage by frost action. Some bricks of relatively high strength and low water absorption are nevertheless only moderately frost resistant. There is no dependable correlation between strength or water absorption and frost resistance.

The two categories of frost resistance, F2 (Frost resistant) and F1 (Moderately frost resistant), referred to in BS EN 771.1:2005 Specification for clay masonry units are classified by declaration. This is as a result of observation after a number of years in use or sometimes for new products by subjecting them to a specified frost resistance test. You can obtain the latest version from the BSI.

Engineering bricks are classified only by their compressive strength and water absorption. There are Class A engineering bricks that have a compressive strength greater than 125N/mm² and water absorption less than 4.5% by mass and Class B engineering bricks that have a compressive strength greater than 75N/mm² and water absorption less than 7%.

Traditionally they are used in civil engineering applications. They can also be used as damp proof course (DPC) bricks and in the structural design of brickwork to resist lateral loads and large vertical loads. A correlation is recognized between flexural strength of brickwork and water absorption of the brick units.

 

[tony1851]

http://www.brick.org.uk/about-the-brick-development-association/ask-the-expert/ says....

Generally all bricks are acceptable for work below or within 150mm above ground

Eh?

 

[noseall]

http://www.brick.org.uk/about-the-brick-development-association/ask-the-expert/ says....

Generally all bricks are acceptable for work below or within 150mm above ground

Eh?

I think what he means is, the most vulnerable bricks are those exposed at ground level and below dpc. Those well below the surface, whilst being subjected to salts ingress are not subject to frost attack.

Probably.

 

[tony1851]

http://www.brick.org.uk/about-the-brick-development-association/ask-the-expert/ says....

Generally all bricks are acceptable for work below or within 150mm above ground

Eh?

I think what he means is, the most vulnerable bricks are those exposed at ground level and below dpc. Those well below the surface, whilst being subjected to salts ingress are not subject to frost attack.

Probably.

'below, or within 150mm above ground' is pretty clear.
This phrase includes brickwork from ground level to 150mm above ground level, which is probably the most severely exposed. This is where bricks frequentliy shale off if not frost-resistant.

 

[RonnyRaygun]

'below, or within 150mm above ground' is pretty clear.
This phrase includes brickwork from ground level to 150mm above ground level, which is probably the most severely exposed. This is where bricks frequentliy shale off if not frost-resistant.

Maybe it means that if the ground drains sufficiently well (such as clean gravel) then the ground won't be saturated and therefore the bricks will be unlikely to suffer from frost attack. Don't know though.

As an aside, my mum's house was built 43 years ago, with rustics above and below dpc. There are no signs anywhere on the perimeter of the house of the bricks spalling. However, there has been some serious frost attack to the garden walls which were also built from the same bricks.
I guess the exposure level of garden walls is much higher...

 

[tony1851]

I guess the exposure level of garden walls is much higher...

Yes, because they remain more or less permanently wet; bricks in the house walls have a chance to dry out - or at least not remain wet for long periods - due to the heat from the house.

For bricks between ground amd dpc level, it does help if the ground is free-draining, though these 2 or 3 courses tend to get wet through splashing of rainwater on the ground.