Using a lintel, where some of it is below ground level

2 Aug 2019
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United Kingdom
I've planned a conversion of an internal garage into living space using lintels to span a 3.6m gap between walls, which is normally a building regulations compliant approach.

I want to put in place floor-to-ceiling doors where the garage door currently stands. The house internal floor is c. 150mm above the level of the garage floor and external ground level. This would approach would require putting the lintels at least below the existing DPC in the walls. This approach would leave the lintels just above ground level, but forming a thermal bridge.

What I would like to do is excavate slightly and install the lintels below ground (the top of the lintel would be level with the ground – ie an existing terrazzo surface that drains away from the house) and have a course of insulating block (eg Marmox thermo blocks) resting on the lintels. However I am concerned there may be issues with building control because the lintels ares below the "frost line".

Do I need to be concerned about building control and a frost issue? Does anyone know of a lintel product that I can specify that would address any concerns?
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I'm building an extension in the SE and my BCO actually suggested I dig pads and build off 1800mm lintels rather than dig the usual footings. This might be because I'm building alongside a 1800mm Victorian wall which has seen better days but lintels will be just below ground level in my application and so be at frost level.
Of course this may not be permitted in your locale, street or town.

If you use two 100mm lintels and create a cavity then you can avoid the bridging? Nothing to stop you putting wall ties in on top of the lintels and putting a decent water repellent insulation in or cut down rigid cavity batts? Or did you mean thermal bridge through the edge of was once your garage floor?

Dig a pad centrally and install 4 in no. 100 x 65 x 1800mm lintels you'll need to tooth out the brickwork at the sides (assuming they're actually sitting on footings) to insert the lintels in 100mm or so which means you'll need a brick space on your pad if that makes sense? then stick a row of engineering bricks to help prevent water ingress to edge of internal floor level then build up your thermalites from there.

Ex. brickwork > lintel <single brick>lintel<ex. brick
[500 x 500 x 500 pad]

Well from your description that's what I would do - but I'm sure someone either disagree, call me an idiot or come up with an easier simpler solution!
Many thanks Isambard,

I understand the below ground issue better now - some (most?) lintels are rated for below ground use (covering sewage pipes etc). In my situation you need to ensure that the ends of the prestressed lintel are encased in impermeable mortar to prevent the steel reinforcement rusting in the coming centuries, and I think that is it.

I may well [have to] draw on your idea of an intermediate pad - if I use a single concrete lintel for the 3600 span, it needs a needs bigger cross section and 200mm bearings on the walls, rather than 100mm, which might be a pain to achieve. All the bricks are staffordshire blue engineering bricks.

The thermal bridge arises if the floor insulation butts up against the concrete lintel where installed above ground. By having the lintel lower with insulating blocks on top that minor issue goes away
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I think you’ll need the pad because I’m not sure you’ll find (or even lift) a 3600mm lintel :)

you can avoid the thermal bridge by trimming off 20mm garage floor and putting in rigid insulation (or even expanding foam). Personally I’d do rigid insulation as it’ll stop mortar etc filling in your all important slot.

This is a typical application of insulation when pouring a slab so I see no reason why is won’t work in this case!

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