Building a lean-to conservatory - how?


3 Dec 2008
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United Kingdom
Hi everyone. My house is a typical victorian end-terrace house. Where the lounge meets the kitchen, there is an 'L-shaped' area outside. See diagram below.


The area has been covered with concrete - about 8 cm deep as far as I can tell. It's about 2m x 5m in area. I am thinking about building a lean-to on it. A single-skin, rendered breeze-block wall along the long side (to match the rest of the house on that side), 900mm high, then d-glazed windows and triple-wall polycarbonate roof. A glass panel + door at the far end. There is already an external door that accesses this area from the lounge.

So, I assume that the concrete is not sufficient for a lean-to, and would need to add foundations. Can I dig under the existing concrete along the edge and pour in concrete to the required depth under where the wall will go? I really don't want to have to remove the existing concrete base. Also, do I have to use neat concrete or can I add hardcore or is there something else I need to do?

Also, any hints about building the wall itself? I've not built a wall before although I can do brickwork. I need to tie it into the existing house, using a wall starter? DPM at top of first level of blocks or at base of blocks?

Basic questions I'm sure, but it's my first foray into building work such as this, so thanks for all your patience at answering my questions ;)

- helen
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single skin walls, even for a conservatory is not advisable.

any full height walls (above 2m) will require decent full foundations.

dwarf walls could be built off a slab but it is something i would not recommend.

it is not to say that the concrete could not stay put, but i would hammer through it and dig a strip footing. this is provided the existing concrete low enough to accommodate insulation, and a 100mm slab.
single skin walls, even for a conservatory is not advisable.

Oh ok. Will rethink this.

dwarf walls could be built off a slab but it is something i would not recommend.

That's what I was thinking. The current concrete slab is low enough to accommodate the insulation layer + new slab. I'll bash the concrete away from the edge and put in proper foundations as you suggest.
There are kits available for conservatories, where the base and foundations are formed of preformed special posts knocked into the ground and beams spanned across them.

The floor and walls are then built as normal
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I borrowed a Hilti, and bashed away the concrete around the edge, where the wall will go. The concrete slab is thinner than I expected - only 4 or 5 cm in some places.

Am now thinking of removing the whole slab, digging down a bit (to allow space for insulation) and laying a new floor. What type of floor is best? Suspended or new, thicker concrete slab.

Any other advice for a beginner?
make shure you do your heat loss calculations as a large conservitory will be unbelivably expensive to heat in the winter months at around 50p an hour £30 a week or £400 for the winter quarter!!!!!
I am not planning on heating it, as it would be too expensive. It just provides cover in the winter for a few plants, and the cats. Once the weather warms up a bit, I hope it will be more useful.
Due to the recent changes in planning rules, where they were changed to make it easier for people to do minor stuff to their house (like my little lean-to conservatory) without the need to the authority to get involved, I now have to get planning permission to do it.

Apparently, if I had started it before last October, it would be fine without.

So that just put a 2 month delay in proceedings and increased my costs. :(

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