Options to flatten top of rocky stone foundation to prepare for timber + bales

Hi all. I have built up my stone foundation with a cob house in mind, but in the end I decided on straw bale. So now I have an uneven surface on top, because that wasn't important before. The house is a one room 15 inside m2 outside 20 m2 with a straight roof declining from 3,5 m to 2,5 m height.

I would like to hear about your experiences making a flat surface from a rocky stone foundation top. I am edging towards a slate or sandstone surface, but I am unsure what to use as a mortar. I'm afraid that the lime mortar will crumble and that the clay mortar will not be strong enough to hold a wall.

I read somewhere here, that you can put a cememt slab on top, but my textbook warns me not to use cement as it will not be flexible and move with the building over time.

Thank you for your input

PS. The house is being built in Copenhagen, Denmark if anyone should be interested

What type of mortar was used for the foundation?  Normally, I would use a similar type as the underlying stonework.  Usually, that would be some sort of lime/cement mortar.  The ratio of lime to cement dictates the compressive strength and adhesive properties.  More lime gives better adhesion but lower compressive strength, higher cement gives higher compressive strength but lower adhesion.  Adhesion is more an issue with the flat rock caps since the irregular surface of the underlying stonework resists lateral movement.  I would probably use a high lime mix like 1 part lime, 1 part cement, to six parts sand. or 1/2 part lime, 1 1/2 parts cement to six parts sand.  Your foundation walls likely have a large area - so compressive strength shouldn't be an issue.

You can improve the adhesion of the stone cap by cutting shallow grooves in the bottom surface with a diamond blade on an angle grinder.  Also, I normally paint the underside of the cap stones with a lime/cement slurry (toothpaste consistency or a little thinner) (without sand) before placing the stone pieces on the mortar bed.  The lime/cement slurry on the stone pieces improves the adhesion significantly.  I would probably go with sandstone over slate.  Sandstone is easier to work and will have better adhesion. The cap stones need to extend past the bales if you want to see them after plastering the bales.  It would probably be a good idea to install pins or anchors that go into the foundation and bale wall.  The pins could go between the cap stone pieces or you could drill holes through the cap stones.  The pins will help prevent lateral movement of the bale wall on the now flat foundation wall.  

A concrete bond beam would also work and it makes it easier to install pins or anchors. If the bond beam is the same width as the bales, it will get covered by the plaster and you won't see it.  

Thank you for your comprehensive reply. I use ready made lime mix (it's very expensive. Next time I will pursue other options).

I am happy to hear that you suggests sandstone, because that is just what I got and I am almost at the point of laying them on top. My plan is to use rubble granite in the mortar to balance out the few but inavoidable uneven places on the top of the foundation.

When I connect the anchors between the foundation and the bales, will they not create some sort of moisture transportation or create condensation within?

I already build in anchors in the foundation to attach to the roof beams. these will not go through the straw and cause condensation.

I'm going to Brighton this week for a workshop. Hoping to get a lot of answers there

Once again thanks

If you have posts anchored to the foundation, you probably don't need any lateral support for the bales.  I wasn't sure if you were doing straw bales only or wood frame with straw bale infill.

I would just use cob as the transition to the bales. Cob Cottage Company uses this technique for bale/cob buildings when transitioning from urbanite to bale. Cob would be better than any cement based transition as it would help wick out moisture migrating up towards the bales. Since the walls need to be thick for the bales, just leave a groove in the cob to insulate like a cordwood wall.