Strawbale foundation toeup details.

Hi there.
Im in the process of building a timberframe strawbale house. Its actually a rebuilding of a old Portuguese Xisto (shale/slate) barn. The ground floor is in stone that been restored and a ring beam added, and then a timberframe  built from sweet chestnut reclaimed from the wildfires.

There are a few details that im struggling to find information/ conflicting information about and I will add pictures to try and explain more.

#1 I will use a metal plaster stop at the base of the wall that is attached to the external toe up. How important is it that this stop and therefor the plaster itself is resting on the foundation underneath, rather than extending out from the wall the thickness of the plaster. In other words, that the toe up is recessed the thickness of the plaster and not flush with foundation wall.

Ive seen from some sources that in the case of load bearing walls the plaster itself plays a big part in the structural strength of the wall, and that it is important that this rest on the foundation.  But other sources and pictures that dont care for this, more so on strawbale infill.

#2 Between the two toeup beams i will be putting Leca, (expanded clay balls) this should be somewhat insulative and stop any water wicking up into the bale. I will put a damp proof course between the stone / concrete foundation and the toe up beams to preserve the wood, but between the two beams I was originally thinking to leave open.

Many sources i have found advocate for a membrane or waterproofing to be but accross the whole foundation, but this to me seems to counter productive as any water or moisture that may make its way to this point, will then be trapped. Its better to gone down into the foundation wall than just sit inside that cavity next to toeups.

Does this make sense, or am i missing something here.

Thankyou for any advice and comments.

What a cool project!

#1
I just plastered my house this summer. 3/4” cement stabilized clay plaster with straw. I used a metal “J” trim plaster stop at the bottom of the plaster, then a drip edge underneath that. This gives a pocket for the plaster to sit, and good drainage. The horizontal part of the J trim had a 1/4” hole every 4” to allow for drainage. Given this drainage, I think you would want the plaster away from the foundation wall so the edge can drip. I did this at windows too.

#2
I mostly followed this one. I would think a sort of membrane would be worth it here. Those are important wood beams and it would be unfortunate if anything happened to them. I’ve not heard of the clay balls. I understand the concept as clay is so receptive of water, but would you want that in a mostly sealed box with wooden walls? Seems like a potential to encourage moisture to hang out between the beams. Maybe build up a nice and smooth mortar bed with sloping edges and that’s your attachment surface. A membrane could work drape on that well and shed  water. Sill Seal is a product used for separating bottom plates and on-grade concrete slabs, maybe that would be enough.

Hi there T!

Beautiful building site!  And good drawing, too—I’ll try to answer your question as though you were building in the U.S., knowing that our codes aren’t applicable in Portugal. Still, you might find some of it useful.  

I won’t get into whether such a tall foundation made of (presumably mortared) slate is adequately reinforced for what will be a very heavy plastered straw bale wall and roof assembly.  Where I build in Oregon this might be considered a top-heavy structure, and with earthquake activity a possibility would require a lot of reinforcement since straw bale walls are five times heavier than conventionally framed, insulated, and sheathed buildings. Once things start moving in a seismic event that’s a lot of weight to restrain.

RE question 1.  The metal weep screed at the wall base is a good idea for a couple of reasons.  It gives you an edge to gauge plaster depth, gives the plaster a place to stop, protects the plaster edge, and if you drop it another inch or so below the joint where the sill plate meets the foundation wall, also protects against wind-driven rain entering the wall while still leaving an opening for water to drain out (heaven forfend that water should ever get in, but in case it does, it’s a precaution taken on structures I have worked on).  

The weep screed and plaster don’t need to rest on a ledge or shelf in the foundation so long as it’s not playing a structural role—most conventional buildings in North America that have +1” thick stucco (render) exterior finishes don’t rest the plaster on a shelf cast into the footing or stem wall.  Even though the plaster alone weighs 15 lbs. per square foot at 1” thick (150 lbs. for a 10’ column!), the lath-mesh stapled to sheathing holds it in place, and the same is true for the lath supplied by the rough straw bale surface.  You may wish to add mesh for out-of-plane force resistance, or employ some other method like paired external pins, but you don’t need it to give the plaster enough tooth to hang on.  

If you’re relying on the plaster skins to supply resistance for in-plane shear, and you haven’t engaged an engineer to calculate whether the staples holding the mesh in place will be enough, then the prescriptive path outlined in our building code requires the plaster to rest on a shelf cast in the footing or some other engineered design.  This is true for both load-bearing and in-fill designs.  Resting the bottom edge of the plaster on the foundation gives it something to bear against.  Seismic tests conducted in the U.S. showed that the test wall fared better when the reinforcing mesh was stapled to the sill plates on a specified schedule and the plaster could bear on a solid surface as the applied lateral force (simulating an earthquake) attempted to turn the rectangular wall into a parallelogram.

RE question 2.  Here in the U.S. we need to place a waterproof barrier between concrete foundations and wood sill plates, even when the sill plates are made with wood treated to resist decay.  That’s probably not a bad thing if it makes allows buildings to last longer.  I’m not sure what our code says about sill plates over stone.  Whether stone allows moisture to rise through it like concrete I can’t say, and given the tall slate stone wall you have I doubt it would rise that far via capillarity.  Still, I’d take the precaution of using a waterproof material under both sill plates and the space between—whatever is commonly used in your country.  In my practice we use a flexible peel-and-stick membrane that we run up the side of the interior sill plate (after that sill plate is secured to the footing with a layer of some waterproof membrane under it), then run that membrane out to the edge of the footing.  The exterior sill plate sits on top of it.  That way if water ever drained into the area between the sills it would flow to the exterior.   This method doubles as an air barrier so long as the interior plaster drops below the top edge of the interior sill.  Some builders cut narrow (1/8” x ¼”) drain channels with a saw every few feet in the bottom of the exterior sill—not a bad idea if you have a tight seal there—but my experience is that the somewhat rough surface of the foundation is uneven enough that water drains.

Scott's detail showing a "Z" flashing over a weep screed or casing bead above a window is one that I use as well.  It directs water flowing off the wall above away from behind the window.  One difference I'll point out is that if you are trimming windows with wood I'd continue the casing bead (also called "J" channel) around the entire window.  That way when the wood trim wears out (gets destroyed by sun and water) it can be replaced without damaging plaster that might have stuck to it.

Jim
Many Hands Builders

What is a 'toe plate' please?

Good question John.  I'm not an authority on the language used to describe a building's parts, and toe-plate and toe-up are not terms I have heard used in the Western U.S., but I understand them to be synonymous with sill plate or mud plate.  In one plan set I saw the term toe-up used to reference a curb cast in a concrete slab/footing upon which a straw bale wall sat.  

I expect that across regions, cultures, nations and building traditions this is pretty common, e.g., render = stucco = exterior plaster (more or less).  

Getting off topic a bit, I wish designers and builders would restore the term "stool" to reference the interior horizontal trim at the bottom of a window. Instead I see it called a "sill," which is also what the exterior horizontal trim at the bottom of a window is called.  So when plans reference widow "sills" without a drawing I need to figure out if they are talking about the inside or the outside.  Then there's "Larsen truss" being used interchangeably with wall truss and ladder truss.  Technically, wall or ladder trusses are the same--vertical framing members that support roof loads.  A Larsen truss is an assembly attached to an exterior wall for the purpose of increasing the wall's insulation, and it doesn't bear roof loads.  Using "Larsen truss" in place of wall or ladder truss seems to have crept into common use in the natural building world--it gets confusing.   I have suggested using a "frieze board" at the top of a wall to some designers and
drawn blank looks--possibly another term destined to become archaic, just like stool.

My 2-cents worth.  I'd enjoy a discussion about confusing terms used by builders--lots to learn.

Jim
Many Hands Builders