Build Your Own Workshop Part 2 - Lime Plaster

This Instructable shows how to work with hydraulic lime plaster on a strawbale building.
Lime Plaster refers to a mortar using only pure lime as a binder and sand as an aggregate.
Lime has a long history of use, serving both functional and decorative purposes.

This is what I'm using as a exterior stucco and interior plaster on this project. I use the St Astier Natural Hydraulic lime and I'm very happy with the result. This is a very old building method, in fact Saint-Astier Natural Hydraulic Lime was used to build the city of Perigueux in France in 30 B.C. I purchased it from PA Limeworks in Allentown, PA.
I used Hydraulic lime plaster because it is one of the "greenest" materials you can use in construction. This is due to its purity, its calcium carbonate composition, it's CO2 absorption, its longevity and potential for allowing the materials to be reused or recycled, and the result of a low energy production process.
Below are some key advantages to using Natural hydraulic lime as a stucco on strawbale .

 Elasticity-Important in minimizing shrinkage and cracking.
Allows for minor movements.If the building moves, a lime mortar will move with the building. Through crystalline bridging, using the free lime content, the lime mortar will heal those cracks that occur. Portland cement can become brittle as a result of it's high burning temperatures

Permeability- Good vapor exchange qualities allow for condensation dispersion.
Allowing structures to breathe reduces or eliminates condensation and rot . Bugs don't like it either.
Hydraulic lime plaster will combine the ability to resist water penetration while allowing the structure to breathe freely

Self healing- small cracks will heal up because of the amount of free lime in the product.
If some cracking occurs in the brown or second coat, you can mist the crack and close it up the next day.

Insulation- The breath-ability of NHL mortars reduces moisture in walls, therefore improving insulation levels.

Reworking- All St Astier mortars can be reworked (8-24 hours), reducing wastage and increasing work rate. This is due to the absence of cement, gypsum, pozzolans or high aluminates.

Recycling- Materials built with hydraulic lime mortars can be reused. the NHL mortar itself may be recycled in a number of ways, such as an aggregate for new lime mortars, fertilizer (NHL is calcium carbonate), or it can be used for water purification to adjust pH levels.

CO2 absorption- The most eco-friendly feature of using limes. CO2 is reabsorbed during the carbonation of the free lime. Also the amount of energy used at the production stage is one half of what is needed to produce cement. Consequently, the release of CO2 into the atmosphere is much less.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Tools Needed

These are some of the tools I use to plaster onto strawbale walls. The hawk is basically just a big flat surface to put your plaster on and hold up to the wall. You can make one. The store bought units have a nice thick rubber ring to cushion the weight against your hand. I got mine at a garage sale for a buck

You will need trowels. The finish trowel is the main unit for applying the plaster. It can really move a lot of material quickly.
The pool trowel is the best for smoothing the irregular shapes you encounter in strawbale walls.
Pointing trowels are for getting into tight places. same with the small flat end trowel.
a Hopper sprayer and the compressor to run it, especially for exteriors, if you can afford it it will really speed things up. You will need a compressor that can push 18 CFM's
A cement mixer is essential on a project of this scale. The one I have is a cheapo unit and I'm always having to jerry rig something to keep it working.
For smaller plaster or concrete jobs you can use a drill powered mixer.
Buckets and tarps and rubber gloves.
ladders, scaffolds and other medieval devices.
scratching or scaring tool which can be as simple as a piece of hardware cloth or you can spend 30 bucks on a designer tool.
A big mortar pan is nice when you're up on the scaffold.
A hudson sprayer for misting. Keep it clean and for water only. I got mine for around 18 bucks new.
you might want a set of Coveralls so you don't get plaster everywhere.
Eye protection: if you're like me, if plaster can go in my eye it will, and it burns.
Helpers! sure make things go faster

Step 2: Final Prep the Walls

Prior to the application of any plaster you should make sure that your walls are ready.
Strawbales by nature are not perfectly smooth, but, the smoother you can make them the easier your plastering job will be. You can use chicken wire to sculpt your curves and arches for door ways and windows. I use a big pair of Tin snips to cut the wire.
If you have any wood framing or bucks you can use a Pneumatic stapler to attach the wire or you can make pins from a decent stiff wire and jab it into the bales.
Wire also helps to smooth any big lumps or bumps on a flat wall. Check to make sure that you don't have any lose straw and that any holes are filled.

If you have any exposed posts or other woodwork it is a good idea to oil them with boiled linseed oil prior to plastering. Linseed oil is a good general purpose finish, pigments can be added to the oil as desired.

Step 3: Mix the Plaster

NHL likes to mix for at least 10 minutes and 20 is better. The trick is getting the timing down so that you always have plaster.
For the first coat I used 1 part plaster to 1.75 parts sand. I had the sand delivered from a local source. Be careful not to use too much water at first or you'll get a slurry that is hard to keep on your hawk or in the hopper sprayer. I run the sand through 1/4 inch hardware cloth screen to get out any pebbles or clay that may be in the sand. For the first coat on the exterior I also added a small handful of fiberglass for extra strength

Step 4: First Coat

Use your finish trowel to scoop out some plaster onto your hawk. Determine how much you can handle. Place the edge of the hawk against the wall. Don't press too hard with the hawk. Take the finish trowel and spread the plaster onto the wall. The first coat will require a little extra pressure to key the plaster into the straw. Catch any plaster that may fall with the hawk. Spread with different stroke directions ...whatever works best.
It's something that will come with practice.

If using the hopper sprayer for the first coat spray on a 3/4" thick "pricking-up" coat of NHL and 1-1/2 parts sharp, well-graded sand which was mixed for 10 minutes in your concrete mixer and
then after being spray-applied pressed into the straw with a stucco trowel
to seal everything up. Straw will most likely still be showing through in
many places. Because of the slower set time of NHL you have plenty of time to work with your plaster.

Step 5: Scratch the Plaster

Scarify or scratch the pressed-in first coat in a cross-hatch
pattern with a scarifying tool. (or hardware cloth)

Wait 7 days and keep the first coat dampened to slow cure the lime
plaster protecting the exterior with burlap from wind, sun, driving rain and by not
doing any plastering when temps will fall below 40 degrees F for 72 hours
from application.
Use the hudson sprayer on the interior coat and mist it a several times a day.
7 days may seem like a lot ,but, by the time you work your way around the walls you'll be ready to start the 2nd coat.

Step 6: Second Coat

Knock off the loose particles of lime/sand, (a broom works well for this). Dampen the plaster and
with no standing water remaining spray or trowel on a 1/2" thick Brown or second coat of 1 part NHL
and 2 parts well-graded sand with the same mix and cure conditions as the first coat.

After the plaster firms up enough scratch the pressed-in second coat in a
cross-hatch pattern with a scarifying tool.

Step 7: Finish Coat

Exterior Finish: Trowel apply an 3/16" thick finish coat by hand with 1 part
NHL 3.5 and 2-1/4 parts sharp, well-graded finer sand with the same mix and
cure conditions as the first coat.

Interior: Trowel apply an 1/8" thick finish coat by hand, (troweled smooth
and tight) with 1 part NHL 2 and 2 parts sharp, well-graded finer sand with
the same mix and cure conditions as the first coat adding no more than 8% of
the binders's weight of your own pigments.

Step 8: Additional Notes:

  • If you want flatter or square walls in your strawbale building, you can repeat the second or "brown coat" after another 7 day cure on a scarified (scratched) second coat.
    • Mist with water and use a trowel to rub out any shrinkage cracks on the final base coat in either case within 24 hours of its application.
This is done to seal up any cracks that would transfer to the finish coat if it were not done.
cure properly for 10 days .

  • Before you plaster any interior walls it's a good idea to install a plaster stop. For the top of the wall I used a strip of wood I ripped on the table saw. I attached it with 18 gauge brads. What this does, is give you a consistent top edge to plaster up to. I planed it smooth and after attachment oiled it with linseed oil. It looks good and goes with the rest of the wood.

*I also used a strip of 1x4 along the bottom edge to plaster up to. the plan here is that it will give something to attach trim to and a straight line is easier to lay flooring to.

  • * * *
Here is a slideshow I built to show the building going up

Step 9: Summary

Lime plaster is a very old world wall covering and does take more time and skill than just slapping up some drywall. The benefits are well worth it; most importantly the green benefits. Any compound that helps to reduce the CO2 in the atmosphere is well worth the time and skill. Anyone using Lime Plaster will not only help in reducing CO2 emissions but will benefit with a healthier living environment. I hope this Instructable has at the very least introduced people to this amazing stuff : Natural hydraulic Lime Plaster. There is a low impact alternative to cement.

Epilog Challenge

Participated in the
Epilog Challenge

Earthjustice United States of Efficiency Contest

Participated in the
Earthjustice United States of Efficiency Contest

Be the First to Share


    • CNC Contest

      CNC Contest
    • Make it Move

      Make it Move
    • Teacher Contest

      Teacher Contest

    18 Discussions


    4 years ago on Introduction

    Excellent job on the shop and on the 'able. I may be trying something much smaller than this in the spring, we have a lot of clay around here, I might try a local clay coating.


    10 years ago on Introduction

    Lime is made by heating limestone, which drives out the CO2. The lime won't absorb more CO2 than it had to start with. As a carbon sink, it's a net wash.

    11 replies

    Reply 10 years ago on Introduction

    My friend that has 20+ years as a concrete techie (yea, there are such geeks) says that making cement is pretty ugly for the environment. As far as this being a 'net wash' on CO2, I would want to see the chemistry behind it to believe anyones claims, either way. Once you have cement/concrete, it is good for hundreds of years if done right, to bad we give most structures less than 50 years to live in this country (USA). The worst thing environmentally in the concrete is the cement. That calcined limestone takes huge amounts of energy to drive off the water. I understand the portland cement industry is one of the highest energy consumers, just in the manufacturing of the portland cement. We use it because it drys/cures so much faster than other kinds of cement. (There is a good article on wikipaedia on cement and portland cement. Cement was used back in the days of Rome, but it did not generate as 'hard' a cement as we have today. Anyway, the article is good reading! I wonder how does the cost Natural Hydraulic Lime Plaster compare to (place your favorite competitor here)?


    Reply 10 years ago on Introduction

    As far as this being a 'net wash' on CO2, I would want to see the chemistry behind it to believe anyones claims, either way.

    Nearly everything that people talk about as if they removed carbon from the air actually only store carbon. Growing trees absorb carbon, but rotting trees and leaves emit it. A mature rain forest releases just as much carbon into the air as it absorbs, as does every mature ecology. If the total biomass is constant, there is no net absorption of carbon.

    The only ecosystem that actually removes carbon from the atmosphere is oceanic algae, which sinks to the ocean floor.


    Reply 6 years ago on Introduction

    Many strawbale building books talk about the net CO2 absorption. Bale and cob houses have been known to be still standing after 500 years. There are still bale houses built in the pioneering days in Nebraska which are still there. If you want references, there are bales web sites that talk about all this. And books GALORE. But the CO2 issue aside, the fact that you are building with sustainable materials, gotten from local sources is huge. HUGE.

    Great job on the house BTW. I want a stable like this when we live in a permanent home.

    So no oceanic algas are not the only things that absorb CO2.


    Reply 10 years ago on Introduction

    Well, that's the problem, isn't it. (Assuming that you buy into this idea that CO2 is a driver of global warming.) When nature does manage to lock up carbon, we go and dig it up again.


    Reply 10 years ago on Introduction

    Worse even: people will did a great big hole in a peat-bog, fill it with concrete and mount a wind-turbine on top - "green energy"?! L


    Reply 8 years ago on Introduction

    Not to mention the resources to make the wind turbine were mined from the rain forest after cutting it down, shipped to America to be refined, sent to China to be made in to the turbine, the sent back to America.

    Strawbale Shopjdege

    Reply 10 years ago on Introduction

    Thanks. I don't recall that I claimed that it will absorb more CO2 than it had to start with. The fact that it absorbs as much as it does, when combined with it's other benefits, in my opinion, makes it an excellent building material.

    vestieStrawbale Shop

    Reply 10 years ago on Introduction

    This also has to do with the relatively high insulative properties of your building concept. It's built to last (no wasting energy rebuilding). And provides a lot of thermal mass to hold and reflect heat. That being said having not constructed the building I am assuming a lot of things and am not and expert on the topic.


    7 years ago on Introduction

    First let me start by saying regardless of the type of shop your going to have (wood, metal, smelting ect) THIS IS FREAKING NICE!! 2nd, I'm in SC where the temps in summer are usually 3 digits on occasion we have below freezing winters. This question may be irrelevant, however in your notes you stated the ambient temp inside was around 70 degrees. Is that a consistent temp or do you require an additional heat/cooling source? My other question concerns rodents; ie raccoon, mice & rats. Does the lime act as a retardant or did you add something to the straw? I'm a disabled vet & have waited years for a shop of my own. Sadly I know I'll never have one as nice as the one you've built..... but a guy can dream!!! Well done sir. Thank you for your post.


    8 years ago on Introduction

    Great instructable!
    As a suggestion to make your straw bale building greener, save money, and likely save time too:

    You can plaster with mud first to smooth out all the walls and make a great surface to then cover with a thinner and easier plaster.
    Mud sticks very well to straw, and lime sticks very well to mud, also protecting and more or less waterproofing it.

    Most any dirt that has both clay and sand in it (silt is for gardening, not building!) but ideally you want clay from 10-40% and sand from 40-89/90%. Sand is the strenght (like aggregrate in concrete) and clay binds it together (like cement in concrete).
    Mix with water and a fibre like rice husks or chopped straw from your bales in about 1:4 fibre to mud.
    Plaster it on, and it's easy to work, easy to fix, if you make mistakes you can use water to turn it back to mud.

    When 100% bone dry, plaster your lime on.
    You can do this much thinner because it's being used more like a paint than a plaster, in fact you don't need to add sand for body (this depends how smooth you got your mud undercoat).

    Local mud is a healthier option for the environment (no or little transportation and the only embodied energy is from the food powering your body to dig and mix it!) than anything else, but cheers to you for eschewing cement!

    One other tiny hint - in a historical cob home book, I saw that builders would hang damp burlap sacks around the lime plaster for a week to help it cure slowly. This might save you time and misting!


    10 years ago on Introduction

    Codini, I haven't tried this technique/building method yet but I find your instructions very easy to follow. Within 18 mos or so I hope to start my small cabin with outbuildings in north central AR. I'm hoping to incorporate lime plaster either for my workshop or my craft studio. Thanks for sharing your experiences & knowledge.

    Mike Pulskamp

    10 years ago on Introduction

    My understanding is that most Portland cement reaches it's strongest point in around 25-30 years, lime on the other hand binds with CO2 slowly and can take litterally hundreds of years to reach it's strongest point. At that point you have limestone again! The temps that you need to make P.C. are much higher than lime, so embodied energy is much lower with lime, but this stuff is brought over from France, so shipping ads to that. The CO2 issue and even the energy issue is moot after you feel what it is like to be in a lime plastered room. Not to mention the hydrostatic wicking that matches native clay better than P.C. And the St Astier stuff is like working with butter!


    10 years ago on Introduction

    Wow that is so cool! I had no idea that plaster could absorb CO2. I loved the look of the plaster on the drywall. Is that the only drywall in your strawbale shop?