May 23rd till June 10th 2011
So, here you go - some notes on our first 3 weeks of work on cob and timber!
COBBING
What have we accomplished in three weeks? A lot! A conventional builder might laugh at the statement, yet we can genuinely be happy about what we have learned and accomplished. Right now, we have built parts of the wall up with more than a meter of cob, over half a meter thick. Our focus has been on the Southern wall, which is lower on the slope and facing the Sun. This will work as an efficient thermal mass to hold the gained solar heat. On the Western side, we have prepared level cob foundations for light straw clay walls and in the East side the plan is to create a wall of bale cob.
We have mixed cob with our bare feet and in that way gotten rid of the large pile of soil that greeted us when we arrived (and consumed quite a quantity of the sand pile on the other side of the site). For the stone foundation laid last year, Paul told us having used about 60-70 tons of stones (4 tractor-trailer loads) from 5 miles away, as well as a couple of loads of urbanite to cover some of the non-visible inner foundation. Cob mix, according to Paul, acquires best quality when done by bare feet. Further along we will be using machinery to make mixes, but this, according to what we’ve been told, will never reach the same consistency in quality, as pure human labour would.
Our mix is 3 parts soil, 2 parts sand, combined with some water and straw to hold everything together. Stomping with our heels and rolling the mixture using a tarp helps us produce an even mix to create the sturdy walls. Each day, depending on weather, we can build the wall up by around 12 inches. The cob then needs to dry in order to prevent putting too much weight on the still damp cob. This will prevent sagging of the moist wall and lessen the need of trimming afterwards.
The initial layers of cob require careful kneading to bind them into the crevasses of the stone foundation. Further along, when laying cob on cob, the process is accelerated quite a bit. Upon finishing a cob surface, it is important to leave it a bit rough so as to bind it more easily to future cob, also for patching and plaster layers. On top of a fresh wall we create a “spine and rib” structure which can vary in height. This supports and works to bind it to the future layer. (This is handy in case there is a delay in building and the fresh layer has dried completely before applying the next.) After finishing spine and rib grid, a good practice is to drill or “thumb” holes into the freshly built layer. This helps to bind the straw vertically in the cob mix, allows air to flow into the inner part of the wall to help drying and creates a good surface for adding the next layer.
In building, we are introduced a guideline of “Inspect, Correct and Construct”. When it comes to cob walls, this means checking the wall before beginning on a fresh cob layer and conducting all the levelling, trim work and patching before moving on to building more. If wall is under-built on the lower part of the structure, it will easily lose more thickness when building goes on. With this in mind, we are always advised rather to overbuild and trim rather than under-build and patch.
Trimming cob walls is made with the help of cob saws (regular old saws shaped into larger, rounded teeth). Patching uneven surfaces on the walls is done with clay slip (screened clay mix with a lot of water to create a fluid paste) and a cob mixture with sticky consistency. Clay slip is applied as a base to attach cob mixture into any dry pre-existing surface (either cob, wood, stone or other). This practice follows the principle of “Wet will not stick to dry”. After finishing a good bit of wall work, we did a fair bit of sculptural work to trim and patch walls vertically level, meeting the foundation with a soft, eye-pleasing “lip”. Any patching that needs to be done beyond the thickness of 1-1,5 inches, should be done in several layers and let them dry in between.
WINDOW SILLS AND MORTAR MIXES
After a certain amount of cob work, window sills needed to be installed before getting further on the walls. We levelled a surface with cob for the sills and laid a foundation of breathable lime mortar with a generous consistency of lime (2:1 Lime/sand) to support the heavy and wide stone sills. We saw to that drip line on below-surface of the sill reached a point beyond the outer parts of the cob wall. This makes sure water will not be damaging the parts of the wall below, but will be dripping on ground instead. Sills must be adjusted with a slight angle towards the ground to let water escape.
For window sills we used 2:1 ratio of lime/sand. As opposed to mixing cob, ratios in a lime mix need to be precise and very thoroughly mixed for about half an hour, water being carefully added after 10 minutes of dry mixing and only in small quantities at a time. The more lime you have in the mortar mix, the stronger and stickier the mixture will be. A regular mortar mix would be a ratio of 3:1 lime/sand – this is the ratio we later used indoors when laying brick foundations for inner cob walls.
As opposed to concrete, our Natural Hydraulic Lime (NHL) mortar mix is breathable. It permeates water but also releases water back into the air as the weather dries and warms up. Lime mortar takes more time to dry than concrete. The lime rated to dry the longest (lower NHL grading - for reference, 2 would be low, 5 would be high) should be used if possible for best results.
When manufactured for consumers, limestone is heated up to high temperatures. The higher this temperature is, the quicker the lime dries in the mortar mix. Concrete is pre-heated to much higher temperatures – thus its capacity to dry more quickly. Lime, as opposed to cement, should not be exposed to direct sunlight when drying. The slower it dries the better.
WOOD WORK
The support structure of our cob house is round wood post and beam. We use logs of circa 20 year old Douglas Fir, harvested from a sustainable forest stewardship site some tens of miles away. Wood could have been acquired from closer by, as Paul points out, but the purchase was an intentional way to support a supplier seriously committed to sustainable stewardship. About 55-60 slow-grown logs were selectively harvested from an individual stand for our structure.
The support structure was mainly finished by last year’s interns, but we have still had plenty of wood working projects for rainy days: window lintels to support walls above window panes, knee braces and vertical support posts which will serve as bases for the windows as well as the wooden frames of which will be sunk into them from each side.
Lintel joints were carefully sawed and chiselled out by Leila and Shawn into an angle which creates a space for three window panes bowed into a slight panorama-like setting. Angles were created by the help of ready-made plywood templates to help the process with the final woodwork. Main tools were the Japanese woodworking saw, flat-bevelled chisels and hammers.
Knee-braces were measured into lengths no less than half the length of the adjacent post or beam (whichever is shortest). Length of a knee brace is measured from its longer side, and adjustments are made, first with pencil lines when the knee-brace is fixed into its place with a ratchet-strap, and then with gouges (round-bevelled chisels) and wooden mallets to work out the marked pieces to shape out a curved, hollow surface to fit the round faces of the timber frame.
Posts for vertical window sides were inspected to figure out the straightest profiles to accommodate the windows. With the help of levels, squares, rulers, chalk line and tape measures we established a horizontal and vertical reference lines. These helped us create an imaginary square within the round log in order to define the placement and depth of the trench for the window frame. In our structure, frame needs to fit in at least a minimum of 2 cm, and the function of measurements was to accommodate the undulations in the surface of the trunk.
TOOL CARE AND SITE MAINTENANCE
Upon our more vigorous woodworking sessions, we have been instructed in chisel care and sharpening. Each of us has created our own sharpening stations with sections to cater for both our curved and straight bevelled chisels and gouges. If honed regularly, as often as once every 2-5 minutes, chisels stay nice and sharp for a longer period of time. Instruction for tool and site maintenance is well organized. Things are orderly, easy to fetch and return to their proper places.
Our construction site is special as well: it is safe for bare-footing. We take care not to have sharp objects lying around, and we barely have any construction waste. Wood cuttings are burned in the cottage woodstove or used for firing the outdoor cob oven for bread or pizza baking. Cob scrapings from trimmings are re-used by rehydrating them into new mixes. In the end of each day, we clear out the site for the beginning of a new, productive day.
