Cob Building Tutorial

Chapter 9: Roofs and Floors

Now that you’ve got all the cobbing done for your walls, it’s time to consider the roof and the floors. There are two ways to approach the roof system. Your choice of these two methods is a matter of personal preference, plus what your local building code will allow.

Cob has about one-third of the load-bearing strength of a cinderblock wall. While standard cinderblocks are eight inches wide, a standard cob wall is two feet wide. This means that if the load is distributed evenly over the width of the wall, a cob wall has just as much compressive strength as a cinderblock wall. In spite of this, some building inspectors who are unfamiliar with cob construction might not want you to make load-bearing walls out of cob. If this is the case in your building location, the ‘work-around’ for this problem is to build the roof as a separate entity, and then build the walls up to the roofline. In such a case, the roof is supported by poles or stone or masonry columns rather than resting directly on the cob. The advantage of this method is that many building inspectors don’t care what the walls are made of if the roof is well-supported. In the past, I’ve gotten a building permit by building a ‘gazebo.’ Once the gazebo was permitted, we just built cob walls around it.

So before you get to the point of building a roof, talk to your building inspector to find out whether the rafters will be able to rest directly on the cob walls or whether they’ll have to be suspended on posts or columns. Incorporate your final choice into the design beforehand. If you’re going to support the roof using posts, installing the roof before beginning construction will allow you to keep the rain and the hot sun off of you while you work.

No matter which type of roof support you choose, you’ll need to allow for at least a two-foot overhang all the way around the cob walls. This overhang helps direct moisture away from the cob.

With either type of roof, it is possible to make a conventional roof using plywood, shingles or aluminum roofing, and insulation. While this is probably easier to get passed by the building inspector, most of these materials are manufactured. I have another choice that I prefer. That choice is to use a green roof.

A green roof is just what the name implies: a roof made of living, green things. The benefit of such a roof is that it contains about four to six inches of soil in which the plants grow. This soil acts as an insulator, eliminating the need for manufactured fiberglass insulation. Here’s how you make a green roof…

To start with, a green roof can’t slope as much as a conventional roof. I probably wouldn’t try to make a green roof with a slope of more than 25 degrees. You may need some extra reinforcement for a green roof, since the soil and plant material weighs up to about forty pounds per square foot. If you’re harvesting lumber from the building site, you can just cut down trees bigger than three or four inches in diameter. Spacing these on sixteen inch centers should be more than enough support for your green roof.

You’ll probably want to use plywood for your sheathing. You could also use tongue-and-groove lumber, but that will be more labor-intensive and more expensive. Once you have the basic sheathing laid, cover it with plastic. If you attach the plastic with staples or nails, you’ll need to seal around the nail holes with some sort of weatherproof sealant. If possible, use glue instead so that you don’t have to poke holes in the plastic. You can also use rubber. This is a bit more durable, but harder to work with. Whatever you use, make sure you have a tight, weatherproof seal before you add the soil.

The next step is to cover the roof with an even layer of soil. You’ll need to make a ‘box’ of the roof by nailing 1” x 6” boards all the way around the perimeter to prevent the soil from rolling off the roof. Next, sod the roof with a grass or other plant suitable for your local environment. Make sure it’s a plant that doesn’t need a lot of maintenance. By using sod or turf squares instead of seed, you assure that the soil won’t wash off at the first rain, before the grass seeds can take root. Check with your local garden supply dealer to discuss appropriate plant choices for your area. They should be grasses that can exist on the normal amount of rainfall you get in your area each year, unless you want to have to water your roof from time-to-time.

Before planning a green roof for your home, check with the local building inspector to make sure such roofs aren’t a violation of code in your area. Don’t stop at the first sign of opposition. It may be that the building inspector simply doesn’t want to pass it because he’s not familiar with it. It could be perfectly legal. You might have to do a little research to find out for yourself.

Green roofs can add a touch of beauty to a home while reducing your dependence on manufactured products. If you’re interested in a green roof for your home, visit www.greenroofs.com for a wealth of information and resources.

There are two methods of adding a floor to your cob home. The first is to add a crawlspace. If you’re adding a crawlspace, this means that you’ll be using regular plywood or OSB underlayment/subflooring with some sort of floor covering on top of it. If you’re not familiar with flooring, you’ll probably need a contractor to do this. Since, if you’re planning to use this method, you either: a) already know how to do regular flooring; or, b) plan to use a contractor, in either case, you don’t really need me to tell you how to do this type of flooring. So I’ll focus on the other way of adding a floor, which is so easy you could probably train a monkey to do it.

The simple way to add a floor to your cob home is to make sure the ground is level where you want the floor, then to simply add the floor on top of the ground. As with everything else involved in cob building, find out if this is allowed by your local building code, then proceed.

The way to level the floor area for your building site is to first get it roughly on the same pitch all over the floor area using a transit level (see Chapter 5: Foundations). Next, you’ll build a frame around the outside perimeter of the floor. It is best to do this before the foundation stones are laid, so you don’t have to work around the foundation to level the floor. If you are building a round house, it’ll still be easier to use a square frame. Just build the square frame so that it encompasses the circle (see Figure 9A).

The way to build the leveling frame is to drive four stakes in the ground at the corners of the floor area. Next, nail one end of a 2”x4” to one of the stakes. Place a carpenter’s level on the board. Go to the other end of the board and have an assistant watch the carpenter’s level while you move the end up and down until the board is level. Once it is level, nail the end down. Repeat the procedure on all four sides of the leveling frame. When you have all four sides nailed, go back around and check to make sure everything is still level. If anything has slipped out of level, you can adjust a little bit by gently tapping on the stakes until the boards are all level again.

The next step is to create a grid by spacing nails at one-foot intervals along each board in the leveling frame. Once you have the nails driven, wrap twine around the nails, making sure that the twine is resting against the boards and not riding up to the heads of the nails. When done properly, the twine should intersect at one-foot intervals, making a grid of square-foot twine intersections over the entire floor area (see Figure 9B).

Now that you have a grid over the floor area, this grid should be perfectly level, assuming you leveled all the boards along the perimeter of the leveling frame. To level the floor area, measure down from a twine intersection to the ground. The distance you measure should be equal at all intersections of the twine, throughout the whole floor area. If it is not, use a spade or a shovel, move the twine aside, and dig or fill in until the distance from the ground to the twine under all twine intersections is equal. It can be difficult to work around all that twine, so you might prefer to simply use two sets of lines, running perpendicular to each other. If you choose this method, you can cut a little jig out of plywood that will allow you to mount the strings along the frame and slide the twine down as you go, checking measurements every foot or so.

Once you have the floor area level, you may wish to pack it down with a soil-packing machine. You can rent these from an equipment rental store. Packing the soil keeps the floor from bucking or moving around because you’re giving it a firm bed to rest upon.

At this point, you could pour a slab foundation, but since a slab relies on processed concrete, I prefer other methods. One of these methods is to use paving stones, or patio stones. You may have enough flat stones around your site to be able to successfully ‘pave’ your floor. If you don’t want plants growing up between the stones, first lay down a layer of landscaping plastic. You may then place the paving stones directly on top of the plastic. For a weatherproof and waterproof seal, you can place mortar between the stones. If you’d prefer not to use mortar, you can also fill in the spaces with sand.

An inexpensive alternative to paving stones is to use salvaged granite. The current popularity of granite countertops in higher-end homes has led to granite shops popping up all over the place. When counter tops are cut out of granite slabs, the leftover pieces aren’t big enough to be useful to the shop owner, but they’re the perfect size for flooring for a cob home. You may be able to work out a deal with a local granite countertop installer to take his leftover pieces away for free, or at a substantially reduced rate. Granite is usually thick enough to make excellent flooring without having to resort to additional sub-flooring. If in doubt, check the local building code.

Probably the most readily available material for making a floor is used brick. If you live in a rural area, there are sometimes brick chimneys out in fields that were once the centerpieces of homes that are long gone. You may be able to work out a deal with the owner of the property to haul away the bricks for free if you agree to tear down the chimney. If you’ve never done this sort of thing before, it’s best to check with someone who has. Such demolition work can be quite dangerous for the inexperienced.

A safer alternative is to check the local trade papers or salvage building supply stores. Used brick can sometimes be obtained for as little as five cents apiece through such outlets. Another possibility is to hang out around building sites where masonry work is being done. A good mason always orders more brick than he will need, so there is usually a few dozen bricks left over after construction is completed. Most builders would be happy to have you haul these away for them so they don’t have to find a way to dispose of them.

In addition to being highly durable, brick allows you the flexibility of many patterns and regular designs. Once you have your design, you can make the floor permanent by adding mortar. If you’d prefer to avoid processed materials you might try natural plasters as a binder.

The ultimate in environmentally friendly flooring is an adobe floor. Such a floor uses natural materials, but affords you more protection from the elements than a natural dirt floor.

An adobe floor is made much like a natural plaster. Its ingredients are clay, sand, and a binding agent. In an adobe floor, the sand acts as an aggregate, and the clay acts as a binder. Try a 1/3 clay to 2/3 sand mixture, adding enough chopped straw to hold the mix together. Use wheat paste or cow manure as a binding agent. Just add enough to hold the ingredients together. You’ll want to experiment with small patches to make sure you have the mix right before you do the whole floor. If the floor cracks, add more sand. If it crumbles, add more clay. Once you have the mix correct, cover the floor area with about 2 inches of gravel to act as a base for the floor and to conduct moisture away from it.

Then spread out the adobe material over the top of the gravel, using a trowel. If you’re making a large floor, you will want to score the adobe at regular intervals (making one square-foot tiles). Leave ¼ inch grout line. This line can be filled with sand. The scoring keeps large adobe floors from buckling or cracking.

Build up the adobe in layers, about 1” thick each. As each layer dries, you’ll want to weatherproof it. To weatherproof each layer, coat with hot linseed oil. You’ll want to heat the oil so that it penetrates better. Give each layer at least four coats of hot linseed oil before adding the next layer. You’ll want to do at least three layers. I’d recommend four layers total. This makes a nice, weather-treated adobe floor four inches thick.

Adobe floors are the ultimate in natural flooring. They’re the perfect complement to a naturally built cob home, as these pictures demonstrate.