The Small House Book (8 page)

Trinity Park, MA (top) & a San Francisco Bungalow Court (above)

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You know you have perfection of

design not when you have noth-

ing more to add, but when you

have nothing more to take away.

-Antoine de Saint Exupery

How to Build a House on Wheels

The Foundation and Framing

With little exception, my first portable house was built by using the most stan-

dard methods of construction. Like any other mobile home, my structure sit

on a steel chassis – in this case, a 7’ x 14’ flatbed, utility trailer. I took most of

the wooden deck off to save weight and put aluminum flashing over the gaps

to safeguard against mice. The floor framing was laid on top of that. I used

two-by-fours spaced about 24 inches apart on center.

Once that framing was assembled, I filled the cavities between the boards

with foam board insulation and spray foam and capped the whole thing off

with some ¾-inch plywood subflooring.

The walls were framed right over the wheel wells using headers just as you

would over any other opening. I used two-by-four studs and rafters spaced

twenty-four inches on center rather than the more typical sixteen inches. This

is a fairly standard practice used to save both money and natural resources.

At this point, I was using it primarily to save weight. My flatbed was rated to

hold 7,000 pounds.

Bracing

Tumbleweed would have to withstand not only the normal wear and tear of

everyday living, but also the occasional jolts and gale-force winds generated

by highway travel. To prepare for this, I used what has come to be called the

“screw-and-glue” method of sheathing. This means that a bead of construc-

tion adhesive was squeezed onto the entire length of every framing member

before 3/8” plywood sheathing was screwed (not nailed) to its surface. This

makes for a structure far more resilient to lateral wind loads than sheathing

secured with nails alone.

before 3/8” plywood sheathing was screwed (not nailed) to its surface. This

makes for a structure far more resistant to lateral wind loads than sheathing

secured with nails alone.

Preventing Condensation

The only other special building consideration, after the foundation and bra-

cing, for a little house on wheels is condensation. Unless they are insulated,

sealed, and vented properly, small spaces are prone to a lot of condensation.

It simply takes less time to fill the air in a small enclosure with the moisture

caused by bathing, breathing, laundry, and cooking than it does to fill a large

one. If that warm, moist air comes into contact with a sufficiently cold surface,

it will condense into water. That is the reason that cars come equipped with

defrosters, and that small houses need to be equipped with the right insula-

tion, vapor retarders, and ventilation.

I used expanded polystyrene foam board as insulation with expanding spray

foam in the seams for two basic reasons: 1) It takes a thicker piece of fiber-

glass batting to get the same amount of insulating power as you get out of a

piece of extruded polystyrene. As I didn’t have enough space for eight-inch-

thick walls, this would have stood as reason enough for my choice. 2) Foam

board is far more resistant to condensation.

With fiberglass batting and other porous insulations, you have to worry about

moist air getting into it and condensing when the moisture gets to the cold

part of the wall. At that point, the fluffy, pink stuff turns to mush, and mush

doesn’t insulate. It rots. To prevent this, you have to use a vapor retarder.

This is usually just a large sheet of six-millimeter plastic hung over the inside

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surface of the batting and sealed at its edges. If your seals hold and your

plastic does not rip, your fiberglass should stay fairly dry.

Expanded polystyrene with an impermeable coating does not need a vapor

retarder. Being virtually waterproof makes it its own retarder. I chose the

white, expanded polystyrene over the pink, extruded poly because, while I

love the pink stuff for its superior insulating qualities, bugs love it, too.

The threat of condensation is also what prompted me to use double-glazed,

insulated windows. The glass panes on a little abode can fog up pretty quick-

ly unless they are well protected against the cold. I’ve found that windows

sold with gas between the interior and exterior panes work pretty well for this

purpose.

The other primary way to eliminate condensation in a small enclosure is by

venting it. I installed a fan at the peak of my loft. It sucks moisture-laden air

out of my living quarters when I am cooking or bathing and helps keep the

place cool during the summer. On cold days, the vent can be sealed with a

plug I cut from some leftover scraps of foam board.

Tools

My tools are pictured on the facing page. They are pretty much all I have

needed to build a dozen small houses. Folks I’ve worked with tell me I’m a

fool for not using a table saw, too. You might want to add one to your list.

1. skill saw, 2. jig saw, 3. plyers, 4. files, 5. miter saw, 6. hammer, 7.

wrench, 8.goggles, 9. tape measure, 10. drill & drill bits, 11. pencil, 12. box

cutter, 13. level, 14. chisel.

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1

4

3

2

14

12

13

5

6

10

7

11

9

8

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Step-By-Step Instructions

1) Buy your materials and

order your windows. Be sure

the trailer will accommodate

the weight of your house.

Cut any extra vertical parts

off the trailer, but leave the

wheel wells intact. Remove

all the decking you can.

Leave no more than 24” be-

tween the remaining boards.

These gaps should be cov-

ered with aluminum flashing

to guard against rodent and

water infiltration. Do not put

any beneath the porch.

2) Assemble the floor framing

in front and in back of the wheel

wells. Then connect the two

sections by framing between the

wells. Use screws instead of

nails for this and all your fram-

ing.

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3) Fill the cavities with your

choice of insulation (in this

case, expanded polystyrene

foam board with expanding

spray foam at the seams).

Once again, the porch area

should be left open to let wa-

ter drain through it.

4) Once you cover the whole

thing with 3/4” flooring or a

subfloor, the exterior wall

framing can be erected all

along the perimeter. Connect

the walls by driving screws

through the bottom plates

into the floor framing below.

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