There are many way to build small concrete vaults. Here I'd like to present a system that can work well for large (24 ft) reinforced concrete shells as well as for small, almost un-reinforced, light weight shells . This system has worked well for me in Mexico and can be adapted to a more mechanized approach for use in the first world.

The basic idea for the formwork is to use two or more layers of welded wire. The first, lower, layer of welded wire is FORMWORK. It is removed and reused.The second upper layer(s) is placed inside the roof pour for reinforcing. Once you understand this you will "get it"

THE LAYERS - bottom to top:

  • 2"X2" and 2X4 pine--formwork, (bamboo works well here) holding up the #66ww formwork It is removed after the pour.
  • 26 gauge non-galvanized plaster lath -- inexpensive in Mexico. (You can substitute plastic mesh onion sacks or burlap on top of reusable chicken wire, for this lath layer, many possible options here)
  • 3/4" Shell-- 6 parts light weight aggregate, one part cement-by volume --this mix depends upon the aggregate used. If you don't have light weight you can use a mortar mix ( 4 sand , 1 cement, 1 lime)
  • #66WW mesh -- it can be thinner or thicker than the formwork mesh -- this mesh stays in the pour. Note that rebar is almost never needed in the membrane of small vaults.
  • 4-8" pour with light weight concrete-- mixed 5/1 (structural) to 8/1 (insulative) OR 2 1/2" hard concrete
  • 1/8" polish -- mix: 3 screened sand /one cement, sponge finish. Polish the same day as the pour, it sticks better and can be thinner.
  • Fill low sides of vault with loose light weight aggregate (or empty soda bottles), to level out for the second floor
  • Pour 2" flat floor for second story-- mix: 2 parts light weight aggregate, 3 sand and pea gravel,one cement--volume

Formwork for the shell -- same idea, two different roofs
Laying on the Initial Shell

The light weight shell and the #66WW are in place -- ready for the roof-pour



Movable Formwork

Leveling out the first floor to build another floor on top


Formwork Needed
for each 4X5 M vault:

  • 10 each 2 X 4, 8 foot, pine (or corresponding bamboo) These need to be straight
  • 20 each 2 X 2", 10 foot pine . Seconds OK here
  • 33 square meters plaster lath
  • 33 square meters #66 WW driveway mesh
  • some nails and tie wire
  • Cost of formwork-- less than $50 per vault, for the first four.The formwork can then be reused an additional 50 times. With a little imagination you can use the same formwork for both, 4M wide vaults and 3M wide vaults.

Drive 8d nails with tie wire, pig-tails into your light weight vertical wall blocks at 1.7 M height from floor (will vary), to tie in the sides of the #6666ww barrel vault formwork. Drive 20d nails (or pieces of 3/8" rebar) into the wall blocks above the formwork to grab the pour.

Once the welded wire is attached to the side walls use the 2 X 2" pine, to push the #66WW formwork ito place to form a nice even vault. Then 26 gauge lath (or alterative material) on top of the WW, and start pouring the shell. This is as simple as it gets for concrete vault formwork.




It's good to have a movable scaffold below and a fairly well attached "skywalk " above when building larger vaulted roofs.

A 3/4" light weight shell goes on top of the plaster lath and before the pour. We typically "lay" this on with brick trowels. Dig mortar out of large buckets and just gently set in on top of the lath. If you try to spread it around too much or completely cover the lath you will find that the mortar wants to fall through the lath. The next day this shell will be semi-hard and you can easily bang out spots that aren't right (warts) and apply another thin coat to cover the bald spots. Applying a little bit at a time is the key, especially when doing complicated shapes or when using very light weight formwork. After the shell sets up for a couple of days a small person can almost walk on top.

Bolsters space the layers of mesh in the pour and are great labor savers. To envision a bolster think of long 3D triangles that form a 3/4" space. I typically cut them into 6" pieces. Place bolsters vertically on the shell membrane.This helps to avoid gaps in the pour near the bolsters and is easy to do when using mesh for reinforcement.

The compression stresses in shells are usually quite low, so the concrete strength is not the most important element in designing the concrete mixture.

The use of lightweight concrete for shells may save some weight, but this saving will result in little reduction in stresses, so there will be little reduction in the quantity of reinforcing. So you won't save on reinforcing but this isn't a big cost in small vaults.

If soil conditions are marginal, the reduced weight on the footings may save some concrete.--- from Ketchum

scaffold Set-up

Top forms are sometimes helpful when placing on nearly vertical surfaces.
See photos on right


Construction joints (between pours) are inevitable with most small, scale,site-built vaults. Keep seams to a minimum by planning for them. Since you will probably be pouring the roof in sections. Have the shell done in advance and start spraying / placing / pouring on Monday (not Friday) and pour a planned section everyday until you finish to avoid "cold" seams. Waiting more than 24 hours between pours is not recommended. Use pure cement slurry (neat cement) at daily seams. Vibrate and consolidate well at the seams .I have observed the cracks on my vaults over the years and I would say that less than a third are associated with seams so most cracking doesn't involve problems with cold joints .

From Ketchum -- Construction joints should be planned and specified. If the shell membrane is thin, usually the stresses are fairly small, and no special reinforcing is required at construction joints. The most important detail is that the screed at the edge of the placing area is carefully fitted to the reinforcing, and that careful preparation of the surface for the next concrete placing is provided. In edge members, the usual good construction methods for beams, girders, and columns should be followed. An underground BV built with cold joints.

Leche means "milk" in Spanish. Lechada is a milky mix of just cement and water. We use gallons of the stuff. It goes between the initial shell and the 4" pour on roofs and is used at all construction joints.

Put 2 gallons of water in a 5 gallon bucket and sprinkle in dry cement. Mix occasionally until you have a milky consistency (thinner than cream). Use a one liter container to splash it on clean, rough, wetted, concrete that is less than 24 hours old. Vibrate and consolidate at the joint.

Apply the lechada and immediately place the concrete. Don't let the lechada dry out on top of the old concrete.

Mix only enough for use within 2 hours. During this time add more water to keep it soupy.

I have seen latex bondo "not work". Instead of sticking surfaces together, it acted as a release with really bad results. Acrylic latex is much trickier to use than neat cement and I now only use it for small patches in a controlled environment.

When we place 3,500 psi concrete for a hard structural shells, the pour should be vibrated in a 24" grid pattern. Shells are thin and vibrations only travel a short distance. Just a touch with the vibrator will do. Vibrating is especially important at colds seams.

For 1,500 psi semi-structural with one layer of WW mesh we basically bang on the mesh so the mesh grabs the mix and cut into the mortar with a brick trowel to fill voids. We don't usually use a vibrator, but give special attention to construction seams. They must be clean and damp and use neat cement binder.

When placing 1,000 psi. light weight for insulation we don't vibrate at all -- splash on lechada as binder and just rake the concrete around to fill voids.

Pour / place the concrete from the bottom of the vault to the top. Don't try to dump all the concrete on top and rake it down. This can result in form blow-outs. Concrete will also consolidate better when placed or sprayed from the bottom up. (I have violated this rule a bit when I've had a good guy on the vibrator and we were getting good consolidation. In that case we slowly placed from the bottom up and from the top down and met in the middle).

Plan scaffold set-ups early on to be able to reach all parts of the roof.

"he selection of the type of placing equipment, whether by pumping, by bucket and mobile crane, by movable wood runways and carts, should be the decision of the contractor, based on factors such as the equipment available, the steepness of the slopes, the form and shape of the shell, and the distance above the ground. - Ketchum

Here in Mexico we usually place using people and five gallon buckets. In the first world, there are other options -- spraying mortar with a small pump rig or raising wheelbarrows of concrete with a front end loader. I have been working on various bucket lifter designs lately -- stay tuned.
Small hopper sprayers (Tirolessa) work well only for very thin concrete shells and not overhead. Try to keep overhead plastering / placing to a minimum -- it is a nightmare. When building vaults it is always easier to place concrete on top of formwork rather than trying to fill from below.

More on placing methods soon. Please send your ideas on lifting and placing. Send an email to Steve at: mxsteve at gmail dot com.

Top Forms

note -- that is what happened -- the mixer tried to lift itself to the second floor-- exciting!

I take down the formwork (de-center) after 3-4 days when using light weight concrete in warm weather. I don't walk on top for another 3-4 days. This delay needs to be planned for from the start. Hard concrete shells can be de-centered sooner. After the formwork is down always put back a few supports and leave them in place for 10 days. To cure the concrete keep it wet and bagged for 10 days.

It may be possible to move forms on a 24 hour schedule if you use early high strength (type III ) cement and the vault is re-shored at once. I have never done this.

You need to keep all new concrete roofs wet for at least 10 days. A 24 hour sprayer system is almost never practical, so cover the vault completely with several medium sized blue tarps (easier than one huge one) and wet underneath, once a day. Ideal cure for hard concrete is 20 plus days but I usually end up needing to move and reuse the tarps within 10 days.

Ideally my light weight pumice (pozzolanic) roofs should be wet-cured for 2 years. This of course isn't practical so I end up having to add more cement to the mix to gain a faster initial strength.

Yes there will be cracks. One of my 4 X 6 M light weight vaults will typically have about 1.5 M (4 ft) of superficial cracks ( less if I use fibered concrete on the final polish layer). Most of these are near the ridge and appear to me to be related to shrinkage while curing. These cracks are visible, but not deep, but they will leak water and need to be patched -- which is easy with light weight concrete.We have had excellent success patching these roofs and patches almost always "take". Simply chisel out the offending crack with a wood chisel, wash out the crack, paint on a cement slurry to bond, and then polish with fibered masonry.

Cracks are more of an issue with hard concrete shells because they can't be easily repaired. Use fibers or lath in the outside layer of hard vaults to avoid them.

Even with a few initial cracks, a well-built and maintained concrete roof should last well over 200 years.

Note: Cracks correspond to the illustrations in Billington's "Thin Shell Concrete Structures" see: Design of Cylindrical Shell Roofs

Walls Photos to come


Sealing: Here in Mexico, we've been sealing light weight roofs with polished, fibered mortar. They are then, usually, painted later with latex roof paint. In harsher climates, one has to resort to more exotic membranes.

Central Mexico --

In central Mexico, we have very few freezing rains (It’s hot during the rainy season). When sealing concrete roofs, freezing rain separates the men from the boys -- and we're boys. When a wet roof freezes -- that's bad. Here, latex roof paint every five years works well. Five year old roofs, painted white, turn to a light gray elephant scale -- that looks OK. So just paint the cracks every five years (or whenever you notice wet spots on the interior ceiling) with the same light gray color. Add concrete to the paint to adjust the color. You can be pretty laid back about the whole sealing thing here in this climate -- when you have a well drained, vaulted roof. Flat, non-vaulted, roofs, on the other hard, are a nightmare.

I would suppose that these ideas for my roofs apply for the same roof built in the climate of India, Thailand, or Hawaii .

My new idea in this climate is to not seal the roof -- to let it breathe. Damp spots only further cure and harden my pozzolanic aggregate -- at least that's what I tell the wife. Or simply patch the few cracks that might develop in the first few years.

Frozen North--

I have several ideas, but no guaranteed / one-size-fits all, recipe for sealing a concrete roof where you have freezing rain .Below are some plans that look good.

  • The Eugene OR, Plan --- (from inside out) interior plaster / structural concrete roof / 4 inches “blue board" rigid insulation / 40 gauge. Pond liner / 1 inch fibered mortar on chicken wire. The pond liner could last for 200 yr. with some maintenance of the concrete cover coat.
  • The Santa Cruz , CA Plan --- this one has been up for 25 years and looks good for another 200 yrs. Interior plaster / metal-fiber, reinforced concrete shell / sprayed, urethane foam insulation / 3/4" fibered cement / elastomeric paint
  • The Eastern WA Plan -- interior / structural roof / glued on rigid insulation / glued on EPDM membrane.
    Edge details important. For high wind areas, optional straw bales (California look) or used tires (Mississippi look) on the roof to help hold down the membrane? Replace EPDM every 40 years?
  • The Utah Plan -- Structural shell / rigid insulation 2 X 2 = 4 inches white Styrofoam / 24 gauge galvanized plaster lath / 3/4" fibered concrete. This roof is still unsealed but will likely get painted with elastomeric paint. The heavily fibered (.5% by weight) mortar will be very good against spalling even without the paint.
  • The Tucson, AZ, SB, Plan-- 1 1/2" structural shell / 40 gauge pond liner / 4" straw compost / straw bales (SB) wired to stay in place / optional blue tarps for rainy season. This has to be the most cost effective, R-50 roof in the world--- even with USA wages in Tucson.-- Hey, this could even work in Iraq-- and maybe in Minnesota??

See Vapor Barriers



10-25cm thick -- Block made of the same light weight volcanic aggregate. 10:1 cement, vibrated. Poured, hard concrete, reinforced, vertical, pillars every 3M approximate. Reinforced bond beam at top of walls, tied to 2" thick, second story floor. Foundation and wall costs are best calculated on site.

Ketchum- the first half of this deals mostly with large shell / vaults-- start half way.

For more complicated forming systems -- see forming system

For inflatable forming systems -- see construction

mxSteve at gmail dot com
You can reach Steve at the following address:

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