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Root Cellar Part 4: Roof

Updated: Feb 14

Getting our root cellar walls built was a huge accomplishment, but now we are moving on to the more complex roof. We did so much research to figure out the best way to accomplish this portion of the project and I am happy to say it has turned out beautifully.

To start with we needed to build a wood form to hold the cement up until it had a chance to cure. A flat roof is a bad idea in a root cellar because there can be condensation. If it is an arched roof, it will not only have any moisture drip down to the sides, it will also be much stronger.

Cut plywood

cut plywood

We started by cutting our 1/2 inch plywood into 8 foot long 11.5 inch strips. We did them this size so we could fit 4 on each plywood sheet and it ended up having a nice arch to it at this size. We then marked the center and cut a 3.5 inch deep, 1.5-inch wide notch out of the top for the backbone. We then used a section of wire fencing to make our line for the arch we placed it 1 inch up from the base at each side and had it arched up to the top of the board and I drew in the line. We then used a jigsaw to cut out the arch. After we created the first one we used it as a template for the other 8 so that they would all match.

Dado Blade

dado blade

We then used a dado blade to cut in a 3/4 inch notch in (9) straight 10ft 2x4s.

Cut a groove in the 2x4.

grooved 2×4

The arched plywood fit nicely in this groove. These needed to be marked and cut for their specific location in the cellar as there were slight variances in the walls and we wanted a snug fit.

Posts with the top runner for the root cellar build.

posts with top runner

We also needed to measure and cut (2) straight 16ft 2x4s for the length on the sides specific for each. These runners are what we placed our arches (inside the grooved boards) on. We also measured each vertical post location to position the edge of the arch at the edge of the wall. We then cut them to fit. We made (5) 4×6 posts on each side.

Upside down arches set across the center of the root cellar.

arches set across the center

After placing the posts upright with the runners on top of them we placed the arches up to connect the two sides. Placing the grooved boards across ties them together and helps provide a little stability. We had already placed the plywood inside the grooved board but if I could do it again I would wait until later. We had a few posts fall before we had it all put together and that was a little scary.

Place the spaces between the arches of the root cellar roof.

place in spacers

The next step is to place spacers between the arches to keep them from moving. It was a little hard to make this frame because the whole time we were very aware of the fact that we would need to take it back apart from the inside. After the spacers were in, it started to feel more solid.

Set the backbone of the root cellar arches.

place in backbone for arch.

We then placed a 2×4 the length of the cellar in the notches we had cut at the top of the arches like a backbone.

Place plywood spacers on the small ledges of the root cellar arches.

place plywood spacers on small ledges

At this point, we added 8ft strips of plywood that were measured exactly for the distance between the plywood arches. These were placed on the little ledges next to the grooves and slid right into place. This keeps the arches in place and helps distribute more of the weight. Then it was time to head to the inside.

Connect posts to the arches and runners to the spacers of the root cellar roof.

connect posts to arches and runners to spacers

From underneath we used scrap to connect the posts to the arches and the runners to the spacers.

Reinforce the base of the root cellar.

reinforce at base

We also used scrap wood to place bracers along the bottom of the posts.

Place additional spacers between the arches.

additional spacers between arches

With the underside stabilized, we then put some more spacers between the top plywood arches. We put these in a staggered pattern to make them easier to put in and get out later. These are not only for spacing but provide a location to attach the plywood sheeting to the top of the arches.

Attach the plywood to the top of the arches of the root cellar roof.

attach plywood top

We decided to place the plywood on starting with the center on the backbone. We would then walk to the end and attach it as it bent with our weight. We had about a 3-4 inch strip on each side that still needed to be added but this kept the center, more visible area cleaner.

Add plastic and foam to secure and prevent leakage.

add plastic and foam

We used 6-mil black plastic to cover the top and construction foam to fill in all cracks.

Grid rebar over the top of the root cellar roof.

grid rebar over the top

We then needed to grid #4 rebar all across the top and connect it to the wall rebar. All of the rebar needed to be 2-2.5 inches from the roof all along the arch so we had to bend it just right. We also placed small bits of cinder blocks under the grid to keep it from sitting too close to the frame. See our video for details.

It was tempting to feel like we were close to being done with this portion of the project when we realized just how much work there was left to do for the exterior concrete form.

Backfill to ensure the cement does not put too much pressure on the forms.


The first step was to backfill. The top of the cellar was about 4 feet in the air, so, instead of building a form that high, we piled the dirt back around it to help hold in the boards of our form. We would need to rebury it eventually anyway. We wanted a 5-inch roof so we reused the 2×6 boards from our floor pour for the sides.

Front view of the roof form.

front of form

The front and the back were a little trickier. Since the top is arched, the center of the front/back needs to be higher. We just used some plywood scrap and made sure it went higher than we needed. For the front, there was no dirt due to the stair/door section, so we provided some diagonal supports. They look pretty wonky but they did the job.

Fill the root cellar form with concrete.

fill with concrete

Because we wanted a nice clean pour we decided to order in a batch of concrete. We had them add fiber to increase the strength of our roof. We also decided to have a pump truck come so we didn’t have to do buckets of concrete to the top. While it was as expensive as the concrete my husband thinks it was totally worth it.

Form removed from the root cellar roof.

outside form removed

We allowed the concrete to cure for 2 weeks before removing the forms. We started with the outside forms. This required a little digging to get to our screws but it came off without any issues. Now it was time for the nerve-wracking part: removing the inner form.

Preparing the remove the interior form of the root cellar.

getting ready to take down the inner form

All removed except the arches.

up to the arches removed

It was kind of funny because the whole time we were taking it apart we were afraid of it falling on our heads but it held up there until it was only the plywood left which eventually came down with a bang.

Arches removed from the root cellar roof and still the top has not released.

the arches are out and it is still not coming down

Removing the first sheet of plywood.

the first sheet was a little work to get out

The remaining plywood sheets remove together.

The next two came down quick. I would not have wanted to be in their way.

Some caution is definitely warranted.

Peel off the black plastic from the root cellar roof.

Black plastic still left

A beautiful finished ceiling of the root cellar.

Beautiful finished ceiling

Next came taking down the black plastic and what it revealed was a beautifully shiny, fully intact, perfectly arched ceiling. I was so pleased. We did some touch-up paint and then began to bury the cellar. Because I was so excited to set it up and my husband was so excited to get garage space back we started to fill the inside.

The next construction post will be on how to finish off the outside.

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