How to Build an SCA Practical Iron Age Celtic Roundhouse

In an attempt to add a more period feel to our camp, my fiancee and I decided to create a more authentic Celtic dwelling for us to live in at camping events. The first thought was to find a period Celtic tent design, and replicate it. After extensive searching, we found nothing useful. It seems that the Celts were not big weekend campers. There are some Roman tent designs from our time period (1st century A.D.), but we wanted something more Celtic in appearance. During a trip to Butser Ancient Farm in Hampshire, U.K. we saw interesting Iron Age Celtic roundhouse reconstructions that fit the bill quite nicely (see Appendix 1). The goal then became to replicate the roundhouse look, with something that was portable, roomy, and still structurally sound. The prominent details that I wanted to incorporate in the tent were that the roof should overhang the walls by a fair amount, the walls should be relatively short, the roof should have a peak at the top, there should be an entranceway that sticks out of the front, and that there would be no center pole to obstruct the usability of the inner space. I decided that a tent with a wooden frame and canvas skin would be the way to proceed. After investigating many alternatives, it seemed that a tent that used many of the construction techniques of a Mongol Ger (popularly referred to as a yurt) would be the most efficient and functional method to pursue. My first step in making a roundhouse was to decide what size tent we wanted to make. Although this description could be adapted to make practically any size tent, we decided to make an 18-foot diameter structure, with a 12-foot peak. We thought that this would be a comfortable living space, and also be big enough to entertain people inside when we chose to.

There are five major portions of the roundhouse that need to be made. These are the lattice wall, the roof rafters, the roof ring, the doorway, and the canvas covering. I will explain the methods I employed in fabricating each of these different parts in the following sections.

1 Wall Lattice

1.1 Calculations In making the lattice, you have to figure out how long and how high the wall would be. The length of the wall is determined by the circumference formula minus the width of the doorway. Since the diameter is 18’, the circumference can be found by the formula, C = p x d where C is the circumference, p is 3.14, and d is the diameter. For my tent, C = 3.14 x 18 which is 56.52 feet. Subtracting 3 feet for the width of the doorway, leaves us with 53 ½ feet of wall to make. Roundhouses have characteristically low walls, so in order to stay in proportion, we made our walls 4 feet high. Figuring that the lattice would be opened so that the openings were roughly square, you can determine the correct length each of the laths in the lattice. To make square openings, all of the laths need to be at 45° to the horizontal. This means that if the height needs to be 4’, the length along the ground is 4’, and the lath length can be given by Pythagoras’ equation shown in the following figure:

The laths could be cut at 5’8” as the figure above dictates, however the topmost rivet will be spaced 3” from the top, which will affect the circumference. In order to compensate for this, I cut my laths slightly oversized to 6’ 0”.

1.2 Cutting the Laths I cut three 4x8 sheets of 3/16 ” thick plywood to 6 feet long, then ripped each 1 3/8” wide lath out of the remaining 4’ x 6’ sheet using a table saw as shown below:

After setting the tent up a few times, I have damaged many of the laths, and when I redo the lattice (hopefully soon), I will use ¼ ” marine grade plywood to avoid these problems and make the lath more resistant to dampness. This plywood will be more expensive, but it will give me more peace of mind. I suggest that you start with the ¼ ” plywood to save yourself the expense of having to buy plywood twice.

1.3 Drilling the Laths Holes are then drilled in the lath at 1-foot intervals starting with a hole 3” from the top.

In order to speed up the drilling, you can stack the laths into tall bundles, and drill the bundle all at once as seen in the following pictures:

Marking the hole locations with a completed lath.

Drilling the holes. The wood clamps are holding a fence that I am pushing the bundles onto to keep the holes in the middle of the laths.

1.4 Assembling the Lattice Once the laths are cut and drilled, it is time to attach them together. I used pop-rivets for my assembly because they were quick and easy, but bolts, knotted string or rawhide could be used. I found that a very tight washer was needed to stop the pop-rivet plunger from pulling through the lath. In my case, 3/16” pop rivets needed 4mm washers. It takes about 200 pop-rivets to complete a 53’ wall section. To assemble the lattice, lay several laths on the ground parallel with each other. Then lay one lath on top of them leaning in the opposite direction. Make sure that all the laths have the 3” hole spacing at the top, and the 9” spaced hole at the bottom. Put a pop rivet through the topmost hole in the top lath, and the first bottom lath, slide a washer onto the rivet from underneath, and “pop” the rivet. Put a rivet sequentially in the remaining holes on the top lath, then put another lath on top, and repeat. Looking at the picture below, notice how all of the laths leaning to the right are behind all of the laths leaning to the left. Without this, the lattice won’t fold.

Open the laths to the angle at which they will rest when the tent is assembled, cut one end square as shown in the picture below and continue assembling laths from one end to the other until there are 16 full crosses or X’s at the top, with three single laths that don’t have a pairing lath. This will eventually be one half of your wall.

To begin the other half, pair these unpaired laths (single 1-3) with the first few laths of the second section as if continuing the wall around, but do not rivet these “single” laths to the laths of the next half of the wall. I find the wall more convenient to lift and store in two pieces. These two wall sections will be connected with nuts and bolts during tent setup and unbolted for takedown. Be careful to continue this wall when making the second half, as it is possible to make two wall sections that will not knit together. Careful comparison of the picture of the real lattice above, and the drawn lattice below it will reveal that the two are incompatible! The picture has the lath closest to the reader leaning left, while the drawing has the close lath leaning right. They have a different “handedness”, and no amount of flipping and twisting will make these two sections bolt together correctly. By the time I learned this, I had wasted around ten laths. Continue the 2nd half of the wall until another 16 full crosses are completed. Cut the far end square in the same way that the beginning of the 1st half wall was cut.

2 Making the Roof Rafters

2.1 Cutting the Top End The next parts to be made were the roof beams. I used 1” x 2” x 12’ vertical grained fir boards, but there are lots of light, strong, straight-grained woods that could work. Cut their ends at 52° by cutting a block at 38° on a chop saw, then using that as a guide, cut the rafters with the chop saw set square (0°) as can be seen below:

Cutting them at this angle will allow them to sit flush to the top surface of the roof ring as will be seen soon.

2.2 Notching the rafters The rafters need to have a notch on them to sit in the crosses on the top of the lattice. This notch holds them in place, and keeps them from sliding. With a handsaw, notch the rafters about ¾” deep, at a 38° angle, 10’ from the top point, and 20” from the bottom. The finished rafters should look like the drawing below:

The 20” of rafter after the notch allows the roof canvas to overhang the walls, and adds to the roundhouse appearance. The roofs of actual roundhouses would overhang the walls to protect the mud daub of the wall’s wattle and daub construction from getting wet. Thirty-five of the 36 rafters need to be finished in this way. The last rafter will intersect the doorway ridgepole, and will be shorter.

3 The Roof Ring

For ease of measurements, my roof ring has 36 sockets to hold 36 roof beams. This means that each beam is 10° from the next one, which makes for nice, round numbers. For a smaller diameter tent, fewer rafters could be used. The ring is a sandwich construction having a top and bottom of ¼” plywood, and an inner core made up of many small pieces of cut wood. In the center of the ring is a threaded block of wood ready to accept the pole that holds up the peak. A cross sectional diagram can be seen in the next figure:

3.1 Cutting Out the Parts The block to the top-left of the above drawing shows how the angled blocks were cut from a 2 ¾” wide board at an angle of 38° set on the miter saw. These angled blocks are then glued and nailed (with a nail gun) to two square blocks, one on either side, the same 2 ¾” length, and 2 1/8” high. The following is a picture of the 36 angled blocks, and the 72 rectangular blocks after they were cut.

The next step is to cut out the top and bottom layers of plywood. First, establish a center point, and then using a piece of scrap wood with a screw in it as a compass, put a pencil through holes drilled in the scrap at 28 ½” and 22 ¾” from the center, and draw the inside and outside circle of the roof ring. The setup is shown below:

After the two rings are drawn, draw a line on either side of the scrap/compass, making a strip 3 ½” wide that passed through the center of the ring. This will hold the threaded block mentioned earlier. The top and bottom pieces are then cut out using a jigsaw, and the edges smoothed with a rasp.

3.2 Assembling the Ring The 36 block assemblies are glued and screwed to the bottom ring circle, starting with the four “corners”, and the rest were glued and screwed down starting with one quadrant at a time. These figures show the process:

Here are all of the blocks in place on the ring. Some of the inner surfaces of the blocks where they touch each other needed to be thinned so that all of the blocks could fit. This could be avoided if the ring was made slightly larger in diameter.

The next thing to do is to put a ¾” x 2 1/8” piece of wood brace on either side of the 3 ½” strip to which the threaded block could be screwed. I drilled large holes in these braces to try and keep the weight of the ring down. The finished ring with braces, threaded block, and top plywood attached is shown in the next picture:

4 The Doorway

One of the classic portions of a roundhouse is the entryway that juts out from the walls. In order to affect this on the tent, an inner doorway had to be tied to the walls, and an outer doorway attached to the inner doorway. For this, I used a pegged mortise and tenon construction to simplify portability and storage. The sides of the inner door are made by sandwiching a 2 ½” wide x ¾” plywood spacer between two ¾” x 3 ½” boards. The spacer was the same thickness as twice the lattice thickness, and was flush with the inside of the doorway. This created a 1” deep by ¾” wide dado in the uprights into which the lattice can be inserted. These dadoes are shown as grey areas in the drawings. This holds the lattice in place while setting up the tent. The doorway parts are illustrated in the following drawings:

The last two parts of the roof/doorway to complete are the two beams that form the apex of the doorway. They involve some special techniques, but are not too hard to manufacture. The beam that connects the top of the outer door to the inner door continues into the tent until it intersects the 36th roof rafter coming from the roof ring. This rafter is cut there to the correct length to fit into a sheet metal socket on the doorway beam. The basic idea is shown in the next picture:

The metal bracket is 16 ga. or thinner sheet steel or aluminum, the large holes are there to lighten the steel, and the smaller ones are to screw or rivet the bracket onto the rafter. The bracket should be laid out as in the following diagram:

The beam above the doorway with its metal bracket has the following dimensions:

The frame as it stands at this point should look like the picture below:

In this picture, you can see the two “belly bands” that are tied to the inner doorway and encircle the lattice; one is around ½ the way up, and the other is at the very top of the wall. These are necessary to stop the lattice from swelling out from the weight of the roof. Each band is, in fact, two straps that join at a tightening buckle at the back of the lattice. This helps to cinch the wall tight, and stabilize the roof.

5 The Canvas

5.1 Layout The canvas I used for the tent is Sunforger flame retardant, water resistant 12 oz. Canvas. I was willing to spend a little extra on the canvas, in hopes that this would be the last time I would have to pay for it in many years. There are many ways to lay out canvas to make a conical roof, but I chose to use the method that had the fewest seams, as I figured that the sewing would be both the hardest and most time consuming part of the project, I thought that I should minimize it. The calculations are probably way too involved for their own good, but I finally got to use some high school mathematics for something useful. To see the calculations in their entirety, refer to Appendix 3. First the cone shape is calculated, then the shape is laid out on flat canvas. The canvas I used was 60” wide, which made the layout relatively easy. The layout is shown in the following drawing:

It took 6 strips of canvas to complete the layout. Once these sections are sewn together, lay the canvas over the already set up frame, and check the fit. Clamp the wedge shut, and see how it fits the frame. It should fit well, without being too tight. At this time also, you can lay a piece of canvas over the doorway, and mark the interface seam between the doorway roof, and the conical roof. It creates a rather complicated seam with lots of fitting, but it can be done. Remember to measure thrice and cut tomorrow. The final roof shape (before sewing the cone seam) should look like this:

With the doorway roof sewn on, the entire, monolithic roof, as viewed from above, should look like the following picture (the three newest seams are shown in red):

Once the sewing is completed, the roof should be placed back on the frame, and the alignment of all of the rafter ends should be marked on the outer rim of the canvas. This will show where grommets should be placed in the canvas to tie the roof onto the frame. Compared to the roof, the rest of the sewing is dead easy. The wall surrounding the lattice is merely a 54’ long, 4’ high, hemmed sheet with grommets around the top to tie it to the lattice X’s and some at either end to tie it to the door frame. The walls of the doorway are two panels 3’ wide, and 5 feet high, with pockets sewn at either end through which the top and bottom entryway beams slide. The cross section looks like the following:

The roundhouse is now pretty much complete. All that needs to be done now, is to set it up, and live in it for a while.

6 Setup

Setting up the roundhouse can take some time, but if things are done in the right order, lots of hassle can be averted. The Doorway is assembled first, including the canvas walls. Insert the tenons though the canvas and the mortises, and then drive the pegs in, securing the joints. Then stretch open the two lattice sections, bolt them together, and tie them into the slots in the inner doorway. The belly bands should be put in place and snugged up as needed during the whole process. Next, someone on a short ladder should hold the roof ring in place while roof rafters are inserted in their correct slots in the ring. Care should be taken to count the ring slots and the lattice X’s to put the correct rafter in each slot. Otherwise the ring will twist, and the rafters will bend. Once all the rafters are in, they should be secured into their X, while also hanging the wall canvas, by knotting a small piece of twine around the wood and through the canvas grommets. Next the roof canvas goes on from the entryway point over to the other side, to keep it from being rotated with respect to the frame. Grommets on the outer edges of the roof canvas can be used to tie the roof under the rafter at the end, and to the lattice. The last step is to put a short, threaded dowel into the threaded hole in the roof ring, and raise the peak. In the future, the peak will be made with a separate cap that can be raised and lowered by turning the threaded dowel to which it has been attached. A drawing of this mechanism can be seen here:

Appendix 1 Reconstructed roundhouses

The Large Roundhouse reconstruction at Butser Ancient Farm in Hampshire, U.K.

The reconstruction at Flag Fen:

Appendix 2 The completed roundhouse

Appendix 3 Roof Math

From the edge of the rafters to the peak of the roof is around 13 feet. If you add 6” for the canvas to roll under the rafter at the bottom, the canvas layout can start with a circle with a radius of 13’ 6”. Anyone who has made a cone before could likely tell you that a cone is made from a circle with a wedge taken out of it as in the following drawing:

The question becomes, how big is the angle ?? The answer can be found using some geometry. First, we need to find the diameter of the cone of canvas when it’s in its final shape. That is, if you were hovering directly over the roundhouse looking down at it, how big would the circle look to you? We know that the roof rafters hang over the by 20”, at an angle of 52° to the vertical. In pursuit of excessive mathematics, the amount of overhang can be calculated in the following way:

If we take the resultant 15 ¾” overhang, and add that to the diameter twice (once for each side) we see that the circle would appear to be 18’ + 31 ½”, or around 20’ 8” in diameter. The circle above has a diameter of 13’ 6” x 2 = 27’. That means, to make a cone with the correct shape, you should cut out a wedge big enough to leave 779” around the outer rim as shown below:

To find the angle ?, the ratio can be set up using angles of a circle. A circle has 360° all the way around the perimeter, and for a giver radius r, the circumference c can be written as c = 2pr. This equation assumes that we care about the total circumference going around all 360°. What if we want to know what ½ the circumference is? This corresponds to the circumference described by a rotation of 180° of the total 360°. For any random angle x, the circumference of the wedge can be described by the formula:

Solving for the angle x, we get:

since we know c = 20’ 8” x p = 779”, and we know that r = 13’ 6” (156”) we can solve the equation and find that x = 286°. This means that ? = 360° - 286° = 74°

Appendix 4 Materials list

3) sheets 3/8” Marine Grade plywood

1) sheet ¼” plywood for roof ring

36) 1”x 2”x 12” vertical grained fir boards

11) (approximately) 1”x 4” wood of your choice (for doorways, doorway rafters, and roof ring blocks)

Scrap amounts of ¾” plywood for the core of the inner doorway

Scrap amounts of sheet metal for the rafter bracket

120’ cotton webbing or rope for the belly bands twine

65 yards of 60’ wide canvas

Optional: material to make a 3’ x 6’ door

Door hinges, handle and latch.