Portcullises

I’ve done a little reading on portcullises and castle gates. This was sparked by a discussion in a Dungeons & Dragons (DnD) Facebook group about whether an Immovable Rod would stop the descent of a portcullis. Which led to the question of how heavy a portcullis is.

I have yet to find an actual figure for the measured weight of a surviving medieval portcullis, or even a convincing later version in the medieval style. I’ll update if I do.

So I think we’re left with estimates based on the size, structure and material of portcullises. I’m not the first person to try this approach.

There’s one here (on page 1 of the thread, by Zophos) assuming they are made of something like six-inch diameter iron rebar. I find this implausible: there may have been iron-grille portcullises at some stage of history, but six-inch bars I doubt. But I link the thread because it was cited in the Facebook discussion that got me onto this.

There’s one here (on page 1 of the thread, by Curmudgeon) using some real-world information from Bodiam Castle and filling in some gaps to estimate a weight for an oak portcullis of closely set 4-inch square oak timbers with iron cladding. This I find much more convincing. (The weight number I arrive at below is lower, because I’ve assumed somewhat wider gaps in the grate, plus the iron facing somewhat thinner and only on one side.)

But, thanks to the wonderful Castle Studies Group and the members of the Medieval Buildings Group on Facebook, I’ve got a serious paper on castle portcullises and front gates surviving in Britain. So in the rest of this post I’ll try my own version of the weight estimate for a wood-and-iron castle portcullis.

Real-world information and estimate based on it

Let us take the example at Bootham Bar in York, described on p256 of the CSG paper (it isn’t anything like that long but it doesn’t start at page 1—I think it’s originally a chapter or article), which is a 14^th-century construction on a city gate.  I’m choosing it because it is described in the most detail. The overall size is 19ft high by 12.25ft wide. The paper reports it is made of 4-by-3in timbers, set 7in apart. A photograph shows that there are 13 uprights (which actually I think suggests that the gapping should be more like 8in, for a total of 1ft per timber-and-gap, with the extra quarter-foot provided by the 13^th upright; I’m going to go with that because a repeating 12in unit makes it easier to do calculations for larger and smaller portcullises). The photograph also shows 13 horizontal bars to the grille, but there are more sunk into the ground slot that the portcullis lowers into. I will estimate 4 more bars, with probably some of the overall height contributed by spikes along the bottom of the portcullis extending beyond the bottom horizontal bar.

English oak has a density of about 0.72 g/cm³. Since I’m working in imperial units here (to contribute to a DnD discussion), that comes out to 45 pounds per cubic foot. So how many cubic feet of timber in the gate?

Let us first work out the front surface area of the portcullis in square feet. (We will want this number again later if we have iron facing.)

Thirteen uprights, 4in wide and 19ft long total in area 82.33 square feet. I could take a bit off for the bottom of each upright tapering to a point, but let’s not complicate things.

There are also horizontal timbers. In the CSG’s photographs, they appear to be jointed flush with the uprights, so we are only adding surface area and volume for the lengths filling the gaps between the uprights, not the entire width of the gate. With 12 gaps each of 8 inches then there is 8ft to add for each horizontal bar, With 17 bars, each 8ft long and 4in wide, that’s another 45.33 square feet.

This makes the total frontal area of the portcullis 127.67 square feet. Call it 128.

So, if the gate timbers are 3in thick front to back, then that’s 32 cubic feet of oak in the gate. At 45 pounds per cubic foot, that’s 1440 pounds. Adding a little for fixings, you could say that a basic wooden portcullis of this size might be 1500 pounds or so.

Some of the portcullises are faced or clad with iron. I have found one photo, of a surviving portcullis at Bodiam castle, that shows the iron in close-up. The iron appears to be only on the front surface of the portcullis (though I think wrapping around the sides and maybe back of the lower spikes), and it appears to be thin compared to the timbers—I’m going to guesstimate a quarter inch, while noting that because iron is so heavy any error here has quite a large impact on the overall weight. Returning to our area calculation for the portcullis of 128 square feet, that would be about 2.67 cubic feet of iron.

But the photo looks to me to show the iron strips overlapping where the timbers join. On our model of 4-inch beams with 8-inch gaps, that’s about 20% of the surface area to be double-plated, so we need to go up to 3.20 cubic feet of plating.

And then the gate is held together by a large iron nail at each joint. We need 13×17=221 joints. Suppose each nail is 4in long (all the CSG portcullis photos seem to show a nail-head whether taken from the front or the back, so I’m taking it that they go all the way through with a head each side and are more like rivets than what I would call a nail), averaging half an inch thick, and square. That would be one cubic inch per nail, or about 0.13 cubic feet of nails. So we are saying about 3.33 cubic feet of iron in the whole structure.

The density of iron is about 7.9 g/cm³ or 491.5 pounds per cubic foot. So our 3.33 cubic feet weigh about 1637 pounds. If we add this to the 1440 pounds of wood we get, for our example portcullis, about 12 feet wide, 19 feet high, made of 4-inch wide wooden beams spaced 8 inches apart, 3 inches thick and faced with iron: 3077 pounds.

Portcullises of other sizes

Not every portcullis you might want to put in your game will be exactly that size, so it would be useful to generalise. Let’s consider a one-square-foot unit. Take the top-left corner for illustration, but this is replicated in every square foot across the structure. There is a 4-inch wide 1-foot long upright down the left-hand side, a 4-inch wide, 8-inch crossbar across the remainder of the top, and an 8x8in empty square taking up the rest of the square foot. (The bars on the right and bottom of the square are part of the adjacent square-foot units.) If you imagine the square foot divided into a 3×3 grid, the 5 cells at the top and left of the grid are bar, and the 4 cells in the lower right are empty. So the area of portcullis in this square foot is 5/9 square foot. If it is made of 3-inch thick beams like our example, it will have 0.139 cubic feet of timber in that square foot of portcullis. If made of English oak it will weigh 6.25 pounds.

So there’s a useful figure: each square foot of a wooden portcullis on this pattern will weigh about 6.25 pounds. You should increase this if you want thicker beams. I think some of the beams in photos look roughly square in cross section—if you go up to 4 inches thick, then you get 8.33 pounds of wood per square foot of coverage.

If you want to face your portcullis with iron, you need about 6/9 square foot of iron plate (remember the crossings are double-plated) and if it is a quarter-inch thick you have 0.139 cubic feet of iron, weighing 6.83 pounds, per square-foot portcullis unit.

You also need a nail, weighing about 0.28 pounds at my guesstimated size of one cubic inch.

So that’s a total of 7.11 pounds of iron. And a total weight per square foot of portcullis of 13.36 pounds. Or maybe more like 15.5 if you want it 4 inches thick.

You should finally add a little for the right-hand upright, and maybe for the bottom cross beam (if you aren’t leaving protruding uprights sharpened to spikes on the bottom of your portcullis). Perhaps in round numbers we could say 16 pounds per square foot of portcullis, and about half that without the iron facing.

All-iron construction

I haven’t encountered an example of a medieval portcullis made entirely of iron. Let me just say watch this space and I will edit numbers in or link to a new post if I find a historical example, or if I fantasy one up.  I will just record my thought that I would envisage any iron-grille portcullis to be of thinner bars than a wooden one, since iron is heavier, stronger and generally more expensive than the same volume of wood.