Make an Ultralight Tent Part 2/3- Bathtub Floor

Part 1, Part 3, Ultralight Tent Pictures, Ultralight Tent Pegs, DIY Short Tarp Pole, DIY Long Tarp Pole, DIY Carbon Fibre Tarp Poles

Bathtub floor


One of the most useful features of a two-skin tent is the Bathtub floor. It is so useful as a design feature that there is no reason why you cannot have one under a tarp or a single-skin tent. The only difference is that instead of being attached to the tent it will be free-standing.

Using a deep bathtub floor in a tarp does wonders for the airflow - it helps to redirect the draughts over you rather than through you.

The design given below can be pegged out directly or can be attached by guys to the corners of the tent so that it is part of the tent. Experience tells me that it is better to peg it out. If you are worried about the weight of the pegs then it is possible to buy Titanium Nano pegs that are 4 inches long and weigh in at 1g each. They might just hold the floor in place.

Fabric Choice

The best choice of fabric for the base is probably 4oz PU coated Ripstop Nylon. However at around 110gsm (grammes per square metre) it is quite heavy as a fabric choice despite being bombproof. SilNylon (Silicone Elastomer coated Nylon) weighs in at around 60gsm but is amazingly slippery.

I have chosen to use 2oz PU coated Ripstop Nylon which at 55gsm is a reasonable weight. It is not very waterproof for this application and I have treated the underside (the side which is not PU coated) with Fabrisil to help. It is not 100% puddle-proof but OK for Summer lowland use.

The fundamental issue with the lightweight PU fabric is that it will seep water under pressure - such as when you are lying on it. This does not happen with SilNylon. SilNylon is very popular for groundsheets Stateside. I've made one in SilNylon and it does work very well. I did however add several spots of SilNet Seam sealant to it to help stop the camping mat slide around. I also machined a little pocket onto the base that the base of the mat fits into.

The cheap traditional and heavy alternative that is 100% waterproof is to go and buy a medium-weight polypropylene Tarp from a builder's merchant and machine it up. It will work well but at 90gsm is quite heavy. It is also bulky.

After lots of testing I have found that the 2oz PU (at least the CRISP-coated stuff that I've found) is not very practical for the wet UK - it's not puddle-proof and will seep when put under pressure. I've therefore moved over to trying SilNylon. This is about the same weight (65gsm) but resists seeping very well. To reduce the slipperiness of the floor I have added lots of dots of SilNet Seam Seal to the 'inside' of it in a rough grid pattern. This makes it a lot less slippery and a lot more usable.

If you really want to then you can use 0.75oz Spinnaker cloth (it weighs in around 49gsm) but it is a very poor choice for durability.

Pattern and Cutting

I have kept the design simple and 'wasted' fabric by using a single length for the floor. This saves having a seam in the middle of the floor and looks neater.


  • 2.5m of Fabric in a 1.5m width
  • 40-60cm approx. of thin (4mm) plastic or aluminium rod (for 4 x 10cm corner reinforcements). (weight 4-6g!!!)
  • 40-60cm of 10mm webbing or other synthetic fabric tape.
  • [optional] 50 cm of seam tape

The Pattern

Bathtub floor


The design shown here is the basic pattern for a 1-man version that weighs 140g in 2oz PU. This is lighter and slightly smaller than the one I originally made. Even so it is sufficient for me at 6ft and still has wriggle room and a place for bits and bobs.

You can shave a bit of weight by reducing the height of the sides. The MINIMUM height of the sides should be 5cm. [This is the point at which the fabric can no longer hold enough tension to create a side].

It is possible to fit a 2-man version underneath the tent canopy but I have not calculated the dimensions.

Cutting and Marking

Let's assume that you are doing the basic 1-man version.

You will need a piece of fabric that is 1.5m wide and 2.25m long. Buy a piece that is 2.5m long and you have a little margin for error.

To mark the fabric you will need to use some draper's chalk or a fabric pen. I would use chalk. If you use SilNylon it is a total bitch to mark and you might need to use a permanent marker if chalk doesn't work.

At one end of the length of the fabric mark a line across the fabric that is perpendicular to the edge and is 1.1m in length. Do the same thing 2.25m along the fabric from this line. [Note the edge of the fabric that you want is actually on the PU coating and is probably the innermost set of pinholes from the loom. It is not the physical edge of the fabric)

Now mark a point at 55cm in from the edge on one of these lines. This is the centre of the foot. Measure out 37.5 cm from this point either side on the line. This now gives you the foot corners. Join the foot corners to the head corners (the other 1.1m line) using a single line.

Note 2: I have a 2.4m/8ft length of wood that I use as a long ruler for marking out. It will be worth getting a single length of wood from a builder's merchant.

The image marked out will look like the one above. Check your dimensions before cutting!!!!!

Construction 1 - Marking the Corners

To do the corners you must first mark a line parallel to each edge and 10cm in from the edge (see picture). I have marked the fabric on the PU side which will be the INSIDE of the bathtub floor.

Note: An easier way to do this is to use a large Set Square that is marked in cm and then align it so that the corner is 10cn from each edge.

bathtub floor corners

I have marked out the corners before hemming. You can see how rough my cutting is. I am not going for artistic merit and hemming will hide most of the roughness.

Construction 2 - Hemming

You should now have a shape that looks like the pattern above. You now need to hem all the edges. It is not essential for SilNylon but will make it look tidy. I use a rolled hem (below) which requires a rolled-hem foot which my machine has (also below)

A 2mm Rolled Hem

rolled hem

A Rolled-hem foot.

rolled hem foot

If you do not have one of these feet then you can fold the fabric over and use an ordinary straight stitch or a zigzag stitch.

I tend to use a #11/#12 needle which is fragile but is ideal for thin fabrics. If you are not careful you will break the needle.

By marking the corners before hemming the sides will be maybe 0.5 -1cm shorter than the nominal 10cm but the internal width will be preserved.

Construction 3 - The corners

In order to produce a bathtub corner what you need to do is to sew a corner.

bathtub floor corner

To sew the corner you take the corner lines that have been marked and fold the fabric so that the two lines exactly overlap each other. This produces the pocket shape shown. Simply machine along the shadow of the lines. It doesn't have to be perfect but if you can do that it helps.

bathtub floor corner pocket

Stitch another line parallel to the first one and around 5mm nearer to the point to form a little pocket. Make sure that the pocket you stitch is wide enough for the plastic rod that you are using.

Repeat for all four corners

Construction 4 - Preparing the Peg-out points

bathtub floor corner cut

Trim the point off the fabric as shown and mark a line perpendicular to the rod pocket and half-way along. This will be around 4.5-5cm from the hemmed edge of the fabric.

bathtub floor peg-out 1

Roll the fabric as shown and then stitch into position. You will end up with a reinforced point that falls in the centre of the rod pocket. Repeat for all 4 corners.

Construction 5 - Finishing the Peg-out points

Change to a #14 or #16 needle - the 11 will break!!!

Cut 4 lengths of the synthetic 10cm tape and seal the ends using a lighter.

bathtub floor peg out

Place the tape as shown with roughly a 2cm overlap on either side of the fabric. The edge of the loop when compressed should be roughly where the bottom of the rod is.

Stitch the tape using the larger needle (#14 or #16) by zigzagging over both pieces of tape several times. The one shown is not neat but it will work.

(You can see on this pocket that I have reinforced the pocket with a second layer of very small stitches. It is sensible to do this with something like PU nylon or SilNylon but pointless with spinnaker fabric - it will just tear more easily. )

Repeat for all 4 pockets and trim any loose ends.

Cut your rods so that each rod will fit in each pocket (typically they will be 0.5cm shorter than the pocket).

Insert the rods into the pockets and then close the pockets using a few hand stitches.

Construction 6 - Extra waterproofing

If you wish you may apply seam tape to the inside of the corners using a warm iron. Test everything on scrap material FIRST!

To apply the tape ensure that the pocket is under tension otherwise the tape will lift when in use.

If you are using 2oz PU nylon you will probably want to apply Fabrisil or another proofer to the outside of the base which is not PU coated.

Construction 7 - Bed-Roll fittings (Optional)

With the type of walking that I do I found that my camping mat was being ripped apart by branches and so on. I did make a simple cover for it but that weighed 26g so I wondered if there was a better way - by turning the bathtub floor into a roll-mat.

The picture shows that I've sewn a reinforcing patch onto the head-end of the bathtub floor. Onto this has been sewn a piece of cord. The cord has a loop machined onto the reinforcing patch and at the other end a loop of cord holds a toggle.

The bathtub floor sides are folded over the camping mat and then tightly rolled up. The toggle is then secured to create a self-contained waterproof unit. The toggle additions add less than 5g to the total weight.


This is the fun bit! Go out into your garden with 4 pegs and peg it out. If everything has been constructed with reasonable accuracy you will have a nice bathtub floor with self-supporting sides.

You also will have a floor that works and is lighter than any Bivy bag you might try and buy. Not even Bozeman Mountain Works can do something for 120g!!!!


All that remains now is the tent in Part 3