Hemp Building Workshop with Steve Allin

In mid January 2012 I spent three days, on Sydney's north shore, at a Hemp Building workshop conducted by Steve Allin. Day 1 was hampered by heavy rain and the workshops started with Steve giving a general presentation on hemp building.

The timber frame of the outbuilding the group would be working on had not been completed and in the afternoon of Day 1 much time was spent by those in the group with building skills completing the roof frame.

Late on the first day we finally got into putting up the formwork, consisting of plywood attached to the frame with coach screws and a spacer in between. The spacers from plastic conduit worked better then the metal ones which tended to cut into the timber.

I  have already acquired a tradesman quality cordless drill to use in putting up my formwork, but discovered that a cordless impact driver will also be very useful to put in the coach screws. Also hammering the coach screws into the timber to give them some bite before driving them in worked well, a tip picked up from the builders amongst the participants.

For the formwork we used one full 1200 x 1800 sheet of ply on one side of the wall and a 600 x 1800 half sheet on the other side. Since we were only filling the walls from one side this meant that we only had to move the larger sheets half as often. Putting the formwork together we often needed scraps of timber to joint the plywood sheets together and hold them stiff as the outbuilding we were constructing had widely spaced posts, that did not line up with the joins in the formwork.  The greater spacing of the posts than in a conventionally framed house with 600mm spaced studs, meant that there was some bowing of the formwork that had to be overcome.This was not from the weight of the hemp mix which was very light, but from the natural tendency of the ply.

For the bottom layer the corners were formed up square, but as the formwork rose up it was harder to match the formwork at the corners. From discussions with other participants I have decided to try using some dedicated corner forms when I start my walls. The difficulties in matching up the screws with the spacers as the formwork was lifted up has made me think about using a system of regular measured screw holes in the formwork for my build.

On Day 2 we got into mixing the hemp with the lime binder. We used a 120litre Screedmix Pro electric pan mixer to make the mixes. Although cost will be an issue for my build, it is good to know that such mixers are available for sale from the Healy Group (ph 02 9525 5522). The binder mix we used was different to that used by Steve in Ireland and we used a lime based binder with no cement in it that the owner of the property, where we were building, wanted to trial.

We used hemp from large 1 cubic metre bales measured out into buckets. The hemp was added first followed by water then the binder which had been measured into a bucket on site from its constituent ingredients. Then the last of the water was added being poured over the centre and edges of the mixer to clean off the hemp and binder into the mix. The mixes were generally wetter and stickier than I had come across previously and were somewhat variable. I remembered the advice of Particia and Graeme who had built with hemp at Billen Cliffs of the importance of consistency in the mixes.

Based on previous experience working with these materials I brought my own mask to protect me from the lime dust and gloves to protect my hands. As an owner builder I will be responsible for safety on my site.

We used wooden tampers that we had made the day before to lightly tamp the mix once it had been poured into the forms and levelled. The tampers had one tapered end to allow then to get into small areas particularly around the spacers. The edges were tamped first, then around the posts, tamping only enough until the mix stopped moving, not using a large force. The centre was hardly tamped if at all.

Once the formwork was removed it became clear that in the thickish layers of hemp mix the top was often tamped too much and the bottom not enough. This showed up clearly as lines of denser and less dense hemp mix, rather than a homogeneous mass.

We were originally advised by Steve that we could remove the formwork after 6 hours, but almost as soon as we got to the top of the first rise, the formwork was stripped off. The removal of the formwork was not difficult as the hemp mix did not stick to it, however due to the size of the form it was definitely a job that required a couple of people. Despite the quick removal of the formwork the walls held their shape and did not slump or move. Unfortunately we did not have enough formwork to go around the whole perimeter of the building, which left some odd spots that would have to be done later.  Given the amount of labour available on site the rate at which the walls went up was limited by the size of the mixer and the speed at which the mixes could be made.

On Day 3 we shifted one side of the formwork up to the base of the rafters. The moving of the formwork required more people the higher it got, particularly as the forms were joined together. A more systematic approach allowing the forms to be moved in smaller sections may have made moving them easier. Although we moved the form we never got up to the area just under the rafters and this looks like it will be one of the harder areas to do. I was also keen to learn more about the detailing around doors and windows, as had been advertised, unfortunately we never got up to this and it did not appear that such details had been worked out for the build.

A small niche was made in the rear wall, surprisingly formed up with a Styrofoam box and a piece of cardboard. Amazingly the wall alcove stayed in place when the form was removed, the top pointed section only having been laid the same morning.

Work on the construction of the walls ceased so we could have a go at some rendering. For the internal render we used a mix with more binder and water. We used the same sized hemp, although we were advised that ideally the hemp would be a bit smaller for a render. For an external render sharp builders sand would be added to the mix.

An even cost of render was applied to the wall by throwing small hand fulls at the wall so that it stuck, but not throwing it with so much force that it splattered. The render was then trowelled flat and smoothed further with a metal round edged swimming pool float. Regular cleaning of the float, with water, made it easier to smooth out the render.

The partly dry render was rubbed with a gloved hand to bring the hemp to the surface and smooth out the render. The render was applied thicker than Steve would normally do it, so as to be the same thickness as the cut out for the decorative border.

Last of all we did a small section of hemp floor, the timber was used as a guide to keep the floor level. The floor used the same mix as the walls with a bit more blast furnace slag in the binder to make it set harder and faster.

Here are some video clips from the workshop.