And So To The Walls

The next stage is to build the walls...


Not really rocket science I accept, but it is the part I have been looking forward to.  The dwarf walls, at damp course height first need a mortar bed - level all round - to lay the damp proof course on.  This is simply a thick plastic roll that stops moisture wicking up the wall by capillary action from the ground, so it is important that it is not breached at any point and that it is at least as wide as the bricks.  I chose 125mm wide dpc so it would overhang the bricks by 12mm each side. The overhang will be masked on the outside by the drip bead at the bottom of the render.


Before the mortar hardened I bolted the wall starters to the existing walls so I could tie the new blockwork in with the old.  The bricks drilled well, but after just three holes my Bosch cordless drill was running out of juice - I think it may need a new battery - so it was back to my old B&D which managed the job well.  I was pleased that the wall starters came with 10mm hex-headed screws so I put them in with a socket on the drill and tightened them up by hand.


Once the mortar bed had gone off and the dpc was set, I started to lay the blocks.  I use a Bricky tool to ensure a constant thickness of mortar and to cut down on wasted mortar.  It is a great piece of kit but it relies on the fact that you can get to both sides of the wall - something I patently cannot do with the wall up against Jean's conservatory.  The Bricky sets a 12mm bed of mortar so I cut a piece of 12mm OSB and fastened a batten to it that would give a 10mm overhang onto the blocks so I could match the bed the Bricky would set.

I discovered one thing straight away; if the bed runs right up against the adjoining all it gives no space for the excess mortar to be pushed out to - so the best bet is to leave a small gap alongside the existing wall for the excess to spread into when tapping down the block.

I am using a 5:1 sand-cement mortar mix and my mixer - the rusty legs of the stand having now been repaired with my rather dubious welding - holds 2.5 bags of sand and half a bag of cement at each mix.  I have been adding a small amount of mortar plasticiser to each mix to try to keep it useable for longer as I am a slow bricklayer.


I had two 3m lengths and 2 2.4 metre lengths of 4"x2" which I cut and nailed together to form a frame to build the door aperture around.  I cut a couple of spare pieces of timber to 45 degrees to brace the corners and keep it square.  the gap itself is 3m wide and will run for the full height of the wall up to the steel lintel which will extend pretty much for the full width of the building.

The insulation, 75mm thick Celotex board, is very easy to cut and slot into the wall cavity.  A basic hand saw does the job with no trouble at all.  It is the same product that I will use for the floor insulation under the screed.  It's not cheap but it seems the best product for the job.

The two different types of block I am using; 7Nm Medium Density Concrete block and 3.6Nm Aerated blocks cut rather differently.  The Aerated blocks cut with a saw to give a neat edge, the medium density blocks, on the other hand, need an angle grinder to give them a neat finish, but will cut with a hammer and chisel, if you don't mind a more jagged edge.   

The Slab

Once the sand was compacted and level I called the Building Inspectors to come and check it.  The inspector who came was very happy with it and was generally complimentary about the quality of the work so far.


I had bought 6 one metre lengths of 12mm rebar to join the new slab to the existing one.  These needed drilling into the side of the existing concrete - about 50cm - then locking in with injected resin.  This took a while but made a good join.  The difficulty then was setting the damp proof membrane on the sand and the perimeter insulation around the edge with the rebars coming through the membrane.  This has meant that the dpm is not watertight and I will need to paint a damp proof course on top of the slab before putting the insulation down.

I ordered 2.1 cubic metres of concrete - which I discovered was about 5.2 tons!  With enthusiastic help from Emily who waded in concrete and Zulum who chose the big barrow, we had the slab laid in good time in steady rain.  

The rain will not hinder the setting of the concrete, in fact slowing the drying process makes the curing better and results in stronger concrete; but it might spoil the finish. As it is not the top screed I'm not unduly worried about the finish - at least it's level.

By the evening it had pretty much set though it was difficult to see the finish.

Now that the floor slab is down I can start to build the walls, there i no need to lay the insulation and screed until I'm good and ready.  So the next phase is to attach the wall starter plates, lay a mortar bed all round on both sets of blocks and set the damp course on top.

Once that is on a second layer of mortar takes the next row of blocks all round, though the 3 metre gap for the bifold doors will leave a significant part of the wall looking rather bereft.  I will make a wooden frame, 3 metres wide and 2.1 metres tall, ensuring the corners are square, to use as a former for the opening.  A second one will be useful for the window over the sink.

More Digging

Now the walls are up to damp course level I can start to think about the floor.  I needed to dig all the existing soil from within the walls down to the bottom of the second bock down - a depth of about 450mm.  I think this would be about 3 tonnes of spoil, though after a few days of rain it felt like 10.

I could back-fill the trenches both inside and outside the walls with some of the soil, but there was still a huge amount to shift and I am running out of places to put it.  I am pleased that it is a relatively fast-draining, sandy soil that digs easily - I'd hate to do the same in the clay-soil we have at school!

I left a small gap in the wall to put planks through as a bridge so I could use the wheelbarrow to move the soil, but eventually I had to brick this up too.  Once down to the required depth I could start to bring back some of the bagged brick and concrete rubble I had stored from the demolition of the conservatory to use for the bottom layer of the floor.

There are several strata to lay:

• First a layer of 150 mm of compacted crushed concrete etc. to give a good, firm base. This is sometimes called Type 1 MOT because it is the standard of material specified by the Ministry of Transport for making the sub base for a new road.  I am going to use the bags of rubble I have made up from the demolition and clearing of the conservatory.  I have plenty of this and when it is compacted using a plate compactor it should be absolutely fine.
• On top of this comes a 'Blinding' layer of 25 mm of sand.  this covers any sharp edges and points that may puncture the damp proof membrane which sits on top of the sand.
• This is covered by either a 100 mm concrete slab or a 75 mm insulation board. It seems there are different thoughts about the order that these go in.  Some websites suggest the insulation board goes on top of the damp proof membrane on the sand, others that the concrete slab goes there with the insulation on top, and there are others that put the damp proof membrane on top.
• Whatever order these go in they are topped with a 50 - 75 mm screed to give a perfectly smooth and level floor surface.

The removal of all the soil from inside the walls took a while but was useful for back-filling around the outside of the walls on top of the foundations.  There was still plenty to get rid of, but I like digging.

I left the part nearest the drive until last because it made a very useful little access bridge but eventually even that needed to be demolished and the concrete drain-surround removed.

The hard-core went in well though the hire of a compactor was not without its problems.  The first one I hired worked for a couple of minutes but then stopped and would not be coaxed back to life even with fresh petrol and a new spark-plug.  So money returned I hired an identical one from HSS - identical in all ways except one - it worked!  Unfortunately this had left me considerably out of pocket and having lost a couple of hours of work time.

The sand I had dug out for the foundation trench would be fine for the blinding layer if it was sieved for stones - so I set about sieving a ton or so of it to lay a 25 mm layer.  The compactor worked well and the base was ready for the building inspectors to view before I lay the damp proof membrane and the concrete slab.