I left the foundations for over a week to dry so the concrete will get to two-thirds strength. It normally takes 28 days for full strength. In some places soil from the trench walls had fallen in due to the rain. Also the concrete had some sort of watery skin that I removed with a steel wire brush. All in all it looked level and clean. Sorry I lost my old mobile phone at the time so I do not have any images to post on the progress here. Soon after I had my 300mm trench blocks delivered.


They seemed much like thermal blocks each weighing about 15kg. This was a new product at the time so I made some calls to the builders merchants and later to the manufacturers to check if they were suitable for below ground. I learnt you can build 3 stories high with these blocks. I went for trench blocks hoping that it would speed up the build. Each trench block would save me using two 100mm concrete blocks with a lean mix of concrete to fill the cavity. I calculated I would only have saved about £150 using concrete blocks however I would have to do twice the amount of work.

I started by placing a trench block at each corner of the build 4m away from the edge of the house. Quite a long distance between them about 12-14m. Halfway I put up my auto levelling laser to get each block at the same height and level. The difference in heights were about 20-30mm. Not bad for an over 10m stretch considering it was my first time I had attempted something like this. I could easily correct any errors with the trench blocks. My bricky mate said that builders sometimes use up to 40mm mortar on the first course to build up the level. This is something I would avoid and build it up over a few courses of brickwork.



A very important stage now, to get the blocks square. I got myself a digital laser measure to work out the exact 4m distance from the existing house wall to the out edge of the block. Quite tricky as the block were under the ground level so I had to hold a spirit level in place. After a few hours I got it right and marked the edge of the blocks on the concrete in place using a pencil. I had to check several times the square using the 48″ folding square. Later on I found that the garage end was about 30mm out, the main corner just 20mm and the other end perfect.


I used a standard mortar mix of 1 cement to 4 building sand with the correct mortar plasticiser. My cement mixer was a rather old battered heavy one with bits of old concrete stuck to the sides. I should have really got a new Belle mini mixer. I used 2 builders buckets filled to the top and one smaller bucket that was half the size of a standard bucket for the cement. I believe this was from an old painters tub. I used half a plastic cup for the amount of plasticiser. I found by throwing in one bucket of sand followed by the cement and another bucket of sand and switching on the mixer made the mixer blend better. It’s best to work out how much water you need to put in and pour a load in after a few minutes once the dry mix has blended properly. Depending on the weather, heat and sun you have to work out for yourself the consistency of the mix. Too sloppy and the weight of the blocks will squeeze it out. Too dry it not be workable.


My first block I mortar in place with a 20mm mortar bed. I found as these block are aerated they suck out the water from the water straight away making the mortar solidify. I had to pour about a quarter of a watering can of water over the blocks to keep the mortar more workable when applied. To level the blocks you will need to use a large rubber mallet as trying to level the blocks with a trowel like you would on smaller blocks was not possible due to the size of these blocks. When I had another block in place I used a string line to make sure then were in line. Once all the corner blocks were in place I double checked them with the laser and spirit level. I had to re-adjust one corner that was not perfectly square once again. I placed a run of blocks all on one side without any mortar (dry run). Peaks in the concrete proved how much some blocks were going to be out. I started bedding some more on each side and eventually done one line in an afternoon. It took me over an hour to get 10 blocks laid on my own, that’s mixing, moving, working in a trench etc…


After laying the first course of trench blocks the day before I placed blocks at each corner and checked the level using the laser. It was no more than 15mm out. This will be corrected on the next course. Before I started laying any blocks, I soaked them with water once again. Within half an hour they would become fully dry due to the hot weather. I started by bedding in 2 blocks at each end and pulled a string line again. On this course I had to make sure that the blocks were also vertically level with the first course. Brickys usually build up the corner fully however I wanted the last course perfectly level before the 3 courses of engineering bricks so I done it my way using the laser level. When finally completed the trench blocks, I planned to build up the corners first. The outer skin would be built with engineering bricks, the inner skin with one concrete block below the dame proof membrane. To match the existing house I used imperial (75mm) bricks. This was a problem, since the inner course would finish slighty lower that the out, when placing the wall ties the tilt would be towards the inside of the house. Build regs state that these should be tilting towards the outside to allow any runs of water droplets away from the inside walls. Later I realised I could have used standard 65mm bricks on the first course of engineering bricks as these will be visible when the ground outside would be filled up. This would have made 10mm difference.


The front of the house, garage and study completed upto DPC.


The rear of the house, Study, Lounge, Kitchen completed upto DPC.

With the engineering course completed it was ready for the building regs inspection the following day. I had rolled out the DPC over the brickwork so the inspector could see it. When the inspector came he was surprised that I used trench blocks. I got the impression that he hadn’t seen these before and just said as long as they are suitable. He could not pass it off since the DPC was not bedding in mortar. Quite silly as this is not possible without placing any bricks work on top of it. I planned on getting the soak-a-way done before calling them in for another inspection. To speed up the progress I called in a bricky. On the first day I arranged for another inspection. A nice lady came in the afternoon while were laying the brickwork. She passed it off and also the soak-a-way which was great news. I had a chance to ask about a few more ideas i had. We stared every morning 7-8am for a full week, it took a further 5 half days to finish to the point where I would need to finish og the internal walls and ends above the garage door and kitchen..