Category Archives: Extension

week 14: intermediate floor starts

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With the blockwork complete, the carpenters were able to make a start. The timber wall plate is bolted to the block using Hilti resin anchors to fix the M12 threaded rods into the blockwork. The joists are then attached to the timber plate using joist hangers. The same fixing method will happen on the existing wall. This approach helps to minimise air leakage. at intermediate floor level – often a key air leakage path.

The floor structure should be finished next week and will be temporarily boarded to give a platform from which to construct the roof.

week 14: blockwork complete

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With the scaffolding up, we were able to complete the blockwork walls this week. Despite the rain, the walls have still gone up quickly, albeit there has been a bit of extra re-pointing here and there to keep on top of the rain as it has washed the mortar away! The walls run to roof parapet height and will be externally insulated with the insulation continuing across the top of the parapet as shown in the detail below.

The insulation will start shortly, but in the meantime the priority is the internal floor followed by the roof so that we can get out of this mad weather as soon as possible!

 

week 13: what a difference a week makes

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They say most the work is in the preparation, and this has been proven this week. The walls for the extension have gone up at an astonishing rate. Even greater progress would have been made if the scaffolders came back when they said they would! Even so, we’re now only approximately 6-7 block courses away from complete construction of the walls.

Internally, the space looks cavernous, but once the internal floor is in it will shrink again. The above photo shows the pantry being constructed, with the retaining wall behind. The internal stairs will be in the space to the left of the pantry wall.

 

The lintels for the rear wall arrived this week and were installed the same day. Two of the four openings are large: sliding/folding doors (3.2m) and the kitchen window (2.3m). These are special lintels and need to be made to order. The rear wall masonry is now complete. The drawing extract above right shows the openings on the rear (please note: some feedback to this blog is that photos/drawings are small –  you can click on any image to enlarge it).

week 12: rear wall construction

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The rear wall was constructed this week up to lintel level. Seeing it built, we realise there’s not much of it really, due mostly to the large folding doors that will be fitted in the dining room. So it only takes a day to build. The rest of the week has been taken up with tanking and below ground insulation (see previous post), land and surface water drainage works and preparation for the front wall.

The front wall foundations were cast on Friday – the last of the foundation works! The scaffolders arrived yesterday (Saturday) and erected the scaffolding to the side and rear of the extension. This will allow the extension walls to be built up next week.

week 12: tanking and below ground insulation

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Fortunately, it’s been a good, dry week to get the tanking onto the retaining wall. The below-ground (substructure) element of wall needs to continue the waterproof and gasproof characteristic of the floor. We opt for self-adhesive Visqueen Gas Resistant Tanking Membrane. The wall was first treated with a bitumen primer (above left photo) and left to dry and the tanking membrane went on the following day. It was a bit cold, so the self-adhesive element needed a bit of help with a roofer’s torch to get it to stick to the wall, but it went on finally.

 

The insulation goes on against the tanking. We are using Kingspan Styrozone H350R. We cannot use any mechanical fixings to attach this insulation to the wall (as it will compromise the waterproofing). I speak to Kingspan to see what adhesives they recommend, but they advised that no grab adhesives of any kind (water- or chemical-based) should be used. We therefore opted for an additional ‘sacrificial’ low-cost fibre board to hold the insulation temporarily in place and to protect the insulation during backfilling. The weight of the backfilled stone and earth, once compacted, will hold the insulation firmly in place. The photo (above right) shows a plan view of the insulation against the tanking as the hardcore backfill was going in – absolutely no air gaps between the wall and the insulation.

week 11: the superstructure emerges

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It’s been a long time coming, but we are finally out of the ground with the new extension. The floor slab had cured sufficiently over the weekend, so first thing on Monday the shuttering was removed. The guys wasted no time in getting on with the retaining wall structure. We are using dense concrete hollow blocks with reinforcing rods. The hollows are then filled with a strong concrete mix. The retaining wall is approximately 2 metres high and the side (boundary) wall ‘steps’ to suit the slope of the land (see photo below taken earlier in the week). Above retaining level, the blocks change to aerated type using Celcon H+H Hi-Strength blocks. Building above the retaining wall is not scheduled until next week, but, even with pretty grotty weather, the aerated blocks go up almost to storey height.

The Celcon blocks used above ground level have a much better (lower) thermal conductivity than the dense concrete blocks used for the retaining wall. With the application of 120mm of external insulation, the walls will achieve an excellent U-Value of 0.14 W.m²K. By comparison, the retaining wall element achieves a slightly poorer 0.17 W.m²K, which is still very good. This would be higher again (0.19) if this wall type continued above ground – the retained earth, in effect, is another layer that helps to minimise the heat loss. By comparison, the current Building Regulations require the U-value for new extension walls to be no higher than 0.28 W.m²K.

week 10: concrete slab poured

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With all the prep work complete, we are ready for the concrete pour. The pump lorry arrived first thing this morning followed shortly after by the mixer lorry.

The pour goes well, but during the levelling out we learn that we are a bit short. The correct amount was ordered (7m³). We find out that we should have ordered and additional quantity of 0.25m³ as there is a residual amount of concrete that cannot be pumped out of the lorry. The difference is made up on site. Finally, the concrete is smoothed over with a power float and left to cure over the weekend. A small celebration as we’re out of the ground from here on (ignoring the back fill works to the retaining wall).

week 9 and 10: preparation for the slab

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With the excavation and underpinning complete, focus turns to the lower ground floor slab.The graded hardcore sub-base is spread out and compacted. Along the edge of the retaining wall, we have introduced a 300 wide x 100mm deep layer of insulation to reduce the thermal bridge at the edge of the slab. I thought it would be quite a struggle to install this detail (shown in yellow on the drawing), as the top of the hardcore has to be compacted perfectly level with the top of the insulant. But the guys manage a precision installation. Given this insulation layer is beneath a structural element, we have had to make sure that we use a product with good compression strength. We could not get our specified insulation in time, so we substitute for Styrofoam Floormate 350 which has an almost identical thermal and compression performance.

A layer of sand blinding is spread out over the compacted hardcore to protect the damp proof membrane from puncture damage. We live in a medium-risk Radon area and so our membrane needs to be both moisture and gas proof. The red (depicting gas resistant) membrane goes in and a lot of care is taken ensuring that all the joints are lapped and taped sufficiently and that penetrations for the column and soil pipes are sealed around and to the membrane.

Finally, the reinforcement bars and mesh are installed in the vicinity of the retaining wall (note the protective ends installed on the mesh to reduce the risk of damage to the DPM) and the form-work completed ready for the concrete pour.

week 8: excavation and underpinning

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The extension area on the lower ground floor has been excavated this week. It has taken just a week to shift 50 tonnes. The photo above shows the excavated area, which is where the new kitchen (part) and stairs will be accommodated.The new pantry will also be in this area – so plenty of thermal mass to regulate the temperature, which is perfect for food storage, and perhaps a bit of home brewing!

On the left of the photo, the new retaining wall blocks are being used temporarily to prop the shuttering for the first section of underpinning along the side wall. The underpinning will continue next week.

The first block course has been laid on top of the ground bearing lintels. This course (highlighted in yellow in the section detail) will act as a shutter for the concrete slab pour. So, just a little more excavation to go and then the hardcore sub base can be laid in preparation for the lower ground floor slab.