DIY BLOG - The ground beneath your feet

Ideal for a gymnasium - ignore the holes in the floor - worrisome for stable footing, the existing floor was posing some problems…

Existing timber joists with joist ends sitting within a masonry pocket. Dampness in and under the floor is a common problem affecting most traditional homes

Behold the ravages of wood worm. The ‘worms’ are actually in the larval stage and are the ones doing the damage, chewing through damp timber and eventually leaving those tell-tale little round holes in the surface. The adults look like little brown beetles

A ramshackle dwarf wall supporting the mid-span of the floor joists

It started with the removal of a couple of old and wood-worm ridden floor boards. 100 years is a good stretch for the timber floor and although I’ve seen 150+ year floorboards in great condition in the tenements of Edinburgh, it’s hard to knock a century in the face of the Skye weather. Of course, it’s rarely a couple of anything, and before you know it, the prybar has a mind of it’s own, prising up everything in it’s vicinity much to the future chagrin of your budget, time and energy.

But what needs doing is worth doing proper, and so began another journey of looking at a little and getting a lot back. The corner of the original part of the house, by the masonry gable, had always seemed a little, gymnastic, and so it was no surprise that the floor joist had given way due to damp. The typical detail on these croft houses was to ‘pocket’ the end of the timber into the masonry as opposed to using any sort of proprietary hanger/timber wall plate or indeed a sub-wall to rest directly upon. The downside to this is that in the most likely event of the masonry getting wet (and there being no damp proofing to the timber) that moisture is transferred into the timber. Over time, this rots the timber and kills its strength, resulting in joist ends that resemble shredded wheat after a soaking in milk. Add to this the issues of woodworm (often not entirely visible on the surface) further reducing the strength of both flooring and joists, and it’s not a far shout from tearing everything up and starting again.

The existing plan is not original, owing to the multiple extensions added to it over the years, however it is not far from the original layout - here seen as somewhat circuitous and cellular

In any project, there will be a point where you question what you are doing - this generally happens when what was a small space becomes a large space and you wonder if putting walls back in is a good idea… in this case, I decided ‘No!’

…here we go again… unfortunately the vast majority of the wall and ceiling lining was in bad condition. I had attempted to retain some of this but it was more than an uphill struggle, so out it came

…the spirits must be appeased! Much in the same way a falaisg renews the ground and improves grazing, burning the rotten timber gives a sense of closure on the old and fertility for the new. It comes with mixed feelings and is equal parts reflective and forward looking

The floor in question is in the original part of the croft house, where the focus of the renovation work has been so far and after this initial inspection and understanding the extent of the damp/woodworm issue, it became apparent that the floor had to go. Once the floor was part removed however, I found that the internal walls were simply sitting on the flooring with no support underneath - not even a joist directly under! This was strange but not entirely surprising, given the simple nature of the existing construction. Add to this a rather piece-meal dwarf ‘wall’ to support the mid span of the joists, it was looking more attractive to clean the slate entirely. In any event, the walls would need to come away if the floor were being removed, and so it became evident that the road ahead involved the removal of pretty much anything that wasn’t cemented in, i.e., the weather-proof envelope of the building. This meant removing the floor, floor joists, walls and all linings.

Wacky framing, orange walls and Bros posters… peeling layers of history away

Removing the internal walls was a relatively quick and painless task, once electrics and errant nails had been navigated. The resultant space was, quite frankly, capacious after decades of being accustomed to the cellular nature of the layout (see plan above). During the initial design phase, and not fully appreciating the scale of the required renovation, it made sense to retain the existing rooms and simply work around that layout - now the open plan space commanded me to not resist. The next step was to remove the remains of the floor, see what lay beneath and find a plan to rebuild.

what lies beneath

Dirt mostly. And rocks… lots of rocks

Ground ‘level’ was anything but - between the highest and lowest point there was more than half a metres difference. In a way this wasn’t so bad as the majority of the rocks could be used to raise the ground level where it was lowest. On the other hand, the highest point of the ground was too high so required some manual digging to level it off. Why too high? Well, the plan had been to install a new suspended timber floor however I didn’t want to be subject to the previous issue of rotten joist ends in damp masonry pockets. Ideally I would resin anchor a timber plate to the wall and hang the joist ends from it with proprietary joist hangers. Unfortunately, the wall was so rough at this level, it would’ve been impossible to get a decent, or flush, fixing into what appeared to be rotten/river rock. This set me back a little and I wondered about filling in the entire hole with concrete - a thought which often returns despite the impossible site access for a cement mixer and the terrifying though of handballing metric tonnes of concrete… so a timber floor it would have to be. To get around this I decided to form two strip foundations, either length of the long walls, and build a concrete block wall to support the new timber floor joists. The benefit of this is that it would be independent of the external masonry and so would not succumb to the previous issue of damp ingress. It would also provide a reasonable solum for ventilation (min 150mm from ground to u/s of timber joists however in this case I kept this around 450mm for under floor access) which would help reduce the issues of condensation once the floor was well insulated. Prior to any of this, the existing ground level had to be levelled and compacted before any concrete could be poured.

Concrete cures all

In modern timber floor construction and indeed in pretty much all forms of suspended ground floor construction, a site topping of around 50mm min concrete on a dpm is put in place to prevent rising damp. In the majority of tenement and house refurbishments, if there is only the ground under the floor, it is usually left as is if there is no damp problem - if it is not broken, it doesn’t need fixing. The ground underneath the floor was surprisingly bone dry and with the exception of one wet corner (a known problem stemming from a cracked external surface water drain) I felt pretty confident that there would be no rising damp issues - this also meant less concrete, less work and less reaching for the wallet.

After clearing, levelling and working back from the finished height of the floor to understand the required build-up, it was time to compact the soil and form the shuttering for the concrete form work.

Having previously bought a pallet of 100mm concrete blocks, I decided to form the strip foundation as required, lay a 100mm concrete block on top, then a timber sole plate that would support the end of the timber joist (not forgetting the dpc!) This sole plate is fixed down, through the dpc, into the top of the concrete block to keep it in place. The entirety of the ground area was compacted using a ground compactor (or ‘whapper’ as I first knew them during my small stint as a labourer) and shuttering was formed from the proposed timber floor joists and ground stakes to keep them in place. Care had to be taken as there were a mess of heating and water pipes crossing the space - a nuisance when using the whapper and the wheelbarrow and resulted in a leaky pipe at the end of it all.

The detail is far from perfect - how do we maintain ventilation to the underfloor space? How do we avoid build-up of moisture in the 50mm cavity? All symptoms of renovating a traditional house… and trying not to introduce any new problems too

Levelling up

Once the strip foundations were poured and all other builder works were complete, it was time to don the tool belt and move on to chipping… thanks go to my half-brother for ‘humpfing’ concrete and generally getting the hard work done with zero grumbles.

Donnie Duncanson2 January 2023Comment