Damp Proofing a Basement by Adding a Damp Proof Membrane (DPM)

The house we are renovating is over 100 years old, covers 4 stories high including a semi-sunken basement that lacks a damp-proof course or DPC. The level of the basement is below ground level, most of the sides are dug out, still there are areas that are underground and it’s significantly below ground level so rising damp and penetrative damp is a problem.

We stripped the walls to the original red brick that (other than an extension from the late 80s I believe) is over 100 years old and so obviously lacks a plastic damp proof membrane you find on newer builds. With the walls stripped to the brick work we found a lot of damp (especially in the corners)!

There’s been an attempt (less than 10 years ago) to use a chemical damp proof course, (the drill a hole in the brick and inject chemical damp proofing, probably silicon) but it had clearly failed (in places you could feel water) and many bricks were water logged. In some areas there was significant concentrations of salts on the external bricks, this is caused by rising damp.

We considered having another chemical damp proof course (was still covered on the DPC guarantee), but the injection damp proofing company that did the work had drilled the holes way to high, in places we had a couple of bricks between our visible floor (concrete which had been raised a few inches) and the damp proof course holes in the masonry and so even if we could get a good chemical damp proof course added it wouldn’t cover the important areas (the watter barrier isn’t where it’s needed). And so we’d still have water seeping into the house to a level that would damage skirting boards etc…!

Since we plan to live in this house at least to our retirement (I’m 38 years old) and so want a high quality job that lasts a long time, we decided on a quite drastic step to remove one layer of bricks from the inside perimeter of the house and internal room walls so we could add a physical (plastic) damp proof course and a new layer of damp proof bricks.

We knew it was a big job to remove so many bricks and add a plastic damp proof membrane, but thought long term it would be worth it (once in a life time job). If it failed we could still have a new injected damp proof course, so other than the time and expense. The materials: bricks, sand, cement and damp proof membrane are cheap so wasn’t a big cost in money terms, was in time though.

Since this was all internal work and all the walls would be covered with plasterboard or cladding it didn’t matter what it looked like after the work was complete, all that mattered was it removed the damp problems.

We set about knocking out a few bricks at a time (no more than 4) cleaning out as much rubble/mess that was left behind and add a strip of plastic damp proof course and new damp proof bricks while leaving a brick gap to pull out the next 3 bricks.

In this way we’d remove 3 bricks, add a strip of plastic damp proof membrane wait for it to set, remove three bricks, over lap the last plastic damp proof membranes a few inches and so on until the entire house was done.

This was very slow/hard work and our eldest son (Cameron, age 15 years at the time, image to the right) did most of the removal of the bricks (did a lot of hard work with a lump hammer, chisel and crowbar, was a proud day for us parents to see him work so hard for us for no payment :))

After Cameron had cleared the old bricks I replaced them with new damp course bricks (my wife and eldest son tried, but they couldn’t get the mortar in as well as I could) and a plastic DPM. I found it quite easy to do this task and other than the odd patch up brick work I’m far from a brick layer.

We didn’t do this in one sitting, Cameron would knock out the 1st 4 bricks, clear out the mess and then I’d add 3 new brick work etc… He’d knock 3 more bricks out which meant we always had a one brick gap to make removing the next 3 bricks easier.

At the same time we were doing other jobs around the house, so all in all probably took 3-4 weeks to replace one line of bricks around the entire house.

Hardest parts was under the basement stairs: had to remove the bottom 3 steps to get into the wall, Was quite damp as well, so glad we did it) but wasn’t so bad as found the stairs had some dry rot and woodworm (sat on the concrete floor, no wood treatment or anything!) and so needed replacing anyway (I built a new section for the bottom with treated wood and laid it on a plastic sheet to keep water out). Was also hard to replace bricks that went between rooms, but Cameron persevered and we managed to do the entire job well.

I wasn’t sure this would work as an effective damp proof course since the house is over 100 years old and uses the cavity free brick work. The brick work is two bricks thick, but no cavity and every few levels the bricks are set the other way around to add stability to the structure. This means even if you damp proof one side of the wall the other side can still draw water freely up the wall and over the new DPC, so I was concerned water could still be drawn into the house through the outside line of bricks that lacked a damp proof membrane.

After the job was complete we found the walls wasn’t as dry as you’d get in a newly built house, but it was much, much better than before so was a success and if we ever get a serious damp problem again (you can’t remove water completely from a 100+ year old house) I plan to do the same sort of treatment to the outside walls (at least for half the house, other half is the neighbours house :-))

We did have one problem area. When we started this area it was drenched, it’s a cupboard that set under the steps to the ground floor and so is fully under ground. We couldn’t check, but suspect the walls are touching the wet soil etc… on the outside. If I was ever building something like this I’d look for products to line the wall prior to back filling (tank it).

This meant even if we removed a line of bricks to prevent water soaking from the ground we’ll still have significant water penetration from the wall that’s underground, penetrating damp.

I did some research and read about systems to tank a basement that’s underground, you pretty much add a waterproof lining to the walls. I couldn’t find a system like this in the UK at reasonable cost, so made my own :)

After removing the line of bricks at the bottom and damp proofing the brick line, I bought some paint on black latex product that’s for coating problem areas. After a few layers of this product there was far less water coming through the wall, but the problem wasn’t solved completely.

Next I used a high grade British Board of Agreement Approved DPM (Damp Proof Membrane Black 1200 Gauge 4m x 15m) “designed to provide an easy to use moisture resistant membrane for use in concrete flooring”. This is the product you use under floating floors etc… on concrete floors to prevent rising damp damaging to the wood floor). It’s not designed for walls, but it’s the thickest plastic lining material I could find for this.

I then built an almost free standing stud wall that had been treated to prevent wood rot etc.. (so if water gets to it, it should be OK for a good few years).

I avoided putting too many screws into the damp wall that had been treated with the black latex paint (had to do a few screws, but not many) instead screwed it to the ceiling joist and a free standing piece of 2″ by 2″ on the floor that after I added some other stuff (like the door frame) meant it wouldn’t move.

I did it this way so very little water could soak through screws etc… into the woodwork, I know a bit of over kill, but it was a small space so wasn’t too much work and I really wanted to do my best to stop water penetration since once the plasterboard goes up there’s no easy way to check if water was soaking through.

After that added fibreglass insulation in the remaining space and added the plasterboard to finish the job. If that doesn’t stop most of the water penetration I don’t know what will.

So far we’ve not had any signs of damp problems on the ground floor except in a small nook that used to be a large walk in chimney. The outside wall to this is one brick thick so didn’t add a damp proof course (I might add one someday from the outside). I don’t think this is the problems, I’m pretty sure it’s because we’ve insulated all the walls in the basement, (2″ to 3″ thick insulated) but didn’t add insulation at this point because we planned to use this nook for a TV and wanted as much depth as possible).

This means when the basement is heated (we are living in the basement while we renovate the rest of the house) that chimney area is going to be a significant cold spot and we got black mold forming (didn’t realise at first as had something in the space).

This isn’t rising damp though, it’s caused by condensation tending to form there because it’s the coldest part of the room. We’ve also not added enough ventilation to this room because the floor above isn’t done and if we add the vents were planned we’ll get dust falling through right now! This room also only has a single double glazed door to the outside (no window, so no window vents) so having a significant cold spot isn’t a good idea (lesson learned the hard way as always :)).

We plan to add some insulation to that area and add another sheet of wood (loose a little depth) for next winter, will loose a few inches of where the TV goes, but will be worth it :-) Also expect to have the room fully vented by then as well.

All in all I’ve found the technique of replacing the damp proof course a success. As I’m an over kill type DIYer I also added the Damp Proof Membrane Black 1200 Gauge 4m x 15m plastic sheeting to all the walls in the basement as well and linked it into the plastic DPM (1200 gauge again) on the floors (over lapped the two) this way the rooms are kind of tanked (theoretically speaking no water should enter the ground floor).

We’ve lived in the basement one winter and we hardly ever needed the heating on, we really went overboard on the insulation. Only problem was condensation issues causing dampness in one area (see above). It’s summer now (end of June) and it’s super cool in here while really hot outside (we live in Skegness, Lincolnshire which is a holiday town). When the house is finished we plan to live mostly on the ground floor, but in summer we’ll probably move back into the cool basement :-)).