Damp Proof Course (DPC) & Damp Proof Membrane (DPM)

A Guide to Damp Proofing Property

All Buildings need to have both a DPC & a DPM, It’s what stops the dampness from outside getting inside.

DPC is a horizontal water resistant barrier that is incorporated into the walls above ground level by the bricklayer.

Forgetting to put in a DPC or putting it in at the wrong height is a very costly mistake that cannot be easily rectified. If you’re driving - Think BIKE, If you’re starting to build a wall - Think DPC

DPM (Damp proof membrane) is a water resistant barrier that is incorporated into the floor.

Let’s start with – DPC. (Damp proof course)

Historically Buildings were built without a DPC. The inside walls, especially in the winter would be damp. -  Moisture un-checked, can & will rise up the wall to a height of approx. 1m.

With the obvious desire to have a dry place to live – Some smart person came up with the idea of incorporating a damp proof course in the walls. This was formed using a water impermeable material like Slate or Clay tile.  Often this was just laid butted together & bedded in mortar. This produced a reasonable DPC.

However, laid in a single course water could still by pass where the slates or tiles were butted together.

In some buildings 2 overlapping layers were used, thus making it an even more effective barrier.

Modern buildings have a DPC that is made using a Bitumen re-enforced strip similar to roofing felt or Plastic; these come in various qualities depending on budget. (You get what you pay for). The best, in my opinion, is a DPC that is constructed using Elastomeric bitumen with a high tensile reinforcement.

(This is something you will only get 1 chance to do – so choose wisely)

 

Stay clear of cheap, plastic DPC’s which over time can become brittle.

DPC’s are laid horizontally & should be a minimum of 150mm above the external finished ground level & protect the full thickness of any solid built wall.

Retro fit DPCs

It’s arduous & expensive, but possible to retro fit a DPC if it has been omitted or become defective.

Using a special chain saw with a masonry blade, the wall is cut through in sections approx. 1 metre at a time. The DPC is inserted by sliding into the cut. The remaining gap is filled with a non -compressible material like slate before finally being pointed with mortar. The new DPC inserted must be overlapped by the next section- the overlap of which should be 150mm. Work round the building 1m at a time until all the walls have been treated.

Chemical DPCs

More popular are chemically injected DPC’s- Usually, these are silicone based & injected into the walls by a high- pressure pump. Ideally, the walls should be drilled from both sides. Often this type of DPC is not entirely effective since it depends on the type of brick being treated & its ability to absorb the fluid. Generally, the plaster on the internal walls needs to be removed and replaced with a waterproof alternative.

Chemical Injection dpc

Chemical Injection dpc

DPMs (Damp Proof Membranes)

Historically, floors were not protected & therefore were always damp.

Later laying clay, slate or stone tiles, as a floor finish would offer some protection.

Modern floors should have a water impermeable layer, usually in some form of plastic sheeting or bituminous product. This membrane is fitted over a prepared over site/ subfloor prior to the concrete floor being laid. Any joins should be well lapped and taped with waterproof tape. At the perimeter walls it should be turned up & finished in line with the horizontal wall DPC.

Damp proof membrane

Damp proof membrane

Always buy a quality product designed to be used as a DPM with a minimum thickness of 1200 gauge or .3mm. It’s available in rolls or packs to suit most popular requirements.

Do not be tempted to use polythene dust sheets or any similar thin sheeting.

Bituminous damp proof membranes tend to be applied on the surface of the concrete floor prior to the floor screed being laid.

If in doubt or you want belt and braces use both a PVC sheet Dpm and then apply a Bitumen coating in 3coats to the over site concrete prior to the floor screed being laid

liquid damp proof membrane

liquid damp proof membrane

They usually require multiple coats to adequately cover. They are very effective since they are not prone to puncture in the same way as plastic sheeting. However, the quality is not in the product. It’s how well it’s applied. In my experience a minimum of 3 coats need to be applied. Any gaps or holes need to be well filled prior to application.

As with plastic DPMs, the coating needs to be applied to the brickwork perimeter, up to the height of the horizontal damp proof course.

Cavity Trays

This is the term which refers to the formation of a waterproof barrier across a cavity wall. It directs any water entering the cavity to safely exit on the external face of the wall. The most common use is over the heads of doorways or openings- All popular steel lintels used on cavity brickwork form their own cavity trays.

In the 1960s & 70s it was popular for flats to have concrete floors that were supported over the full width of the walls – However, if they were not well protected with a properly installed cavity tray, most were not. Any water entering the cavity, would find its way onto the concrete floors of the flats and thus causing much damage & misery for the occupants.  This error cost Council Authorities millions of pounds to rectify and led to the over cladding of walls on many blocks of flats. Cavity trays that need to be continuous should be lapped and sealed on all joins, and correctly terminated with stop ends at any intersections or openings.

Correct installation of cavity trays will avoid this.

Correct installation of cavity trays will avoid this.

 

Image credit: Bryn Pinzgauer and Tim in Sydney