Moisture & foundation
Plinth felt and damp-proof course
Two-layer principle, height above ground, slope and radon tightness
The damp-proof course (masonry felt/membrane) at the foundation is the bricklayer's most important moisture barrier: without it, rising ground moisture is drawn up into the masonry. In cavity walls it is laid as two layers, and together with an airtight layer it can also contribute to radon protection under BR18 § 332. BR18 sets performance requirements — the dimensions stated are guideline industry practice.
Choose the DPC width according to the wall thickness
Damp-proof course is supplied in widths of 5 / 11 / 15 / 20 / 30 cm. Choose a width that matches the wall thickness, so the membrane covers the entire wall cross-section where it lies.
Supplier widths5 / 11 / 15 / 20 / 30 cmLay the bottom layer on the foundation
Place the bottom layer across the full width and length of the foundation. It can be taken under the ground-floor slab and combined with radon protection (protection against moisture rising from the ground, BR18 § 337), so that the damp and radon barriers are continuous.
Run the upper layer up the inner leaf
In cavity walls, the upper layer is taken from the front of the outer leaf to the back of the inner leaf and at least two courses up the inner leaf. This directs moisture that hits the membrane out towards the outer leaf instead of into the structure.
Up against the inner leafmin. 2 coursesGive the upper layer a fall towards the outer leaf
Lay the upper layer with a slope of at least 20° towards the formwork wall, so water is led out and not in. The fall is recommended practice (Mur & Tag) — not a legal requirement in itself, but crucial for the membrane actually to drain.
Slope towards the outer leafmin. 20°Keep the height above ground level
Place the damp-proof course at least 150 mm above the surrounding ground level as protection against splashback and capillary suction. The figure is an indicative industry measure — BR18 sets a functional requirement, not a fixed level — but 150 mm is widespread practice.
Above ground levelmin. 150 mm (guideline)Tape joints and make them airtight
Overlap joints by at least 100 mm and bond them with bitumen adhesive. Make the membrane airtight at all penetrations — this is both moisture and radon sealing, cf. BR18 § 332 (radon concentration max. 100 Bq/m³).
Overlapmin. 100 mmRadon (BR18)max. 100 Bq/m³Place drainage holes in the membrane
Leave open perpends (weep holes) directly on the membrane in the outer leaf so that water on the membrane can drain out of the cavity. Take care that the holes are not blocked with mortar during bricklaying.
Source
Built on Discebo bricklayer «Standard solution: plinth felt» + «Damp-proof course and plinth felt»; Mur & Tag (mur-tag.dk/fugtspaerre-ved-fundament), BR18 § 332, SBi Guidelines 233, BR18 § 337.