Weatherproofing

WEATHER PROOFING

Design and detailing to prevent moisture crossing the cavity are of paramount importance. The following recommendations are intended to prevent moisture crossing to the inner leaf of the wall.

a. Use the correct designation of mortar as recommended in PD 6697:2001 and hEN 1996:2006. See page 37.

b. Take all necessary design precautions to minimise the effects of movement. See page 38 and39 relating to movement control.

c. Provide weep holes in the outer leaf immediately beneath the ground level damp-proof course. Weep holes can be provided by omitting the mortar in a perpend joint.

Suitable spacing for weep holes is at intervals of every 3rd perpend. Clear weep holes should be provided wherever there is extensive bridging of the cavity to safeguard against a build up of water and thus penetration to the internal skin.

d. Ensure the horizontal and vertical damp-proof courses are correctly positioned, adequately long and stepped outwards. Damp-proof course should extend through the full thickness of the leaf and preferably project beyond the face and should not be bridged by pointing or render. Failure to lap damp-proof courses reduce their efficiency. Recommended minimum lap is 100mm as per PD 6697:2001 and hEN 1996:2006. e. Ensure chimneys have effective d.p.c. to eliminate the possibility of water penetrating the fabric. See right for effective waterproofing of chimneys.

f. Use the brick stretcher calculation table on page 35 to ensure the position of window/door openings etc. correspond with brick work.

EFFECTIVE WEATHERPROOFING FOR CHIMNEYS

Build them right: build them watertight.

All buildings for habitation require an effective damp-proof course (d.p.c.) to eliminate the possibility of water penetrating the fabric. This is a well known building requirement, but where problems sometimes occur is in the provision of a suitably effective d.p.c. around a chimney.

Unless the d.p.c. is fitted correctly, waterproofing will be insufficient and moisture is likely to run down the inside of the chimney.

To prevent this happening it is important to lay the dampproof course tray horizontally across the brick or masonry of the chimney and then cover with the chimney flashing, as shown in the illustrations opposite. Shortcuts will result in problems and unnecessary expense.

But don’t just take our word for it! The National Housebuilding Council (NHBC) remind us of how to get it right:

“Above the roof, chimney d.p.c.s should link with flashings. Two d.p.c.s should be used at suitable levels when the roof is steeply pitched, that is where the difference in level between the lower and higher intersection of the chimney with the roof will be more than 450mm. Lead trays should be protected with a thick coat of bitumen or bitumen paint where in contact with mortar. Plastic d.p.c.s are not suitable.”

Extracted from NHBC Standards, Chapter 6.8: Fireplaces,

Chimneys & Flues (Details of weather- proofing are included in NHBC Appendix 6.8-D). Illustrations taken from NHBC Standards.

Chimney on Outside Wall

d.p.c. at front apron level

Combined with d.p.c. Tray and Flashing