Best Practice- Metal Trays
In Brick Chimneys

Brick Chimneys

The chimney that will give least trouble from a rain penetration point of view is the rendered stack with a pre-cast concrete capping. The rendering, in the form of plastering or dashing, acts as a water-proof barrier and prevents the chimney stack from getting wet.

In the case of a brick chimney not incorporating cavity construction, a metal tray is required across the full width of the chimney construction. This will prevent the downward path of any water that is absorbed into the brickwork. It is very important that this tray is built in at the correct level in the roof and that the construction incorporates an upstand tight around the flue, an upstand around the edges of the tray, weep holes, and associated stepped flashings. Photographs 1 and 2 illustrate the effects of water penetration where metal trays have been omitted, or installed incorrectly.

Metal Tray Construction

HomeBond requires that trays be constructed of a non-ferrous metal - lead or stainless steel. Photograph 3 shows a lead tray manufactured from one piece of Code 5 lead (5 lbs./sq ft), and incorporating a minimum 50mm upstand tight around the flue, around the edges, and turned down at the front. To prevent corrosion of the lead and to avoid staining of adjoining materials, lead trays should be coated with a bituminous paint where in contact with mortar.

Photograph 4 shows a stainless steel tray of similar construction, which is available from a number of manufacturers.

Galvanised trays or plastic DPC trays should not be used.

Upstand

All trays must incorporate a minimum 50mm upstand, tight around the flue and the edges of the tray as shown in the adjoining photographs. This is necessary to prevent any water that builds up on the tray from flowing down the side of the flue or over the edges of the tray.

Photograph 5 illustrates a chimney in a dormer roof, with the back of the tray in a habitable room. An additional upstand is required in this case, in order to prevent any water ingress into the room.

Alternatively, the tray should be positioned at a higher level, with associated cover flashings.

It is vital that the drainage channel at the front of the tray is free of any mortar as shown in Photograph 6. This will allow any water to flow towards the front of the tray and discharge over the lead flashing. If this channel is blocked or filled with mortar, water may build up and may discharge over the edge of the tray and into the attic space. Mortar beds should be limited to the area directly beneath the course of bricks and not across the whole section of the chimney tray.

Weep Holes

The discharge of any water collected on the tray is achieved by means of weep holes at the front of the chimney. These "holes" can be formed by leaving open perpend joints in the brickwork, as shown in Photograph 7. Alternatively, proprietary plastic we hole vents can be inserted in the perpends as the brickwork is constructed, as in Photograph 8.

Some manufacturers now include integral drainage channels in the tray construction. This feature, and the weep holes in the brickwork, ensure that any water drains away freely.

Photograph 1
Water staining to chimney breast

Photograph 2
Water staining in attic space

Photograph 3
Lead tray with 50mm upstand tight around flue and edges. Lead tray to be painted with bitumen before building in.

Photograph 4
Stainless steel tray incorporating drainage facility. 50mm upstand at flue and edges.

Photograph 5
Chimney in dormer roof construction. Additional upstand is required at back of tray as indicated.

Photograph 6
Close up of drainage
facility shown in photograph 4. Keep drainage channel free of mortar.

Photograph 7
Weep holes provided in brick joints. (Perpends left open).

Photograph 8
Weep holes formed by using plastic
inserts in brickwork.

Photograph 9
Chimney tray built - in at incorrect level - too high.

Position of Trays

Whatever the position of the chimney on the roof, the metal tray should be located 150mm min. above the lower side of the chimney, immediately above the front cover flashing. The correct position of the metal tray is shown in Figure 1. Photograph 9 illustrates a tray that has been built - in at the wrong level.

A stepped lead flashing in Code 4 lead should then be dressed around the sides of the chimney, the top edges of same turned into a 25mm deep chase in the mortar joints. The folds of the flashing are wedged in with small pieces of lead, and the brickwork repointed. Figure 2 shows a stepped lead flashing.

Figure 1
Metal tray position,
chimney in middle of roof.

Checklist

Brick chimneys of solid construction require a metal tray.

Use Code 5 lead or stainless steel trays. Galvanised or plastic dpc trays are not acceptable.

Lead trays must be coated with bitumen paint on both sides.

Ensure upstand is minimum 50mm, tight at flue and around edges.

In dormer roof construction, an additional upstand may be required.

Install tray upright, level and in correct position.

Provide weep holes in brickwork and ensure these are kept free of mortar.

Fix stepped lead flashings correctly and ensure brickwork is adequately chased (25mm) to receive same.

In areas of severe exposure the use of brick chimneys is not recommended.

Figure 2 Stepped cover flashing on flat interlocking tiles