Cavity trays

[Oz07]

Is there really a need for cavity trays? When talking cavities 150 or 200mm how likely is it that moisture bridges the cavity. I'm thinking about stepped cavity trays where single storey garage abuts a house. Such a pain to do and if using cavity batts the insulation is never going to be as good around the tray detail. How likely is it any significant amount of water is making its way past the brickwork let alone getting to the inner skin. 

[Mike]

30 minutes ago, Oz07 said:

How likely is it any significant amount of water is making its way past the brickwork

Very likely, unless the brickwork is rendered. In an exposed elevation it can run down the inside of the external leaf pretty freely, especially if the joints aren't well filled.

[Mr Punter]

If the brickwork faces south west and is a large wall, a fair amount of water will get in.  If a stepped tray is difficult, do it horizontal and a coat of Stormdry on the remaining triangle of brickwork for belt and braces.

[Oz07]

I just can't envisage that much water getting in. How many weep vents do you ever see dripping water. How do you know its running down the wall inside?

[Oz07]

Found this on YouTube demonstrating the problem i suppose blown insulation avoids the problem IF it gets in around the trays ok.
 

Probably best to avoid this type of abutment if the insulation can't be done well and water really does stream down inside of cavity. 

 

[ADLIan]

3 hours ago, Oz07 said:

I just can't envisage that much water getting in. How many weep vents do you ever see dripping water. How do you know its running down the wall inside?

I've worked on the testing of cavity walls and cavity insulation to resist wind driven rain. Once the outer leaf brick is saturated the rain will drain down the inner face - add the effect of increasing wind speed and the water can actually 'spray' across empty cavities (or attempt to through the insulation and any joints).

[Mike]

12 hours ago, Oz07 said:

How do you know its running down the wall inside?

Some of the work of a company I worked for was fixing NHBC insurance claims. One of them was retrofitting cavity trays that had been omitted by the original builder; stage 1 of that was cutting out bricks to verify the cause and identify the extent of the problem, so I've seen myself just how much can get through - a lot. Never omit cavity trays.

 

Edited December 10 by Mike

[Oz07]

Crazy I'd never of thought so. I had a garage with a section of 4" wall and piers. Part of it faced south and was quite exposed. Never once had water streaming inside although the wall did look a bit damp after a prolonged period of wet weather. How do we explain that?

[Mike]

5 minutes ago, Oz07 said:

How do we explain that?

Exposure extent, rain quantity, rain duration, wind speed, brick permeability, quality of the mortar joints, low height of the wall...

[Oz07]

Low exposure area, midlands. I would say high exposure relative to the location. Opposite a park at the top of a small hill, no soffit overhang. Rain quantity high we had emergencies declared early in the year lots of local floods. Stretcher bond so average amount of joints. Height of wall only 2.5m. Has anyone ever actually seen water running down the inside face of a 4" wall? It probably does happen in more exposed areas but I've never seen it. I've always used trays in the past just thinking out loud. 

[Mike]

58 minutes ago, Oz07 said:

Has anyone ever actually seen water running down the inside face of a 4" wall?

As per my post above, yes.

[Oz07]

So with a single storey garage adjoining a 2 storey house would you bother with the stepped tray or not. It's going to disrupt the continuity of insulation, im not even sure if blown insulation would overcome this. The garage wouldn't be integrated so would have good ventilation. I wonder if bco would make you put the tray in or in this case let you omit. 

[ADLIan]

There should be a cavity tray at this point. Perhaps it’s not specific in the Approved Document but the Building Regulations require that a wall resists rainwater penetration. In this instance the external wall becomes an internal wall below the roof and under severe wind driven rain the internal wall may become saturated if no cavity tray. I would imagine BCO will insist on one but you need to check as their interpretation & enforcement may differ.

[Mike]

1 hour ago, Oz07 said:

I wonder if bco would make you put the tray in or in this case let you omit.

I suspect that they will, but you'll have to ask. While it may be a garage, few garages these days seem to get used for parking cars - and whatever it gets used for instead may be disrupted / damaged if rain does start dripping in.

[Oz07]

25 minutes ago, Mike said:

I suspect that they will, but you'll have to ask. While it may be a garage, few garages these days seem to get used for parking cars - and whatever it gets used for instead may be disrupted / damaged if rain does start dripping in.

Yeh but if the other walls of the garage are 4" brickwork with pillars what would it be achieving. Logically it wouldn't have any benefit?

[Oz07]

2 hours ago, ADLIan said:

There should be a cavity tray at this point. Perhaps it’s not specific in the Approved Document but the Building Regulations require that a wall resists rainwater penetration. In this instance the external wall becomes an internal wall below the roof and under severe wind driven rain the internal wall may become saturated if no cavity tray. I would imagine BCO will insist on one but you need to check as their interpretation & enforcement may differ.

What if the other 3 walls of the garage 4" brickwork

[Mike]

2 hours ago, Oz07 said:

What if the other 3 walls of the garage 4" brickwork

Sheltered under the eaves / gable & with no brickwork above, so no dripping water even if it the inner face gets wet.

[DannyT]

On 10/12/2025 at 10:45, ADLIan said:

I've worked on the testing of cavity walls and cavity insulation to resist wind driven rain. Once the outer leaf brick is saturated the rain will drain down the inner face - add the effect of increasing wind speed and the water can actually 'spray' across empty cavities (or attempt to through the insulation and any joints).

I’ve seen this happen on my parents house. I built the house and while working inside with house weather tight I could hear dripping in the cavity onto a tray. 
knocked a block out and the back of brickwork was soaking.

Rain had been driving across open fields for 2-3 days.

Mainly coming through the mortar joints.

Coated the brickwork with a breathable sealer.

Cavity trays are a must.