Lintels, weep holes and render

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I found the notion of lintels in cavity trays difficult to visualise.
The reason I'm looking into lintels is that there is a damp patch that runs along the lintel over the front door.

Brick houses built today will have cavity trays and weep holes inserted into the design.
You will spot weep holes these as either gaps in the mortar or where the mortar is replaced with a perforated plastic insert as below.
CIGA state that no water should enter the cavity is properly maintained building. If that is so then why does building control mandate a cavity tray.

Typical modern cavity tray with weep holes sitting on a steel lintel.
The purpose of a cavity tray is to ensure moisture that gets in the cavity when driven through the mortar joints or other means is directed back to the outside wall and out of the building.
If you didn't have a cavity tray or some sort of damp proof course then moisture would collect on the flat lintel and then flow off the sides of the lintel or soak into the internal wall causing wet spots on the internal plaster.

So what about my house? Houses do not unfortunately come with a manual or a service book. The home buyer's surveyor had stated that timber lintels were likely due to the age of the property (1930s). The house was originally rendered and there are no weep holes.

According to https://fet.uwe.ac.uk/conweb/house_ages/period/print.htm for 1930s house:

Concrete and steel lintels support loads over openings. Segmental arches and timber lintels still common.
So that doesn't help a huge amount. Also  there's always the possibility that lintels were replaced when the doors or windows were changed.
I ran a stud detector over the lintel and it lit up like a Christmas tree all along the length and a bit more either side which suggests it's steel or at least re-enforced concrete.

There are a few different different style of lintels in use in 1930s as per here:
https://fet.uwe.ac.uk/conweb/house_ages/evolution_walls.pdf

Do rendered walls require weep holes?

So there's a chance that my 1930s house had felt, bitumen or lead cavity tray or possibly even none.
There are certainly no weep holes in the rendered surface.
As it was originally intended to be rendered there may be little in the way of a cavity tray. It was assumed that render would protect against the majority of wind driven ingress and that the small amount of moisture in a cavity tray would usually be ventilated away by the open cavity.

In regards to render. Building regulations sate that
 “weep holes need not be provided if walls have a rendered external finish."  and here it states :This relaxation is based on the assumption that the small amount of water that may penetrate the outer leaf and collect in the cavity tray will dissipate by natural ventilation. This exception only applies when there is no unrendered brickwork above the rendered section.

 I assume if there is a cavity tray then it looks something like this:


 Now if you pump a load of wool into and above this tray then it will not function correctly as any water collected will no longer evaporate as the cavity is now unvented.
Worse than that any wool in the tray has the potential to get soaked and stay soaked from either progressive water ingress or a single event like a bad storm or a broken gutter.

I really couldn't find much information on this issue. Only a report that says that the cavity trays should be free of mortar before proceeding with CWI.
If there is no tray an it's a flat lintel then the wool on top of the lintel getting wet would always transmit some moisture to the internal skin.

If there is a cavity tray. I'm not a building expert but it seems like an incredibly bad idea to pump water absorbing wool into a cavity tray. And I wouldn't trust the Cavity man to avoid drilling holes in in the tray.


Comments

  1. Geoff27 November 2018 at 12:15

    Very good research. I was not aware that anyone was suggesting cavity walls don't get wet inside as this is completely untrue. The cavity wall is misnamed it should be called a double wall as this tells you how it works. All bricks get wet and so we have an outside wall that can get as wet as it likes and an inside wall that then stays dry. The cavity is simply the gap between these two walls. if you fill the gap between the two walls you get a solid wall and one of the principal design defects of a solid wall is that eventually it will let moisture through to the other side. You may find that if insulation has been installed just above a concrete lintel that the relative temperature between the wall and the lintel is different enough for condensation to occur only on the lintel and this would give the staining that you have described. When a concrete lintel holds up the inner and outer walls it then Bridges the gap and therefore can become as cold on the inside as it is on the outside. This is called a cold Bridge and is very Common in older houses and in homes of the 1930s as yours is.

    A cavity tray must also be turned up at the ends to make sure that any water running down the outside face of the cavity does not simply drain down either side of the window. This is a very common building defects

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