Lintels, weep holes and render

-
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

    ReplyDelete

Post a Comment

Popular posts from this blog

The controversy over Cavity Wall Insulation 'topups'

-
Image
In this article I talk about the practice and problems of topping up insulation when voids are discovered, why it is being done and the potential violations of building regulations. I can only put forwards what I have understood from public documents and personal correspondence  So I welcome corrections or clarifications from the industry on this important issue for home owners and will happily update this post to reflect any inaccurate statements. We know that 100s of topups are done every year.   From what I can gather : If the BBA have not approved these topups then the relevant local authority building control should have been notified prior to topping up as an 'off licence' use of the product.  It is not clear that the correct procedures are being followed in all cases. Background :Voids are common A void is simply an area cavity between the walls which is missing insulation.  If a void is large enough then the insulation will not be compliant with the meter square
Read more

CWI, Impermeable masonry paint, hard cement and breathability

-
Image
Waterproof or let's call it plastic masonry paint made with silicone or acrylic sounds like a fantastic idea to keep that pesky rain away from the wall. Unfortunately it can easily cause the opposite in terms of moisture management and inflict devastating damage to old solid walls which were originally made with a breathable building envelope. This article serves as a discussion to highlight some of the unexpected problems which show up in the real world verses the limited testing of the BBA certificates and I welcome discussion from anyone interested in this subject. For a good 'primer' on the problems associated with plastic paint in solid walls, Peter Ward from Heritage House has a number of videos of detailing the sacrilege caused by often well meaning tradesmen applying these aggressively marketed products. I recommend watching all his videos especially if you are having any sort of damp problems so that you can avoid the multimillion pound interventions sold by 
Read more

Discrepancies between Internal and external DPC (damp proof course)

-
Image
In a cavity wall construction, one would expect that the damp proof membrane to be at the same level on both walls. It turns out this is not always the case and my house is a good example where the internal wall's DPC is more than a brick lower than the external wall DPC. I didn't even think to check this before reading the CIGA Technical note 14 . This clearly states that if the distance between DPC and ground level is within a brick (as was the case with my property at time of install) then 'numerous other factors' must be assessed before declaring the dwelling as being suitable for CWI. CIGA  Technical note 14 . One of the factors is the relative level of the inner and outer leaf DPC. This is exactly what I found in my property. Ive used a floorboard to show the position of the internal floor. The DPC is clearly a brick below the floorboard and less than brick above the block paving. External leaf DPC is 1 brick below the floor level.
Read more