Render with a Mechanical key.

[Terry M]

Hi guys,

I have been reading the render threads but I would like some more information if you would be so kind.

Due to re-ocurring small cracks in a bedroom wall (internal), I have decided to removed the plaster back to brick.
In the line of the cracking the mortar is nearly all missing as well as in other areas of the wall.

So to add some strength into the wall I am thinking of the following process :-

1. Raking out all the brickwork mortar joints (40mm depth)
2. PVA the wall
3. Fix 12mm battens (vertical)
4. Sand + cement (4 in 1) for a single float coat 12mm
5. Skim wall at later date.

My understanding is that by raking out the mortar joints, I will have created a mechanical
key for the 12mm float render coat, so the scratch coat could be by-passed.

Is it ok to use this mechanical key instead of doing a scratch coat ?

Will a single 12mm coat be ok or is it too thick in one go ?

I would love to hear you comments on my plan (particulrly the mechanical key) as well as
any other suggestions.

Many thanks.

 

[Terry M]

Guys, could you please spare me a minute or two and take a look at my post (above). Many thanks.

 

[23vc]

Address the cause of the brickwork cracking first, then you could do as you say but it would be easier to dot and dab it or hardwall it.
When I've rendered internal brickwork I've never given it any additional key but still always do a scratch coat. If your worried about it moving again you could mesh it (eml) which would also give you your key

 

[Terry M]

Thanks 23vc.

A roof purlin is sitting on that internal brickwall (in the loft space, 6 courses into a pyramid).
This small number of bricks has moved.
Knowing this I wanted to check-out the small crack in the plaster above the door opening in the bedroom below.
Once exposed, the brickwork above the door hadn't actually moved, it was just completely missing its mortar in 45degree (5courses) running down the timber lintel, (which also hadn't moved).

However as the timber lintel did have a split going length-ways, I have since replaced it with a concrete lintel.

In the loft-space I will remove the small number of bricks, then :-
1. first lay a concrete lintel (1500mm) to spread any load so that the door opening is not a weak point.
2. rebrick the small number of bricks to catch the purlin.
.
Info : Acros already in place supporting the purlin.

To do the rebuilding/reinforcing work in the loft, I just wanted to make sure that the wall below it (in the bedroom), was as strong as I could make it, hence the rendering.

Other than the crack in the loft and the missing mortar above the door, there are no other cracks/damage in any walls internally or externally.

 

[vinn]

OP,
Before going any further with your work you would help yourself if you posted photos of the issues you've raised above?

Has any part of your house ever had a knock-through?

Your original post was probably ignored because there are so many back threads on these subjects and very few OP's research them.

 

[Terry M]

Thanks Vinn,

Your original post was probably ignored because there are som many back threads on theses subjects and very few OP's research them.

I am sure you are right but this was not the case, with me. I did spend quite sometime researching but was unable to find info on my specific query regarding the 'mechanical' key.

In fact I have been using this forum for reference for many years for various trade queries (building, plumbing, tiling, etc.)

Has any part of your house ever had a knock-through?

There is a knock-through but it is in central wall 90 degrees to this wall and it is on the other side of the house. This was done prior to me owning the house (over 10 years ago). No cracks or movement on that wall.

I have uploaded some photos. Grateful for all help, advice and suggestions.

 

[vinn]

What appears to be happening is the roof plane is pushing the purlin(s) sideways which is transmitting to the wall below.
The doorway doesn't seem to be distorted so no need to change the lintel.
Neither is there any need to rake out and render the wall. Simply point up where necessary.
Use EML across the cracks.
You could firm up the wall with a rake-out and render but I dont think thats needed.

In the loft you could re-build the existing purlin support, and extend the supporting brickwork towards the centre of the house using EML in each course bed. Creating a kind of buttress.
There are two difficulties with this approach:
1 - the binder beam has also been moved presumably by the same diagonal pressure, and is now slightly tilted.
2 - how to support the purlin(s) while doing the work.

What about the purlin on the other roof plane?

Given that your roof cover lacks an under felt and was originally torched, it might be best to consider doing the work when doing a re-roof?

I hesitate to give further long range advice where such structural issues might be involved - why not call a builder in for a site opinion?

 

[Terry M]

Unfortunately in my haste, I have already raked it out. What a mess that created. I won't be repeating that task in a hurry.
I now see that your idea to use EML and render was very sensible and the right option.

I have called a builder who was not unduly worried. He commented that there was not huge amount weight in the plane of the roof.
The builder was not keen to take on the job because of the awardness of working space/access.
I agree it would be so much easier to do it whilst its being re-roofed but unfortunately I do not have then funds for that at this time.
The front plane appears ok.

2 - how to support the pulin(s) while doing the work.

Once the disturbed brickwork has been removed, I am planning to lay a 1500mm concrete on top of the bedroom wall (butted up to the tilted binder), to help spread the diagonal/downwards force.
I will then brick-up on top of the lintel to catch the purlin using Class B engineerings.
I was also thinking of installing some re-bar (vertically) within the engineering brick support to prevent any future lateral movement.


Left Purlin (photo 2, #6), I am guessing that the span of is over 5m, which I think is quite some distance without any midway-support. Also the brickwork that this end of the purlin is sitting on is only butted up to the external wall and NOT keyed-in and appears to be precariously sitting on only
2 x ceiling joists, (see below).
As there is no solid wall running up into the loft (apart from the disturbed brickwork), I am thinking of installing a 152x89x16UB 3.5m (length) from central wall to the rear external wall and then bricking-up to create the purlin midway support. To this I will have to notch the underside of the binder beam to run the UB across.

I do always take the belts and the braces approach.

Overkill or reasonable. What are your thoughts ?

Many thanks.

 

[Terry M]

Vinn, where you gone mate ?