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Making A Structural Opening In A Wall: How To Do A Knock through

In an earlier post we mentioned that we would write a little on the process of making a structural opening in a wall in order to open up two rooms. To be honest the process or procedure is similar for doing a knock through as it is to perhaps creating an opening in a wall for a new window or other. However, this is not to say that the same materials will be used as each individual job will warrant its own set of rules or building requirements.

 

Determining Whether A Wall Is Load Bearing

One of the first things to be considered is whether or not the wall you intend to alter is load bearing or not.  It is not possible to just simply look at a wall and know for sure if it is load bearing until some fundamental and very necessary checks have been made prior.

 

What Is A Load Bearing Wall?

 

A load bearing wall is a wall or part of the structure that is taking the weight of an additional structure or load from above. The wall you intend demolishing or altering may be supporting brickwork above on let’s say first floor level or indeed and more commonly, the first floor level floor joists. In a typical layout of a house with a rear dining room and a living room at the front, the rear room joists commonly run from the external walls, rest on the intermediate wall dividing dining room and living room, and then another joist/s would run from the intermediate dividing wall to the front elevation external wall.

 

This wall carrying those joists is therefore load bearing and will need support before any major alteration can be made. Typically this would be done with the use of acrow props and timbers ‘needles’ or steel supports called strong boys to take the load whilst alterations are being carried out.

 

A common misconception that is sometimes made is that if a wall is made out of stud (timber) then it is not load bearing. Don’t be fooled by this as there are many load bearing walls that are made out of stud!

A thorough check should be made to ascertain the status of the wall.

 

When A Wall Is Non Load Bearing

 

One of the greatest telltale signs is by looking at which way the floor or ceiling joists run. If they run parallel to the wall then it is unlikely the wall is carrying any timber joists etc, however, this is not to say that the wall does not continue above to form the partition wall between bedrooms at first floor level. This can be easily checked by knocking out a small section of ceiling to inspect.

 

For most knock through jobs however, you will be making your opening at a standard 6’ 6” height leaving a good 2 foot or so from the ceiling so there will be a load to support (the bricks between opening and ceiling height) even if the actual wall it self does not carry any additional load from above ceiling level.

 

The Procedure

 

Now that you know the load bearing status of the wall, you can start to prepare the job ahead. You will need to prepare some fundamental things first. Are there any radiators on the wall that need removing? Are there any electrical appliances or sockets that are obstructing your work? If so, you must have your plumber and electrician ensure any wiring or plumbing modification is done prior to the work being carried out.

 

Structural Considerations

 

Before you are about to embark on any structural alteration you must have the load calculated by either a competent builder or structural engineer. Different openings and sizes will require different lintels or supports. Typically on a standard single leaf knock through in a domestic  dwelling with lets say a 6 foot opening in a load bearing wall, a steel RSJ would be put in place to take the load above. The dimensions of the RSJ will vary as per the job it needs to perform and also by your building inspector’s requirements.

 

Your local building inspector will want to be informed of any alterations you intend to carry out to ensure all work conforms to current building regulations. You should also be issued with a ‘completion certificate’ when the work has been satisfactorily completed.

 

Setting Out The Job

 

Mark out on the wall the openings you will be making. Also mark out the position of where the supporting RSJ or lintel will go. The lintel must rest on a concrete pad stone and have a minimum 150mm bearing on either side so measure accurately and mark out on both sides of the wall. I personally prefer to use 200mm minimum bearings.

 

With a hammer and bolster chisel you can now start to chop out the plaster from the wall to expose the brickwork underneath. Do this on both sides. Now make one hole in the wall, for every metre, above the where the lintel is being placed. This is where you will fit your supports and acrow props which will add temporary support to the wall whilst you make the opening. The opening can be chopped out by hand or other mechanical tools can be used such as large diameter angle grinder for cutting through brick. Before placing the acrow props, put a scaffold board or similar on the floor to spread the load induced from the wall.

 

When the opening has been made, ensure the site is clean and free from any debris and prepare to fit the RSJ or lintel. It is wise to set up trestles and platform beforehand to ensure easy manipulation of getting the RSJ in place as they can be very heavy according to size etc. This will be either a 2 or 3 man job depending on the sixe of RSJ you are putting in. It is also wise to put any noggins or fixings in the RSJ, prior to bedding it. The noggins you put in will be used to fix your plasterboard, etc to.

How to do a structural knock through in wall with rsj

How to do a structural knock through in wall with rsj

 

 

 

 

The RSJ should be pad stones with 150 mm minimum bearing and bedded on a stiff mix or mortar at a ratio of 3:1, (3 sand; 1 cement). Once the RSJ is in place you must check for level and plumb and when happy build up any necessary brickwork from above to form a tight fit between existing masonry and RSJ. Any gaps should be wedged tight with slate and then pointed up with mortar.

 

Leave the acrow props in until the next day to let the mortar set. Only then can the props be removed and on doing so, fill in the holes you made for the props with bricks and point up and slate if required.

 

The only thing left to do now is make good the plasterwork from your new opening.

 

Carrying out structural alterations is not a job for the inexperienced and should only be undertaken by a competent builder. 5KC Limited have many years experience in structural alterations and are more than happy to inspect any project you are hoping to undertake which is situated in the Northwest.

 


 

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