Extensions...Advice and Ideas From A Structural Engineer
About Our Experts - Hogan & Associates, Chartered Engineers
Hogan & Associates, directed by Michael Hogan have over 25 years experience in home and commercial building projects. Our services include consultancy, design, fault diagnosis, project management, surveys and compliance issues.
First of all, are there any dangers in building an extension? And, what are they?
First things first, the structural design must be right. That's vital for both comfort, and safety. The main items be considered from a structural angle are:
- Tying in to existing structure. Exactly, how is this going to be done? Does the existing structure have any issues that could complicate this such as subsidence, cracking or poor construction.
- Next thing to consider is the foundations and drainage. Many older houses in Dublin don't actually have foundations. They are built on a "wide wall" which may be undermined by adding an extension. Older houses also often have inadequate drainage systems which will need to be replaced or upgraded.
- Finally, we look at loadbearing walls in the original house, and the impact the extension will have on the house structure.
After structure, the next thing I would consider is the comfort of the extended house. Comfort and liveability are an absolute priority. For example, a badly designed extension can make the original building darker. So position, and size of windows in the extension need to be carefully considered.
Could the extension have any other impact on the original house?
Absolutely, the next thing to consider is the various mechanical and electrical systems within the house. Will they be able to support the new extension. For example, it's important to review the following:
- Is the heating system capable of handling additional load, or will it be overloaded? Older systems may have low level water leaks in pipes or at radiator valves. When new heating equipment is added, and the system is repressurised, these leaks may cause trouble.
- Will there be adequate hot water? This is particularly important when new power showers, appliances or bathrooms or being installed.
- Will the water pressure be adequate?
- Are the existing heating controls sophisticated enough?
- Is the wiring capable of handling additional loads? New extension will mean additional sockets, lighting, applicances. Particularly, in older house, the existing electrical infrastructure may need to be upgraded.
What about the actual construction. What are the potential pitfalls here?
Like design, the construction also needs careful planning to prevents problems "slipping in". The types of problems that can occur include poor workmanship, unsuitable materials, inexperienced personnel, lack of awareness of current regulations and standards, health and safety shortcomings.
On a positive note, all of these problems are preventable with proper planning. For example, a professional project specification will include exact details of materials to be used. And as part of our tendering process, we check that health and safety procedures are in place, and personnel are trained in those procedures.
What else can be done to reduce the risk of problems?
Of all areas, probably the one that has the potential to create the most stress during the project is contract administration. Who is controlling costs? Who is liable for defects? Who is liable for costs in scheduling overruns?
Again, the key here is in the initial planning. For example, a careful contractor selection process will identify a contractor with the experience and track record for your extension. Likewise, a well thought out contract will ensure all main eventualities are planned for. These include:
- Terms & conditions of contract
- Tendering/contractor selection process
- Cost control
- Value for money
- Defects liability
What else can be achieved through good, early planning?
Virtually all heat loss is through structure - floors, walls, roof, windows or doors. All of these can be designed for best energy performance whether with retrofit or more effectively when new elements are being added. With proper planning, very good BER ratings can be achieved - even close to Passive House standards if required. Adequate underfloor insulation, external wall insulation, 'warm roof', thermal bridging treatment, airtightness - these are all aspects that have to be addressed at the earliest stages of the structural design - for new build and retrofit.
Is there any other key areas where advice would be needed
Yes, compliance issues need to be understood. Again, if they are addressed correctly, you can expect no future problems. However, the following issues may need to be considered:
- Planning limitations
- Building Regulation requirements
- Conservation considerations
- Exempt developments
Many homeowners already have a single floor extension, but would love to extend up stairs too. What's your advice?
Usually the most obvious and cost effective approach to extending a property is to do so at ground level. However extending over an existing single storey area can often be more useful and practical - in particular to provide additional or enlarged bedroom areas or bath/shower rooms.
A prerequisite for such a development is a careful assessment of the structure of the existing and its capacity to carry additional loading. A structural engineer is absolutely necessary for this assessment.
Most homeowners now want single"open plan" room between extension and original kitchen, without any beams where the old and new rooms meet. Is it possible to achieve a single clean, "beam free" room?
Very often downward protruding beams appear or remain after building work. These can be unsightly and tend to divide rooms and break up a potentially clear ceiling area.
It is usually possible to locate such beams above or integrate into floor or ceiling units. However, forward planning and careful structural design are needed to do so. Also the opportunity to do so cost effectively can pass if not considered early enough, preferably before any building work starts.
Great improvements can also be achieved for existing situations. With proper analysis and design, it's possible to perform a type of "keyhole" surgery, inserting a new hidden supporting beam, and removing the protruding beam. Very often these improvements can be designed and arranged to cause the minimum of disturbance and disruption to walls and ceilings. We've currently carrying out two projects where this type of "surgical" approach is being used to create a seamless extension, without a major disruption to the existing ceiling.
Hogan And Associates | Olive Mount House, Olive Mount Road Windy Arbour, Dublin 14
I wanted somebody with an engineering background because we were taking out walls and a lot of structural steel was needed...Yvonne O'Driscoll from Mount Merrion, Co. Dublin