Whose advice should I follow - my builder or my engineer?
Query: We want to build a small extension for two en suites onto our 1970s bungalow. Our engineer says we need to pile for foundations but our builder says floating a foundation and dowling it to the existing foundation would be grand. Both were at the digging of trial holes so know what the ground conditions are. There is a significant price difference and I don't want to bury good money (literally) if I don't need to.
Answer: To try to answer your query as best I can, I am assuming you engaged both the contractor and the structural engineer on the basis of each being best placed to provide you with the service you require from each.
The contractor has responsibility for construction but not for the design of the foundations, which is the responsibility of the engineer. The engineer could check the proposal by the contractor to establish its feasibility but, ultimately, it is the engineer who advises on the foundation design and takes responsibility for it.
The engineer is trained and qualified in calculating and detailing structural works, such as the design of foundations. Their designs will be based on calculations which tell them whether or not a particular solution will work, with a tolerance built in, in a given circumstance. Obviously, with foundations, some assumptions will have to be made with regard to ground conditions, even with the benefit of soil tests and trial holes and, accordingly, the engineer may err on the side of caution. In this instance, with the extension works being relatively modest (two en suites attached to a bungalow) they must have significant concerns surrounding the ground conditions if they are proposing a pile foundation detail.
Design of the structural elements of the build is why you have engaged a structural engineer in the first place, and, accordingly, in this instance I would suggest the advice be followed, as this is what you are paying for. You note you don't want to "bury good money" and if you follow the advice, you will likely never know if you did, but if you don't follow the advice, you may one day find out that you should have!
At the end of the project, the structural engineer should issue you with an Opinion on Compliance with Part A (Structure) of the Building Regulations. If the foundations are not constructed to their design they may not be in a position to issue this to you. Accordingly, it may prove to be a difficulty if the house is ever to be sold or, more seriously, if the structure fails, your insurance policy may be compromised and the cost of remedial works may be significant.
Do you have a design dilemma we can help you with? Email your problem to firstname.lastname@example.org. Advice is for guidance only and readers are advised to seek professional assistance for any
- Gareth Brennan is a partner in Brennan Furlong Architects & Urban Planners, brennanfurlong.ie
- If you are considering changes to your home, work with a registered architect. Check on riai.ie, the registration body for architects in Ireland.