Section A: Pre Construction
BCAR Building Control Amendment Regulations
You should inspect the site and surrounding areas to get a complete overview of what you are buying. This should only take a couple of hours. To ensure a complete and thorough site visit, the following should form part of your inspection:
Check places names
Does the place name tell you anything about the area? For example:
Look at nearby buildings
a. Any signs of damage? If there is visible damage investigate possible causes.
Diagram A1 - Visible damage on elevation
Please note the following with respect to cracks:
Table A1: Extent of Structural Damage in Low Rise Buildings - Reference: BRE Digest 251
b. Is the site a sloped site? What effect does a slope have? Reason for any gaps?
Building a house on a sloped lot presents unique challenges that you need to be aware of to ensure the rewards for you are worth the additional costs and longer construction time.
Diagram A2 - House on a sloping site
What is the reason for the gap between the houses?
Will the house you wish to build have any impact of existing houses?
Will any existing houses impact on what you are proposing to build?
Gather Anecdotal Evidence
“This area used to be a floodplain but they filled it in.”
By taking such a simple step as speaking with people who are familiar with the area you may uncover important information relating to previous problems on this site and possible problems that may present in the future.
After first obtaining the landowner’s permission, walk through the site and other adjoining land. It is not sufficient to drive by or stand on the boundary and look in, you need to walk through the site and inspect it carefully. The following should be investigated:
- What type of ground is it?
- Are there any trenches or uncovered areas that show ground strata?
- Is there vegetation on the ground? Could mean marshy ground.
- Is the ground raised? Was the site used for another purpose?
- Are there rivers nearby? Is there a risk of flooding?
Diagram A3 - Things to look out for during a site walk through e.g. Marshy Ground
Test Ground – Trial Pits
Whether or not you are confident that you have a detailed knowledge of the site which you wish to purchase it is still advisable to undertake some ground investigation work. With the owner's permission you should employ the services of an excavator to dig a number of trial pits. Trial pits to a depth of 4m are considered a cost effective way to uncover relevant issues that would impact buildings up to three storeys.
Photo - Undertake a detailed site investigation
Photo - Dig a trial pit to determine site specific conditions
Number of trial pits required
There is no fixed number of trial pits required, the number required depends on the changeability of the ground. The correct number of pits have been dug when you are satisfied that the nature of the ground below the surface is accurately depicted.
Given a site of 7 houses:
If conditions do not vary 4 or even 2 may be sufficient.
If conditions vary to a large extent, it may be necessary to dig 7 trial pits.
It is never advisable to just dig one trial pit as one pit is almost never reliable.
Locations of trial pits
It is not advisable to locate trial pits in the area the foundation is likely to be as the trial holes will create soft spots in the ground. In the event that foundations are at varying levels and/or bear on different layers of subsoil, specialist foundation design may be needed.
If the trial pits show the ground to be very changeable, proving borings can be taken as well between trial pits and with the dwelling area to provide extra detail.
Importance of a ground investigation
When contractors are tendering for a contract on a client’s own land it is very important that a ground investigation be carried out as outlined above in order to minimize disputes causing unforeseen delays.
Health and safety
Ensure the appropriate precautions are taken before inspecting trial pits.
Reasons for site being undeveloped to date?
Photographs and historical mapping can provide useful information about the history of the site and its characteristics. It is wise to see what maps and photographs are available, even if you possess a good knowledge of the site already.
Diagram A4 - Sample OS Map
Ordnance Survey (OS) Maps
Study the most recent mapping available. Studying this map may answer the question as to why the site is not already built on. Older Ordnance Survey maps may be useful also as they may show buildings, rivers, ponds, ditches, floodplains etc., which may have subsequently been removed but may have an effect on building.
OS maps are available from the Ordnance Survey Office, Phoenix Park, Dublin 8.
Building on sites located in floodplains or prone to flooding should be avoided.
Diagram A5 - Foundations on a site with a high water table
Signs that the water level is likely to be high are:
The vegetation indicates the area is prone to being flooded
Even in dry weather the ground remains damp
The site is surrounded by higher ground, the levels of surrounding houses are raised.