Site set-up through to foundations is a broad and mucky process which is made infinitely easier by some good planning and well sequenced work. Most of us will be working in urban locations where generally densities are more intense and site boundaries quite tight and, if it’s just a single dwelling project (as most will be), you won’t have the luxury of borrowing some space as the works are phased.
So good planning before you start on site is essential. Last month we looked at the preliminary costs which included provisions for secure storage, safety in unloading lorries, welfare facilities and meeting spaces. All of these functions are going to need an allocation of adequate space and, as many self builders will choose to do, some sites may also need to accommodate a mobile home. From a cost control point of view, the principle has to be about getting the sequencing right, avoiding the double handling of materials wherever possible and to reduce the need to endlessly move facilities around on site all of which is a non-productive use of labour.
Many of us will be replacing a dwelling and, unless you’re lucky enough to be moving the siting or curtilage, this is going to involve its demolition and clearance before work can start on the new house. Don’t be fooled, nine times out of ten, this will be a cost to you. Yes there will be some salvage in the house being demolished but generally not enough to off-set the cost of the demolition process itself. Salvage materials that have value include old clay roofing tiles, old bricks which have been laid in lime mortar (as this cleans off very easily), stone slabs, good quality floor boards, period metal framed/leaded windows and so forth. [Picture opportunity of a salvage yard with their typical materials on view]
However, it’s a trade off as, to retain value, additional time is required to dismantle rather than demolish and salvage companies will have to factor collection, loading and unloading again before securing their profits on selling these materials. Best practice advice is to tender the demolition to a specialist company who will calculate any salvage value, offset this against their costs, and give you a fixed price. You should ensure that this includes the removal of all old foundations as well, plus the backfilling and levelling on the disturbed area of the site.
For those of you who would like to manage the demolition yourselves, there are additional health & safety considerations where all demolitions must be notified to the HSE, regardless of the project’s status/definition, and Building Control as a regulatory procedure. And don’t forget to have all the existing main services properly terminated prior to the demolition work starting! [Picture Opportunity of any demolition in progress]
Where there are no existing buildings to be demolished, the site will probably still need some clearance which will include any unwanted vegetation (grass, weeds, bushes and trees), old rubbish and debris and then finally the top layer of top soil which you will almost certainly want to keep.
Regarding trees, you have to make sure that none are protected by a tree preservation order (TPO), which you can do by consulting your local Council records, or that your new planning consent has not included discrete conditions regarding tree management on site. It will be expensive in fines, and possibly time, if you breach these conditions.
Stock piling your topsoil is a good idea, if you can make the space available as, you will definitely need some of this at the end of the project. If there is any old metal laying about on the site then call in your local scrap dealer as this will have more value than you might think. And for general rubbish, don’t be tempted to bury it as this may cause you problems in the longer term; use skips or muck away lorries to get rid of this unwanted stuff!.
Once the site is cleared it’s time to mobilise your infrastructure. We’re looking at drainage and services next month but, if you’re moving a mobile home onto site, you may need to address these as a priority. Of course, if you have demolished an existing building, you’ll already have been in contact with the water, electricity and gas utility providers and may well have obtained quotes for temporary supplies. Certainly from a building perspective, water and power will be needed one way or another.
The preliminary costs for site set up which include the hire of temporary security fencing, metal storage containers, WC’s, welfare etc., and the provision of a robust site access and hard-standing area can be expensive and, once installed, you don’t want to have to move them if you can help it. So, as a basic checklist, make sure you can get the maximum size lorry onto your site for a safe unload, make provision for a crane if you are planning to use off-site pre-manufactured components and think about where your drainage and service supplies will both access and terminate the site so all your trenches can be dug without interruption. You’ll be using lots of skips and these will need to be positioned for an easy exchange. Finally, think about where your contractors can park their vehicles!
There isn’t scope in this article to give you much information on foundations suffice to say that there are really three options which have broad financial categories and which will be deployed as a direct result of your site’s ground conditions.
The first option is known as ‘strip footings or trench fill’ and is the most common form of foundation and generally the cheapest. Essentially trenches are dug under your loadbearing wall locations, either 450 or 600mm wide (dependent on your superstructure design), hopefully to a typical depth of 1m and filled with either mass-fill concrete or a mixture of concrete and foundation blockwork. However, the final depth of these trenches will be dependent upon the composition of the soil and you may have to keep digging until you get to a level where the resultant strata is sufficiently compressible to support a foundation. Sensible developers will have had a soil test so that they can plan for assumed depths prior to starting their build.
It costs about £12/m³ to excavate your trenches, about £25/m³ to cart the material away and then anywhere between £80 and £110 to fill it with concrete. At 1m in depth and with 600mm wide trenches, a typical house at 10m x 10m may have 36m³ of foundations which, using the above rates, would be a cost of around £5,000. And so if it proved necessary to take these trenches to a 2m depth, then this would add an additional £5,000 straight out of your contingency.
With poor quality soils, which might include a shrinkable clay or made up ground, rather than digging very deep trenches with the associated costs as indicated above, you could consider the second option of a ’concrete raft’. Here the ground floor slab and a thickened area under loadbearing wall locations is all reinforced together with appropriate reinforcing bar such that the whole slab acts as one combined unit. The reinforcement has the effect of helping to spread line loads over the whole slab which acts much like a raft sitting on the ground. There are no standard rafts, per se, but instead an Engineer will consider both the superstructure’s design and the prevailing soil conditions and then design the raft to an appropriate thickness and with reinforcement to suit. It’s more complex for your contractors to build and costs generally make this a more expensive option than the simplicity of digging simple foundation trenches, but there will always be a tipping point where the raft becomes a more sensible option.
On some sites, the soil composition will be so poor that the only option will be to bring in a ’specialist piling contractor’ who will sink concrete piles, based on an Engineer’s design, to depths where the strata can either provide support or where they are in sufficient quantities to let friction do the work. This is expensive and these costs should be reflected in a reduction in price for the site upon purchase. This is why the golden rule should always be to have a soil test prior to buying your site!
This article was written by Tim Doherty, and originally published for Build It Magazine. Tim is the Director and Principal Surveyor of Dobanti Chartered Surveyors, a building surveyors based in Tunbridge Wells, Kent. Get in touch for more information about Dobanti’s property and building services, or read more online now. Further articles and blog posts can be found here.