Repairs: drainage

If you need to relocate the sewer, in addition to the checks highlighted earlier, you will also have to:

• establish that there is sufficient invert depth for the new drain run to operate effectively; and
• consider appropriate treatment of any old drain runs that have now become redundant, to prevent them becoming infested with vermin - pipes need to be either removed or properly and effectively stopped up.

Solution 1

When carrying out works next to older buildings it is common that a new drain run will end up being carried past the edge of an extension. Where the drain is lower than the foundations, appropriate care needs to be taken.

• Where the edge of the trench is less than 1m from the edge of the building main wall footing, the pipe has to be encased in concrete up to the level of the bottom of the footing.
• Where the sewer pipe trench is more than 1m from the edge of the main wall footing, the trench should be filled to the equivalent distance from the footing edge to the trench side less 150mm.

The Building Regulations set out guidance for flow/gradient of different pipe capacities. For example, a typical 100mm diameter pipe underground needs to be laid at between 1:10 and 1:70 gradient (the latter for up to 7 appliances connected).

When planning the works, and after the drains have been inspected, consider ongoing risks:

• Is the soil a very shrinkable heavy clay?
• Does the soil contain a high proportion of gravel or other fine material that could wash out and consolidate if there was a failure and leakage?
• Are there a lot of trees in the area which, combined with a clay soil and subsequent drains failure, might lead to risk of foundation damage?

If the drain run is a sectional clay pipe and you do not propose to replace it because the condition is more or less adequate, you may wish to consider carrying out relining some critical 'at risk' sections, such as those running adjacent to trees to prevent tree root ingress in the future.

Resin-bonded polyester sleeve linings are comparatively straightforward to install, particularly in deep drains, which avoids digging and trenching.

Modern plastic drains are quite adaptable. There are a variety of fittings such as flexible plastic couplings that can be connected onto existing drain runs using a large version of the conventional hosepipe clip, all reducing the amount of digging required – provided the rest of the system is up to specification.

Once complete, test the system. If it is an older drain it may be more appropriate to use a water pressure test rather than an air test (which is notoriously difficult to apply).

Ensure the drain run has a proper soaking first, so that any very minor leaks in the pipe joints are saturated and so is the soil surrounding it, to help provide an appropriate seal under working conditions.

COST SEQUENCE - Drain repairs/relocation

Assumed a drain depth averaging 1m below ground level. Drain assumed to be in pitch fibre or 100mm diameter clayware.

1. Excavate trench to expose defective drain.
2. Set aside spoil.
3. Carefully break out 4m defective section of drain and excavate below soffit of pipe.
4. Fill the trench in granular fill, minimum depth 150mm below the pipe soffit.
5. Install proprietary plastic foul drain pipe with flexible couplings where taken over to original pipe run and secure with stainless steel straps to pipe couplings. BandflexTM or equivalent coupling.
6. Ensure pipe drains level with no back fall within the repaired section.
7. Fill trench min 150mm above pipe crown with granular fill material maximum 10mm size per shingle. Consolidate.
8. Refill trench.
9. Assumed make good to ground surface; allow for reinstating of, say, concrete drive 150mm thick across 4m length of trench.
10. Remove all arisings and surplus material from site and cart away.
11. Costing assumes that the drain run can be isolated, there is no shared drain run or over pumping of neighbours' drains required during this operation.

Drain repairs - COST £540 (100mm diameter); £650 (150mm diameter)

Small works pricing

Since the original price estimating in 2008 the economic turmoil of the last few years has caused some disparities in pricing - if you are reviewing these figures in the London area they may look laughable - if you are located in Hull they might still look a bit expensive. Please treat them with caution and of course always consider obtaining more than one quote against which to test prices. Reference to the BCIS or a small works pricing book such as Spons or Laxton's will help assess costs and the hourly labour rates upon which they are based.