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Groundwork & Foundations

Water in subfloor void causing condensation leading to mould

Hi my semi detached bungalow has a sub floor void of about 1m and when it rains heavily, the void near the back end of the house fills up to about 5cm. The land slopes towards the back of the house. I've had all three sides pointed where brick meets ground. Will be installing new plastic air bricks. The front is half tarmac/drive and a lawn. The back has a large garden, however the gardens floor height is about 50cm above the sub floor void. There is a patio which is further above the sub floor void sloping away from the house.

A specialist surveyed and recommended I install a drainage pipe going into the garden and to a soakway. However the land is clay below ~50cm resting against the back walls. And I'm suspecting the rest of the garden is clay too below a certain level.
After excavating a channel running from the house to the garden about 1m deep, the subfloor around that area and adjoining room drained away through the lime mortar, however this is about 60cm below the gardens ground level. The next two days it rained and water pooled in the channel and has soaked back into the house to about 100mm standing water now.

Two builders have recommended I fill the void with hardcore and cement it, which will cost alot taking into account the size of the bungalows plot. All the floor boards in the house are bone dry throughout the year. Any suggestions would be much appreciated. I was thinking of lowering the patio as it seems the previous owner has layed patio on top of an older patio preventing the house from breathing. Even with the old patio, the sub floor void is lower than the gardens ground level.

Update: 24/10/17 - I haven't dug the soakway yet as I'll need to dig about 4m down since the gardens above the sub floor void and it seems like the water from the garden soaks back into the house due to build up of debris @ the end of the garden creating a hill. And due to the water coming back into the house when it rained and filled the channel with water. The flood pattern indicates it comes back into the house from the garden side as the front 2 rooms have dry sub floors. I might try and remove the old patios completely and angle the slope slightly away from the house and create a channel/drainage going to the back of the garden. Then next year hire landscapers to lower the gardens height so it's all level but sloping away from the bungalow.

Update 26.10.17 - Dye test was performed by united utilities and found no leaks.

2 Answers from MyBuilder Groundworkers

Best Answer

Have you dug the soak away far enough away from the house? Is the soak away deep enough? My recommendations would be around 2m deep due to being clay. Did you fill the bottom with hardcore? Did you use any polystorm crates in the away and then shingle around? As water should not be finding it’s way back up the soak away pipe. And lastly has the pipe got a sufficient fall down into the soakaway and away from the house?


Answered 24th Oct 2017

There's no mention of anyone checking your existing drainage around the property . Could this be a blocked rain water gully or cracked drain that is causing water to run under the sub floor? If this hasn't been looked at then a simple dye test would prove if the drain is leaking or not


Answered 25th Oct 2017

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