Water/moisture on top of raft foundation

[beneix]

We moved in to a house earlier this year. It was built in the early 1950s and I am told it has a raft foundation. The other day, I opened the access hatch to the crawl space underneath our suspended timber floor for the first time to take a look.

Upon opening the hatch, there was a distinct smell of mould. I investigated the whole area under the house and found that most of it seems to have an intact, flat concrete foundation (albeit with years of debris collected on top from various previous works - cable ends, plastic pieces, pipe insulation that has fallen off heating and hot water pipes, pieces of brick...). However, in one area the slab seems to potentially be broken, as there is a small amount of gravel laid there and new concrete seems to have been poured in a small area, too, going slightly higher than the surrounding flat slab.

Some of the concrete slab was completely dry, but some areas were damp. It seemed to be damp near the vertical brick support walls and dry in the middle between these brick walls. The beams and floor boards did not show any obvious signs of moisture damage.

Wet near walls

• beneix
• 21 Oct 2016

Raft foundation, mostly dry but wet near the walls

Today I took a new look and now I see actual water in several places - not high, but enough to create a reflective surface or water when looking down through the hatch. (Didn't take photos of this.)

The brick walls that cross the space underneath the house and on which the floor beams rest have some form of membrane on top of them (looks like tar felt), between the top of the brick walls and the beams.

All of this suggests to me that if this is indeed a so called raft foundation, it was not built with a damp-proof membrane undeneath as would be the case today.

Questions:
1. Should I worry about the water coming in to this space?
2. Is there any remedial work that would be recommended to address the water/damp?

 

[vinn]

Are you in a radon area?
What did the mortgage survey report have to say about these conditions in the sub-area?
Are you on a hillside?
Do you have air bricks all around the house?
Are there any signs of damp in the living area walls?

FWIW: the "beams" are floor joists & the "tar felt" is a DPC.

 

[JohnD]

nice to see that the so-called "rising damp" only wets three or four courses of bricks.

If you look carefully at the base of the sleeper walls, can you see a mortar joint then the bricks standing on top of the slab?

Or could it be oversite concrete laid after the walls were begun?

 

[beneix]

Are you in a radon area?
What did the mortgage survey report have to say about these conditions in the sub-area?
Are you on a hillside?
Do you have air bricks all around the house?
Are there any signs of damp in the living area walls?

FWIW: the "beams" are floor joists & the "tar felt" is a DPC.

The survey didn't comment - they never opened this hatch (it is rather well concealed in the floor of the cloakroom).

We are neither in a radon area, nor on a hillside. We are, however, on the edge of a moss and I believe for this reason all houses in the area are built on rafts. We have air vents around the house (on the photos, you can see the daylight through them) but not on all sides; the house is a semi and on the end there have been two extensions. In the 1970s, a single-story garage was added to the side. This section has a much more shallow foundation with just enough room underneath for four inches of insulation. Then in the 1990s, they added a single-story kitchen extension behind the garage, which also has a very shallow foundation - this one a suspended timber floor without any insulation. Looking in at this section through one of the air gaps, that area of concrete foundation looks dry.

There are no signs of damp elsewhere in the house - but as everyone knows, a house on the market tends to have such things painted over...

Thanks for the terminology help

Today I understand raft foundations are built with a dpm under the cement - was this not the case in the 50s, and was it instead common practice to put the barrier between the sleeper walls and joists?

 

[beneix]

nice to see that the so-called "rising damp" only wets three or four courses of bricks.

If you look carefully at the base of the sleeper walls, can you see a mortar joint then the bricks standing on top of the slab?

Or could it be oversite concrete laid after the walls were begun?

I'm going back down there during the weekend so will have a look. It's a bit of a squeeze so one almost needs to be a contortionist!

 

[vinn]

I tend to agree with JohnD that its probably not a true raft but oversite concrete - check the thickness where the edge is exposed.
A giveaway is the spectacular true rising damp that would not be there if the walls had been built off a true raft.

DPM's became Bldg Regs in the 1960's but were used on some earlier builds.

AAMOI: to all those who doubt the existence of rising damp look and learn.

 

[JohnD]

on the edge of a moss

what's that?

 

[beneix]

what's that?

Sorry, meant to say bog - a raised mire peat bog, hence high water table, but interestingly not prone to flooding, just more challenging to put standard foundations in.

 

[vinn]

What you might do, after isolating the power, is to clip all dangling cables to the joists & re-attach the insulation to the pipes - use tape to secure the insulation or it will split and drop again.
Remove all wood debris from the oversite.

I suspect that the best that could be done with the moisture is to ensure strong through ventilation - using 10 x 6 plastic air bricks will help, maybe adding more of them. And honeycombing the dividing walls.
I'm not a believer in locating active electro-mechanical appliances in sub-areas but some utilise them.
The water table is perhaps against you.