Best way forward in filling a trench inside the house?

[Stephen H]

firstly a bit of background.

I bought a house last December.

It originally had a semi-detached double garage with a supporting wall in the middle. (one of the bedrooms is above one of the garages.)

A previous owner had the garage that was under the bedroom converted to an extra room, and knocked through a door way from the hall.

So the double garage became a single garage & extra reception room. (there were two separate single garage doors, one of which was removed and replaced with a wall and window)

The garage floor was 6 inches lower than the Hall floor when they knocked the door opening through from the hall.

Now at the time, there was a steel gas pipe running along the garage- hall wall (on the garage wall side to the hall) above the then garage floor but below the hall floor level. (kitchen is to rear of garage)

So the builder built a wood frame around the gas pipe which is 12ft long and 5 inches wide to protect said gas pipe in the garage being converted.

They then put down a layer of sand, then a blue plastic damp proof membrane and filled the garage floor it with concrete till it was level with the hall floor. They then put a piece of 12ft long 5in wide plywood to cover this trench containing the gas pipe. This meant that the gas pipe was accessible at all times.

Now for the problem:

The gas pipe has since been decommissioned (it was corroding badly) and a new copper gas pipe put in that takes a different route to the original gas pipe.

I have removed the old steel gas pipe and the supporting wood frame and the plywood cover. I wish to fill this in with concrete to match the rest of the garage floor and the hall floor.

This has resulted in a trench that is 12ft long by 5 inch wide.

There is damp proof course across the doorway that was knocked through and this was covered with concrete.

Now whats the best way of dealing with this 12ft long by 5inch wide now empty trench? There is a surplus amount of blue DPM material which went under the plywood cover but is perforated with gripper rod nails and carpet tacks even though it would reach the existing wall.

Can I put a strip of DPM material down this trench and then fill it in with concrete or will there be insufficient overlap between old and new DPM's given the fact that the trench is 12ft long by 5inches wide?

Is there a water resistant concrete I can use to fill this trench in that will not be affected by the nail holes in the exisiing DPM and prevent any rising damp problems?

Regards,

Stephen.

[harryagain]

A lot depends on the subsoil as to how good your DPM has to be, so no-one can be entirely sure. I would paint the side of the existing concrete with a few coatsof bitumous paint . I would put a strip of DPM in the trench turned up the sides, cut off just below the surface. You can buy chemical to add to concrete mix makes it water repellent if you want belt and braces. You can also buy chemical to paint on to existing concrete the makes it water repellant. However it won't stop positive water pressure so make sure your trench doesn't fill with water in wet weather.

[stuart noble]

Adding SBR to the concrete mix would probably be your best bet

[Jeremy Nicoll - news posts]

Depending perhaps on what you're going to use the ex-garage for, I might have been inclined to leave the trench alone, or perhaps to reinforce it so that a decent thickness of floor-plank could be laid into the top of it so it would bear weight ok. It sounds as if it's one of those potentially useful cable ducts that can make running cables around houses so much easier, even if, say, cables from rooms next to to it come through the wall at one point, run along the trench, then go back through the house wall.

[Phil L]

snip

Just cut a piece of DPM to cover the bottom and one side of the channel and place this over the top of what you already have, then concrete.

Don't worry about nail holes etc, DPM's get damaged every time they have hairy-arsed blokes running barrows of concrete over them, that is to say that your DPM is already damaged all over the place as is everyone else's - it makes no difference whatsoever, concrete won't 'suck' water up through these - the entire slab would have to be sitting on pooled water for decades before any made it to the surface

[Stephen H]

Update:

Now that I have exposed the trench and removed the wood and old gas pipe, I am now ready to fill the trench in.

There appears to be a blue DPM under the existing floor. Under this blue DPM, there appears to be a layer of sharp sand. This is whats on top of the old concrete floor of the garage. (the garage was converted into a room, and garage floor was lower than the hall floor)

I happen to have some building sand in the garage. Can I use this for the trench or must it be sharp sand?

I will then place a DPM in the trench on top of this sand, linking up with the DPM under the hall floor and the DPM under the existing slab of concrete.

After placing this DPM, I then will fill in with concrete. What should this concrete be? 1 parts portland cemenet to 6 parts building sand or 1 parts portland cement to 3 parts building sand?

Regards

Stephen

[NT]

You can fil it with anything you want. I'd choose polystyrene for bulk fill rather than sand, and sand can take up the reamining gaps.

whatever you want really. I'd put stone in the mix too.

NT

[Andrew Gabriel]

I found plywood covered trenches under the carpets when I moved in and ripped up the old carpets. Very useful they were too. Now filled with ethernet, speaker, telephone, alarm, home automation, and aerial cables, as there was plenty of spare space around the radiator pipes in them.

No way would I get rid of such a useful feature.

Mine are actually formed from galvaised steel troughs, with lips along the top to take the plywood lids.