Suspended Timber floor...sleeper walls, oversite questions.

26 Jan 2010
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Tyne and Wear
United Kingdom
Now I have the drains all in on my ground floor extension it's time to put the oversite down.
To get the required 150mm min of space between the top of the oversite and the bottom of the joists the oversite would have to be about 75mm lower than the outside ground level.
The BCO told me that if I put a damp proof membrane in below the oversite that I can have less than the 150mm space so I think this is what I'm going to do, it means that the sleeper wall will only be 1 brick in height with a 2" x 4" wall plate, then the 6"x 2" joists.
Do the joists & wall plate need to be pressure treated timber?

If I can squeeze it in would it be ok to use a concrete block on it's side (100mm) as the sleeper wall?

I've read that the wall plate isn't fastened to the sleeper wall it just sits on a bed of mortar, if this is the case how doesn't it twist all over the place when it gets damp from the mortar? ....I was intending to fasten it down to the blocks to stop it twisting.

Any thoughts?

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As you are going onto dpm, I'd use a full or half concrete block under each joist, to stop it all from sliding around. No problem using mortar under joists as it will dry out by the next day. Despite the dpm on the oversite, still use some dpc under the joists/wall plate.
Yeah I was intending to cut the concrete blocks in half length ways to get 2 out of 1 and put a strip of DPC under the wall plate.

I've read that the wall plate should be sat on a bed of mortar with the DPC sandwiched in the mortar on top of the sleeper wall. I'm unsure about the wall plate being bedded directly on mortar as when the wall plate for the roof timbers was bedded down on mortar it had twisted like a propeller by the next day. I had to remove it and put a new one on this time sitting it on a DPC to stop the moisture from the mortar getting to it.

You must have had a naff length of timber. Mortar bed is standard practice, go easy on the water.
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Using 47x145 treated and regulised joists at 400mm centres you will need sleeper walls at max 3.17mm centres. These will support an imposed load of 1.5KN/m2 and dead load of 0.25KN/m2. Your max dead load of flooring will be 0.13KN/m2.
If joists are 600mm centres sleeper walls need to be max 2.77mm centres.
If you wish you can cut joists down to 47x97 treated and regulised 400mm centres with sleeper walls at 2.03 centres or .joists at 600mm centres and sleeper walls at 1.59 centres.
Your choice blocks or bricks, providing they are 50mm honeycomb. Bed DPC, then bed plate to level required.
You have naff timber if she twists after being bedded.
Regards oldun
If you use one course of bricks and a 4x2 as the plate that will give you about 150mm to the bottom of the joists anyway, or have I read it wrong?
Normally you have to lay the oversite to fall to a drain if the external ground level is higher than the oversite.
Hi, sorry for the delay with the reply….

@ theoldun,
The extension is only 2.3m internal so the span isn’t a problem with 47 x 145mm joists, there is a small section where the span will be 3.9m but this will have a sleeper wall at approx mid span.
I did think about using smaller joists like you mention but I need to have either 100mm of kingspan or 150mm of rockwool insulation between the joists, I’m favouring the rockwool as it’s a lot cheaper.

@ stuart45,
Yeah your right it won’t be far off 150mm with a single brick and a 4"x 2" by the time the mortar beds are in etc, that said I still intend to put down a damp proof membrane.
I've worked out that my oversite will be about an inch below outside ground level, I guess that's not anything to really worry about, the BCO didn't mention it being a problem when he looked at my layout the last time he was here for the drains.

Regarding the drainage, I plan to put a soak away around the perimeter of the extension and the side of the house, this will be a trench lined and covered with landscaping fabric and filled with pea shingle which will run into a freshwater drain. The bottom of the trench will be at the top of the foundation level.
I feel this is necessary as we have a lot of heavy clay in the ground around here and our garden is often water logged, add to this the rain water running towards our house from the small field at the side and you will see why I need some form of drainage. The soak away should keep any surface water well below my oversite level.
I also plan to put some land drainage in the garden that will run to the freshwater drain as the garden is very slow draining taking a good few days to dry out after rain.

whenever i do floor replacements i always nail dpc to the underside of the wallplate with clout nails,then bed the plate down on the mortar.

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