Timber suspended floor with an RSJ sleeper wall

16 Jul 2017
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United Kingdom
I have a room I'm currently renovating which is approximately 3.5m sq. It is the latest room in the ongoing house remonstration, oops I meant renovation. It has a suspended timber floor with a void of 300mm from the top of the concrete oversite to the top of the joists.

- Here's the plan -
I'm installing underfloor heating using the 50mm insulation boards already grooved for the UFH pipe which are fitted between the joists. Under this will be 100mm of Celotex. So in effect six inches of insulation between each set of joists. The building regs gap of 150mm from oversite to the bottom of the joists/insulation would be maintained.

- What I've found -
After taking up some of the floor boards I found the 400mm centred joists are supported with a honeycombed sleeper wall with DPC and a wall plate at either end of the room (these walls are parallel to the wall with the air brick). I expected these. However, in the middle of the room the sleeper wall is an RSJ bedded on mortar with a layer of DPC and a wall plate. It appears to be a single long RSJ.

- The problem -
I expected a honeycombed sleeper wall in the middle of the room. From a ventilation point of view the RSJ has not been a problem as air could flow between the joists as there was no insulation present between the joists. However, now I plan to install insulation between the joists there will be no route for the air to flow past the middle sleeper wall.
I don't think there is any cross ventilation under the floor.

- Solutions -
The ideas I can think of are:
1. Modify the middle RSJ sleeper wall so it is in three/four pieces by cutting a four inch gap to allow ventilation through to the other side of the room. This would involve cutting through the wall plate and RSJ so there are three/four separate sections of sleeper wall/RSJ each supporting around three joists.
2. Remove the RSJ steel sleeper wall and replace with a honeycombed sleeper wall.

- Questions -
Can you think of why an RSJ has been used in this way?
Would you adopt either of my ideas above or another approach?
Is idea 1 a bad idea as I'd end up with smaller segments of wall plate. Presumably, a longer continuous wall plate spreads the load.

Thank you for reading this. I appreciate any help and guidance you can offer.
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Get rid of the suspended floor and do a solid floor?
Thanks for both replying.
freddiemercurystwin, wouldn't the number and size of holes I'd need to drill to allow sufficient ventilation in the steel make it weaker?

Tomfe, a solid floor was my plan as the the underfloor heating would be more efficient. However, after discussing this with a few people I've decided to continue with the suspended floor as this area does have a high water table and I've read about the difficulties where the dampness/moisture is displaced usually up the walls.

My current thinking is replacing the middle sleeper wall with an engineering brick honeycomb wall is possibly the best solution. I cannot think of why the RSJ was used or why it would need to be retained in it's current form.
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No, it'll be fine.

Until it rusts away?

OP, by the time you have messed about with it drilling numerous holes to get the required amount of air gaps, or cut it, you could have ripped the thing out dropped a couple of course in and bedded a wall plate, and still have time to spare before love island starts.
I'd like to see you cut some 100mm holes every 3m in a UB. :D
I'd like to see you cut some 100mm holes every 3m in a UB. :D
I never said it would be easy, but it doesn't need 100mm holes, a really good hole cutter (say 30mm) not some ****e from screwfix or wickes, a good drill, slow speed, cutting fluid (could be tricky I would concede, prolly need a second pair of hands to feed the cutting fluid with say an oil can and a tray to collect/reuse the fluid), some patience, awesome. We won a war you know?

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