New suspended timber floor - oversite questions

[SilverSaloon]

Hi

I need to replace my timber floors throughout my stone-built 1850's cottage due to there currently being no void under the floorboards so the joists are rotted though/eaten by the worms.

My plan is:

- dig down to outside ground level
- add airbrick vents in various areas
- and add sleeper walls around where the end of the joists will sit
- fit new timber joists and boards on

I've done a good bit of research into the methods advised but these dont seem to take into account older properties.

My original plan was to:

- after digging down, level with some sand, and then lay a polythene Damp Proof Membrane sheet across the whole void, going up the sleeper walls.
- putting a thin layer of concrete over the top of the DPM.

However I'm worried about moisture then being forced up the walls.

Should I leave the void to just have some sand on the top of the earth and not install a DPM? should I install a DPM without the concrete, with maybe some shingle on the top?

Any advice with an older property in mind would be appreciated! I will be doing all the work myself by the way, no builders involved.

thanks

Derek

 

[Owain]

I would be wary of using DPM for the reason you suggest, unless you can tie the DPM in with the damp proof course. If you don't have a damp proof course in the walls, and don't intend to acquire one ...

Provided the water table is sufficiently below oversite level, and you have good drainage around the perimiter of the house, the earth below the building shouldn't be at all damp. With ventilation it will dry out.

The advantage to concreting might be to deter animals burrowing up.

Probably worth spraying all the wood with anti-beetle insecticide a couple of times before installing.

 

[Flyfisher]

Hi Derek. I like the sound of your current plan and doubt there is any need for your original plan.

No void under the floor means no ventilation means moisture build up, which is a bad thing. But I think you know that.

We have a suspended wooden floor that must be well over 100 years old. It sits about 6 inches above bare earth with the floor joists supported on bricks sitting on pamments and is in good condition apart from a small area of rot where the inglenook fireplace was bricked up with cement to create a smaller fireplace (now removed). The small amount of rot was where cement had surrounded the floor joist, presumably trapping moisture.

If you're raising the floor slightly and providing adequate ventilation underneath, then I'd suggest there's no need for a DPC - especially if you're using treated floor joists. Based on our floor, I'd say your plan will be fine - just be sure that any sleeper walls don't restrict the ventilation. If you can insert airbricks on different faces of the building then so much the better, as the wind will create pressure differentials that will greatly improve the underefloor ventilation.

Of course, a draughty underfloor void may be good for the floor timbers but not so great for heat retention, so you should also be thinking about insulation, but keeping it within the floor joists should not disrupt the ventilation.

 

[Matt Green]

Probably worth spraying all the wood with anti-beetle insecticide a couple of times before installing.

Use pre-treated timber instead: vacuum impregnation is far more effective than after market spraying. Brush treat any cut ends.

 

[SilverSaloon]

I plan on using treated C16 timber to the correct size for the spans.

Since I need to build sleeper walls to rest the joists on, would I be best to put the damp proof membrane down between these, leaving about 150mm around the edges of the earth for things to breathe a bit, and therefore I'll still have no damp up the walls since it should breathe before the walls but also benefit from the lack of open soil throughout?

If not laying a concrete screed ontop of the DPM I could put sand down onto the earth to get it level, lay the DPM, then put a coating of sand back ontop, then with some shingle.

How about that as an alternative?

I'm really keen to have a DPM down since it would stop bugs/growth but worried about putting the concrete and DPM right up to the solid sandstone walls due to the moisture/damp finding its way out up the wall....

 

[AMc]

I suspect you would be better served by excavating down to a few inches above external ground level, you don't want to inadvertently go lower and end up with moisture moving to the lower level inside? High external ground levels often cause damp and the general advice is to lower them at least 6" below internal floor height?

My house >150 years old has joists over bare earth supported in sleeper walls and some limited ventilation - rot free when we lifted the floors to insulate during our renovation. If the underfloor is ventilated the joists should stay dry and dry wood can't be eaten by any of the wood boring insects or fungi that usually cause problems. Unless there are big holes that can allow vermin in then there shouldn't be any problems with tunneling/burrowing either as there will be nothing to eat. I can't see how anything will grow under your floor if it's ventilated (therefore dry) and covered (so dark).

If you lay a DPM across the surface of the floor you reduce the amount of surface area which can evaporate any rising ground water and increase the risk of that moisture finding another route into the walls.
I might be tempted to lay the joists on dpm where they meet the sleeper walls to discourage any transfer up, but I don't think a DPM across the floor is necessary or desirable.

Of course if you dig down and find the soil is very wet you may have to look at more aggressive approaches but it doesn't sound like that's an issue.

We used foamboard insulation between the joists which cut down on draughts and made the rooms much warmer.

 

[Flyfisher]

I guess that putting some DPM under any new brickwork will do no harm, but the bigger area you cover the more you limit evaporation of any damp in the soil so it'll probably concentrate in the open areas. It might not be a problem if there is adequate ventilation, but I don't know how to calculate such things to prove a design before building it.

The soil under our suspended floor is very sandy and feels dry to the touch. The pamments sit directly on the soil and the bricks on the pamments and all feel dry (well, the ones I can reach from the front of the inglenook fireplace. I put this down to the air gap and ventilation. I don't know how much of either I could reduce before causing a problem, all I know it that it seems to work.

There is nothing wrong with DPMs, of course, when a house is built entirely on top of them, but in this case the house wasn't built with one and relies on ventilation to prevent any ground moisture from accumulating. Restrict that ventilation and any the moisture will inevitably build up. Of course, we may be talking about the difference of the new floor lasting for 100 years or 150 years so it may all be irrelevant in practice. The trouble is, it's very hard to know for sure.

At this point in such discussions I usually bring out the old rubber glove analogy. Unless you've just washed your hands, I'd bet they feel dry to the touch. Now put a rubber glove on one hand for about half an hour and I'd bet it will then feel damp and clammy, while the ungloved hand will remain dry to the touch. Now imagine the soil under your floor being like your skin . . .

 

[SilverSaloon]

Thanks for all the replies.

The rubber glove analogy prooves a very good point.

I think I'll keep with my original plan of installing sleeper walls where the joist ends will lay on (with a DPM between the timber & masonary) and just coating the solum with a thin layer of builders sand + some of the black small gravly stuff (which is under the floors currently) to tidy it up a bit. With plenty of new airbricks to allow it all to breathe.

I plan on using some plastic netting (barrier fence stapled to the underside of the new joists) to hold up loft insulation between the joists.

This is what I originally thought but keep on reading about the DPM and concrete but they tend to be for newbuilds etc which have a DPC in the walls, which I dont have. A chemical DPC was installed in the 70's but as we all know in sandstone solid walls they do nothing and prevent the wall breathe.

the internal room solum is currently about level with the external ground, maybe marginally (few inches) higher.

I've already dug down in 1 room to the max that I wanted to and the soil is dry/sandy like. Its not wet at all, although after being exposed (been dug for about 2 weeks now) its very cold to the touch but I wouldnt say damp even after snow/rain we have had.

This is in a room next to broken guttering too so once I fix the gutters it can only get better.