Breather membrane or DPM below garden building subframe

[andyscotland]

I'm erecting a SIPS garden office. It's going to sit on a timber subframe on ground screws.

 

I've seen some suppliers recommend a DPM between the ground screw heads & the timber to protect from moisture.  But I'd think that could be worse - if any water does get in there (running down the side of the building/condensation) then the DPM would hold it against the timber?

 

I'm thinking it would be better to use breather membrane so any moisture has a route out.

 

The timber will be about 50mm above ground and I'm planning to put uPVC fascia around all sides to protect from splashing water and close the gap. Then drill some holes with insect mesh cover plates to allow airflow through the void.

 

What do you think?

 

[andyscotland]

E.g. my plan as per this very rough sketch

[Iceverge]

You probably don't need anything. 

 

A permeable ground membrane of some type to stop weeds and gravel over it to protect. 

 

I'd probably dig some kind of shallow drain around the edge to make sure water can't pool anywhere underneath. 

[andyscotland]

@Iceverge thanks very much.

 

Do you think definitely a permeable weed membrane? I was planning that but now wondering if a DPM (as per the solum under a suspended timber floor) would make more sense.

 

A drain round the outer edge is a good plan.

[Iceverge]

Weed membrane for the purpose of keeping weeds down and the gravel to help protect the membrane and prevent any splashback on the timber.  

 

A mini french drain will keep the localised water table from pooling under the structure in times of heavy rain. 

 

The highest lightlyhood of decay is the metal wood interface where condensation will form in cold weather on the ground screws. You must ensure this is very free draining. I'd consider a "shoe" of a more durable hardwood or plastic to sit the softwood joist on to ensure theres no risk of condensation forming and pooling where the softwood will be sitting in water. 

 

I'd be interested in other people's take on it. 

 

I hate plastic DPMs under sole plates etc. It only traps moisture as far as I can see. 

 

 

[andyscotland]

1 hour ago, Iceverge said:

The highest lightlyhood of decay is the metal wood interface where condensation will form in cold weather on the ground screws. You must ensure this is very free draining. I'd consider a "shoe" of a more durable hardwood or plastic to sit the softwood joist on to ensure theres no risk of condensation forming and pooling where the softwood will be sitting in water. 

 

Excellent point, thanks. I have some offcuts of uPVC fascia that would probably do the trick for packing the timber off the screw head.

 

1 hour ago, Iceverge said:

I'd be interested in other people's take on it. 

 

I hate plastic DPMs under sole plates etc. It only traps moisture as far as I can see.

 

I guess I was just thinking with a french drain round the perimeter and then the only major source of water would be from the soil upward so a DPM would trap that below leaving the void above dry other than airborne moisture which should generally blow through the cross ventilation?

[PeterW]

25 minutes ago, andyscotland said:

I have some offcuts of uPVC fascia that would probably do the trick for packing the timber off the screw head.

 
uPVC fascia isn’t as strong as you think when compressed - it’s a thin plastic skin on foam.

[andyscotland]

1 hour ago, PeterW said:

 uPVC fascia isn’t as strong as you think when compressed - it’s a thin plastic skin on foam.

 

Ahhh - should have been more specific. What I have is 9mm utility board which I think is solid, my recollection is it was much tougher to cut than the fascias I had. I will double check when I dig it out tho.

 

Failing that I'll find something more rigid - perspex maybe?

 

I don't think the loading is huge, even with live loads & snow load included my rough calculation worked out less than 300kg per screw, so if I have my maths right over the joist bearing area it's something like 10 kg/cm2 / 140psi / 980 kPa.