Help with DPM/DPC for garden room/workshop build

[lookseehear]

Hello all, new to the forums and hoping for some advice.

I'm building a 5.65x3 garden room / workshop at the moment and have very little prior experience, so have been looking online and asking experienced friends and family along the way.

I've cast a 150mm concrete slab 100mm longer and wider than the main structure will be, with 1200 gauge dpm underneath and running up the sides to the surface of the slab (I cut it flush with the shuttering - this may have been a mistake). I have then laid two courses of bricks set 50mm in from all sides of the slab. I plan to build a timber frame on top of this which will be insulated. The floor inside is currently planned to be the bare concrete slab, as I may have some fairly heavy tools in there, but with a view that in the future some insulation and a wooden floor or screed could be laid on top of the slab to turn it from a workshop into more of a garden room.

I've included a diagram below of the cross section of the slab. My main concern is that I don't know what to do about the DPM and the gap to the DPC which is 50mm of horizontal concrete and two courses of bricks. How much of a problem is this in terms of damp ingress into the structure?

What would be the best solution for dealing with this given where I'm at now? Ideally I don't want to screed the floor inside now as it will primarily be a workshop so I'm trying to retain as much of the 2.5m height allowed under permitted development as possible.

With the power of hindsight I would have made the slab the same size as the structure so the bricks were flush with the edge then left enough DPM to run up the sides and join up with the DPC but I didn't do that. Any input would be very valuable.

 

[DIYnot]

(bumped because the thread was missing for a while!)

 

[lookseehear]

Thanks! Also to add to my first post, someone suggested that I coat the outside of the concrete, bricks and mortar with some clear waterproofing paint. I know this isn't a long term solution - any thoughts on this?

 

[John D v2.0]

You don't want any water splashing on the 50mm of concrete outside as it will be on the inside of the dpm so the concrete will absorb water.

 

[Notch7]

I would be tempted to:

1. Form some haunching on the 50mm flat, to create slope.

2. Lay dpm, or liquid dpm across dpc on outside, down bricks across haunching and over existing dpm.

3. When doing timber framing, set framing flush to outside of brickwork. Fix 50mm insulation to outside of frame. Tape joints.

4. Fit 50 x 25mm battening, finish just 50mm above bottom brick -you will now be outside of the haunching so rain will miss and any water getting behind cladding will hit dpm on haunching and run away.

 

[^woody^]

45° mortar fillet at the base then, a few coats of clear water repellant - on the fillet once dry and the bricks.

But that detail is going to trap moisture within the floor slab. You really want to return the DPM under the wall or flash over it and the external brickwork.

You should form a tray and drip on the frame to drain moisture from the cavity and off the base of the wall

 

[lookseehear]

Thanks for the responses.

I would be tempted to:

1. Form some haunching on the 50mm flat, to create slope.

2. Lay dpm, or liquid dpm across dpc on outside, down bricks across haunching and over existing dpm.

3. When doing timber framing, set framing flush to outside of brickwork. Fix 50mm insulation to outside of frame. Tape joints.

4. Fit 50 x 25mm battening, finish just 50mm above bottom brick -you will now be outside of the haunching so rain will miss and any water getting behind cladding will hit dpm on haunching and run away.

My issue with this is that I've gone right up to the permitted development limit of 15 square metres, so using this method (insulation on the outside) will mean that I'll go over the limit unless the walls are about 200mm thick. There's also not a lot of space behind and to the sides of the structure so ideally I didn't want to cut down on that space further. Got to be able to get behind to clear out leaves etc.

I like the idea of haunching with liquid dpm though, to help any water drain away.

45° mortar fillet at the base then, a few coats of clear water repellant - on the fillet once dry and the bricks.

But that detail is going to trap moisture within the floor slab. You really want to return the DPM under the wall or flash over it and the external brickwork.

You should form a tray and drip on the frame to drain moisture from the cavity and off the base of the wall

This sounds good, although that would leave one course of bricks exposed and my understanding is that ideally I should have 150mm between the ground and the timber frame to reduce water splashing back onto the frame. I'm planning on a roof overhang of probably 300mm or so, so that should help to reduce the amount of water directly reaching the slab.

Here's a picture of progress so far:

You may notice that I may have made another error in going with untreated timber, other than for the sole plates on all four sides. Obviously ideally the frame won't be getting wet, but a few mates have given me grief about this decision, making me worry the frame will rot. Any thoughts?

 

[^woody^]

I should have 150mm between the ground and the timber frame to reduce water splashing back onto the frame.

The splashing you refer to is reference to splashing above any DPC.

You should have a formed drip and flashing/tray to protect the frame, and detail the cladding so that it dries off and casts off water regardless.

 

[John D v2.0]

Regarding permitted development, you can cover half the area around the original house with outbuilding that don't touch the main house. So don't forget to take into account the front and any side garden.
The only 15 limit is the building regs exemption, and that's on internal floor space not external.