Foundation and damp-proofing advice for timber garden office

[DanHW]

Hi folks,

 

I'm in the early stages of building a 4m x 4.5m timber-framed garden office, and hope you can help with some questions about damp-proofing and attaching the frame to the foundation slab. I'm a fairly experienced DIYer, but this is the first time I've done something on this scale!

 

Proposed construction is as follows (Drawing attached):

1. Foundation: 150mm concrete ground slab, on top of DPM, sand blinding, and 100mm MOT Type 1 hardcore. I'm planning for the slab to be approx 100mm larger than the office floorplan all around, to allow for a margin of error. The finished slab height will be 100-150mm above ground level. 

2. Floor insulation: 50mm celotex (or similar), infilled between 50mm timber battens laid over the concrete base. Overlaid with OSB, and internal floor covering (probably laminate/engineered wood) over the top.

3. Walls: 4x2 timber frame, for base wall plates/stud walls/top wall plates. These will be overlaid with OSB, vapour control layer, and clad over the top.

 

So, my questions are:

1. Is it ok for the concrete slab to be 100mm larger than the walls? I've heard conflicting advice on this - some saying it creates a surface area for water to pool and get under the building, while others saying it's structurally risky to fix wall base plates right on the edge of the slab. Thoughts?

2. Do I need a fall on the concrete slab? I was intending for it to be perfectly level, but have had conflicting advice from contractors about whether a fall is needed to prevent any moisture from pooling.

3. As mentioned above I'll have a DPM laid under the slab. What should I do with the excess that comes up the sides? Fixing this to the timber frame (perhaps underneath the cladding) springs to mind, as a way to avoid any moisture getting to the slab where it is larger than the walls. But that also seems like a potential route to funnel moisture up the walls, which wouldn't be great.

4. Do I need to lay a DPC between the wall base plates and the concrete slab, if I already have a DPM underneath it? I was planning to do so, as a belt-and-braces in case the DPM fails down the line. But keen to hear views.

5. If a DPC is needed, what type is best, and how can I best fix the wall base plates without ruining it? I was thinking of using a standard plastic DPC, and then using concrete bolts/screws to fix the walls to the slab. But I'm worried that piercing the DPC when doing so will make it less effective. Should the holes be sealed somehow, before fixing?

6. Do I need a similar DPC (or perhaps DPM covering the whole top of the slab), for the 50mm battens that support the internal floor and insulation?

 

Cheers in advance for your help!

[gc100]

Dpm normally comes up to the inside of the wall and dpc from higher within the  wall and overlaps the dpm. You are normally fixing a wall onto some form of foundation which extends past the wall outward however the slab sits inside this. You want to put some form of breathable membranes over your OSB on the walls (outside).

From my understanding 

Edited February 23, 2023 by gc100

[Russell griffiths]

Your wall needs to extend over the concrete slab, what you are creating there is an area for the water to sit and run under the walls/ floor. 

[markc]

9 minutes ago, Russell griffiths said:

Your wall needs to extend over the concrete slab, what you are creating there is an area for the water to sit and run under the walls/ floor. 

Oh yes, rookie mistake I made building a workshop at my first house, wall set in only an inch or so but any water running down the wall is pulled in under the bottom block

[saveasteading]

I would cut the dpm on the concrete line, after construction. Dampness will get into the concrete but dry again.

Agree slab should be just inside the shed wall to allow dripping to ground.

Dpc between timber and concrete is advisable but not essential as the concrete will breathe and dry.

[oliwoodings]

I am in the process of doing a garden building and made some mistakes in this area. I hope you can benefit from my learnings!

 

1) Don't make the slab bigger than the final finish of the building. I did, and I've had real issues stopping water running under the walls back onto the slab. Had to run stixall round the perimeter, and I might also run some flashing tape from the breather membrane to the slab DPM to add some extra protection.

2) No, no fall or your floor won't be flat

3) Leave it long and bring it up the either between the breather membrane and the OSB sheathing, or the inside of the walls. The idiots who did the slab for me cut it to the ground before I could stop them, otherwise my water issues wouldn't have been so bad.

4+5) I laid dpc under my sole plates. I got stuff that was 2x the width of the plates, and just slid it underneath after positioning the timbers. I then concrete bolted the plates down. Once the walls were up, I stapled the dpc up on the outside inbetween the breather and the OSB. On the inside, I 'infilled' with more dpm on top of the concrete, taping the dpc and dpm together.

6) You can massively simplify your floor layup. Unless you plan to use this for a gym with heavy weights, or have a grand piano in there, you don't need to batten on top of the slab. Just put your celotex straight on the slab, tape it well to create a good vapour barrier, and then lay 18mm OSB T&G, gluing the joins together as you lay it with 5 minute wood glue. I would also increase from 50mm celotex to 100mm on the floor. This is what I did, it worked really well and was suuuper quick.

 

DM me if you want a link to my Google photo album of the build, it has pics of all of this.

 

Disclaimer: I'm amateur, it's my first time as well doing this.