Using runners under joists or not

[sampson]

We are about to buy a sectional wooden workshop to fit a newly created 16x12 concrete base. The base is going to be that size, no bigger, to minimise run-off from the protruding shiplap cladding. Options:
1. Company "A" says we don't need runners and the joist just go directly on the concrete.
2. Company "B" would use perpendicular orientated runners between the joists and concrete.
3. I've also heard of companies using a damp proof material between runners and joists.

What would you recommend ?

Thanks.

 

[That would work]

Surely some dpc between the timber and the concrete regardless?

 

[topchippytom]

Did you use a dpc membrane in your slab/base

 

[rafezetter]

DPC every time - even if the base has a dpc, because the top of the slab unless completely covered can still wick moisture and rot the underside of the wood.

There's really no reason not to, and trust me when I tell you jacking up a side of a wooden structure enough so you can cut out a rotted runner ("Base Plate" as its known) and replace it is no easy task; been there, done that.

I would also go with company B - using BP's under the "joists" which I am assuming are referring to the vertical supports. Using a BP has 2 benefits - 1 is you can bolt down the frame to the slab, the other is should a BP somehow rot or be otherwise compromised, it can be replaced - a real faff but not impossible. If no runner is used and the bottom of a vertical support gets compromised... well that's some fairly complicated surgery for an average diyer to fix.

BP's also help stop the bottom of the supports from splaying over time as the wood moves, and it will.

TBH I can't imagine why any wood framing company would even suggest NOT using BP's - in the 20 odd years I've been watching american TV of that genre, every single wooden framed building I've ever seen built, and that's many hundreds if not thousands, I can't recall ever seeing them deliberately not using a BP; it brings so many benefits and the extra cost is minimal when considered to the overall expenditure.

If you do go with BPs (and I strongly advise you do) and they are being bolted to the concrete slab - drill the hole through the wood oversize and essentially fill the hole with silicone before inserting the bolt and washer into the slab - this should greatly reduce or prevent water wicking up the bolt from the slab and into the wood where the bolt compromises the dpc under the wood. The fact there is minmal slab exposure still won't stop the slab getting wet even if you plan to clad the sides of the slab as well, because it'll still absorb ambient moisture.

Drilling the holes oversize will also allow the wood to move over the concrete, and the concrete slab to move according to the seasons as well.

 

[Dibs-h]

TBH I can't imagine why any wood framing company would even suggest NOT using BP's - in the 20 odd years I've been watching american TV of that genre, every single wooden framed building I've ever seen built, and that's many hundreds if not thousands, I can't recall ever seeing them deliberately not using a BP; it brings so many benefits and the extra cost is minimal when considered to the overall expenditure.

Same here - seen perhaps too many North American programmes. :lol:

And have noticed they put something between the concrete and the BP. Some kind of membrane.

 

[RobinBHM]

IMO the best option is to lay down bearers first, with plastic packers underneath -glazing shims or some 9mm upvc trim.

That allows you to level up the bearers but also keep them off the concrete.

Im not a fan of laying timber on dpc -I think it increases the risk of condensation and keeping the timber wet.

 

[MikeG.]

At the risk of being tedious, there should be no timber at all within 150mm of the ground, and the sole plate should be sitting on a DPC. Further, there should be an inch gap between the frame and the back of any cladding. Failure to follow these basic two principles means your shed is starting to rot away as soon as you get it.

 

[beech1948]

I'm with Mike G on this. Consider 2 or 3 rows of brick + DPM under the shed walls to prevent rot, damp and whatever goes with those issues.