The Brew Cabin

I'm working on a project in Colorado, US. 2-part question: 1. Smaller portions of a terrace of exterior flagstone paving will have radiant heating underneath for snowmelt to have winter use. Other areas of terrace will not have radiant heat. Will a control joint or expansion joint be needed in the stone where the radiant floor stops?
2. Does stone need to be thicker at driveways than areas with only foot traffic? Or does the bedding change? thanks, Mark J, Chicago, IL. Great site. very informative.

We don't have radiant heating beneath our pavements/driveways in the the UK (apart from a few 'more-money-than-sense' eejits in very affluent parts of the country) as we tend not to experience the extremes of weather found in continental climates, such as yours. Accordingly, I can't really offer any advice on what works and what doesn't. My guess would be that you would need some form of exansion joint between heated and unheated sections of paving, but, in all honesty, you're better off asking someone in your own area.

As for your second question, it's normal to use thicker stone for vehicular areas. If sand or mortar bedding is used, the thickness of said bedding is usually the same - around 35-50mm thick, regardless of the application, but a flagstone that's 75mm or thicker would be used for a driveway, while we might use a stone only 30mm thick for a patio or pathway that isn't trafficked by vehicles.

As we don't experience frost-heave, we would normally lay driveway flagstones on a full concrete bed rather than on a sand or mortar bed, but I know this isn't always possible in your climate, so you may need to use a stronger sub-base, say 150-300mm thick, then the bedding material and a 75-150mm thick flagstone.

Tony,

Thanks for the reply. A local subcontractor indicated that if the flagstone is sealed properly, then an expansion joint will not be necessary above the stopping point of the radiant floor. However, freeze-thaw will be occuring from below the subgrade and not just from water penetrating the flagstone and joints, so I have not decided on the control joints there. The construction of the terrace will be flagstone on thinset on moisture barrier on concrete slab. Is sand needed below the thinset? The slab itself will have control joints, and these will be translated up to the flagstone, so a control joint at the radiant floor separation may not hurt- Mark

No: I don't think you're gaining owt by using sand beneath the thinset mortar bedding or between the DPM ("moisture barrier" in Americanese) and the concrete slab. It's just complicating the construction.

If you continue the control/movement joints up through the bedding and into the flagstone layer, you will end up with a long straight joint: how will this tie-in with the finished appearance of the paving?

I'd be keen to hear what you use to seal the flagstone. Sealing natural stone is not something that's done a lot in the UK/Ireland, but it;s one of those questions that comes up on this forum with predictable frequency. Do you know whether it's a solvent or water-based sealant?

Tony,
The contractor suggested a solution that I believe helps mitigate the freeze-thaw issue. The flagstone will be set in a 2-inch mudset on a 4-inch mesh (unclear on this particular product) on drain fabric/slip sheet on 5-inch reinforced concrete slab on 2 layers of 2-inch poly rigid insulation on 4-feet of 3/4-inch gravel.
The slip sheet should be loose laid and should help on preventing the concrete movement cracks from translating to the stone. We were concerned about having control joints in the irregular pattern of the flagstone so the solution above helps solve this. The flagstone will be covered in a water-based sealer.

The Rocky Mountain Brick Institute did not recommend the mudset method and instead recommended a sand bed method. However, given the large area of the terrace, we felt the sand bed method would not be practical. The 3/4 inch gravel will solve the heaving during freeze-thaw.
thanks, Mark

That's one helluva construction! It makes me grateful to be living in these soggy yet mild islands of ours! :)

I'm not sure what this "mudset" stuff is. We don't have anything by that name in Britain or Ireland. The "4-inch Mesh" is probably a steel reinforcing mesh, although it might be a polymer alternative, depending on what this mudset jollop is. The "drain/fabric slip sheet" is what we call a Drainage Composite and is used to drain the bedding material.

The concrete slab is what we might use for a driveway construction but would never consider for a standard patio unless there were exceptional loading or ground conditions. The 2 layers of insulation are similar to what would use beneath a floor slab for a house, and then 4 feet (1200mm) of a sub-base - our motorways don't have that much of a sub-base!!

How much does all this lot cost?

Yes, the construction is a little excessive. The terrace itself is large (approx. 3300 sf - bigger than my own house!) wrapping on 3 sides of the house and built on the side of a mountain. This large area is why we were hesitant to use a sand bed. There is a lot of gravel subbase, but given the slope of the existing terrain, there is a lot of backfill needed at the house foundation. Thanks.