Drilling into flagstones

[Trillipse]

I have recently bought a large parasol which needs attaching to flagstones.  How do I go about drilling the holes and what sort of screws would I use.

[Clanad]

Flagstones here in the U.S. are fairly large (18 to 30 inches in diameter) irregular shaped sandstone about 1 inch thick.  This type of rock is quite fragile.  Most patios here using flagstone are bedded on concrete that is about 4 inches thick.  If yours fits this description, you have to drill into the concrete for any strength.  The larger hole to recieve the parasol center tube (if neccessary) will have to be drilled at least 4 inches into the concrete, better if it were drilled 10 inches or so into the earth under the concrete.  A receiving support can be inserted to hold it.  By screws, if you mean a plate on top of the flagstones to help support the parasol, then these will have to brilled into the supporting concrete as well.  A mortar lag screw, at least 3/8 inches in diameter and 4 inches log will hold it.  Once the holes are drilled for the screws there's a metal insert that goes in each screw hole which expands when the lag screws are tightened into it. The smaller holes can be drilled with a mortar drill bit available from any good DIY store.  You may have to rent a heavy duty electric drill unless you have one...  Best of luck!

[jake-the-peg]

I think mortar lag screws are similar to (same as?) what we would call shield anchors in the UK they look like this:

http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=12851&id=14258

You can get them from mosy D-I-Y stores you do up the bolt and they expand and grip the hole.

When you say flagstones do you mean stone ( which as Clanad so rightly says can be tricky to drill) or do you mean concrete paving slabs? With these you just need a masonary bit and a hammer action drill.

[moomoojaj]

Tha problem with shield anchors is that they expand in the pre-drilled hole and therefore there is a possibility that they will shatter the slab. There is a new form of concrete anchoring system on the market called the 'multi monti'. This actually cuts a thread into the concrete (after drilling a pilot hole) to create a firm anchor. These are available from most DIY/Builders merchants.