Thread: minimum screw depth

It depends on a lot of things but you're screwing into long grain (not end grain) so you don't need as much screw length. I don't have an outdoor table made that way but I have a cover for a fire pit made from merbau. It consists of several slats held together by four cleats, screwed from underneath with 30mm gal decking screws. I countersunk them to maximise the amount of thread into the slats. I made it about 6 years ago and it's still holding together.

Or you could try the Camo system, which screws a small-headed screw into the edge of the board at an angle:

http://www.camofasteners.com.au/

When I have needed to do this, I have used a large diameter screw, so the there is more surface area of thread used. 12 gauge I think..

Used to make loads of SS framed outdoor tables and benches with 19mm slatted tops. Normally used 20 x 4 SS strap drilled for screw holes and welded to ends of frame, and 19x19x5SS angles in the middle for larger tables or ones with brolly holes in the deck. Used 20mm round headed SS screws through the strap/angle from below so about 16mm penetration into slats, 6 gauge screws from memory but not certain. Never had any issues with screws coming through the slats, slats coming loose, or slats splitting. Holes in SS about 1mm clearance around screw shanks to allow seasonal timber movement, pilot holes drilled in slats to accept screw without splitting. Probably around 600 tables and 800 benches over 5 years.

If you are using 19mm timber cleats obviously you need to allow extra length to screw through the cleats, but RH screws and oversize holes in the cleats would remain helpful in controlling issues with timber movement.

Originally Posted by BaileyBoy

Hi, I have a few spare hardwood decking boards and is thinking about making an outdoor table.

But unlike an outdoor table, I want to screw them from the underside to achieve a cleaner look.
But my decking boards are only 19mm thick. So how do I attach them I like to use screws but thought screws need 20-30mm embedment.
Anyone care to shed any light on this?

Cheers.

the usual rule of thumb is to have at least 2/3 of the length of the screw in the material being held -- so assuming the screws go about 15mm into the decking boards, then the max length of the screw used would be about 20mm

Thanks guys. The reason I don't want to screw them from the top is dirt, water will get into the holes. My plan is to screw from the bottom via another decking board so we're talking about 19mm + 15mm so I'll need 35mm screws. Get worried that if I countersink them too much, I don't have more room for error.

Anyway, thanks for the advice. I'll do some trials.

Originally Posted by BaileyBoy

Thanks guys. The reason I don't want to screw them from the top is dirt, water will get into the holes. My plan is to screw from the bottom via another decking board so we're talking about 19mm + 15mm so I'll need 35mm screws. Get worried that if I countersink them too much, I don't have more room for error.

Anyway, thanks for the advice. I'll do some trials.

35mm screws is not a good idea. Essentially a screw works by holding a thick bit to a thin bit -- if you do it the other way round thin bit held to a thick bit, over time the pointy end of the screw looses its grip.
far better to counter bore the battens to around 10-12mm and then use 25mm screws.

Originally Posted by malb

Used to make loads of SS framed outdoor tables and benches with 19mm slatted tops. Normally used 20 x 4 SS strap drilled for screw holes and welded to ends of frame, and 19x19x5SS angles in the middle for larger tables or ones with brolly holes in the deck. Used 20mm round headed SS screws through the strap/angle from below so about 16mm penetration into slats, 6 gauge screws from memory but not certain. Never had any issues with screws coming through the slats, slats coming loose, or slats splitting. Holes in SS about 1mm clearance around screw shanks to allow seasonal timber movement, pilot holes drilled in slats to accept screw without splitting. Probably around 600 tables and 800 benches over 5 years.

If you are using 19mm timber cleats obviously you need to allow extra length to screw through the cleats, but RH screws and oversize holes in the cleats would remain helpful in controlling issues with timber movement.

Off on a tangent, but I reckon that I have two (2) of your tables.

Originally Posted by ian

35mm screws is not a good idea. Essentially a screw works by holding a thick bit to a thin bit -- if you do it the other way round thin bit held to a thick bit, over time the pointy end of the screw looses its grip.
far better to counter bore the battens to around 10-12mm and then use 25mm screws.

Ian's advice resonates for me ... it is what my father and granddaddy taught me when building boats.

Originally Posted by cava

Off on a tangent, but I reckon that I have two (2) of your tables.

We made to customer requirements re table and tube sizes and materials, and made frames fro about 8 furniture factories to fit out, plus made them for direct order customers as well, fully finished. There were at least 30 in 316 marine grade for coastal regions, including quite a few for the Peninsula, some sandblasted, some brushed and some mirror polished.

Originally Posted by ian

35mm screws is not a good idea. Essentially a screw works by holding a thick bit to a thin bit -- if you do it the other way round thin bit held to a thick bit, over time the pointy end of the screw looses its grip.
far better to counter bore the battens to around 10-12mm and then use 25mm screws.

Agreed that this is a better approach, use oversize pilot and counterbore holes in the cleats to allow some scope for seasonal movement in the slats.

Originally Posted by BaileyBoy

Hi, I have a few spare hardwood decking boards and is thinking about making an outdoor table.

But unlike an outdoor table, I want to screw them from the underside to achieve a cleaner look.
But my decking boards are only 19mm thick. So how do I attach them I like to use screws but thought screws need 20-30mm embedment.
Anyone care to shed any light on this?

Cheers.

Maybe a bit late, but here are a couple of additional ideas

There are coarse threaded grommets (~8 mm in diameter, 10 mm long) that have a fine inner thread that then takes a bolt. you drill the 8mm hole in your boards to take the grommets and then bolt the boards to the base using the fine thread that has been created. Relatively expensive, but we are talking a few dollars here not hundreds...

The other thing is, why use screws at all? you could cut a shallow rebate (3 mm would be plenty) on the underside of the decking boards with a saw or router and glue two or more cross pieces the width of the rebate - INTO the rebate - Now you can you use these cross pieces as the fixing points - eg sliding bolts or keys in slots etc. You can even screw into the cross pieces in a few places to hold the top down, but now you have double the screw bed depth and you don't have to prepare (pilot bore, bore and counter bore) so many screw holes, because depending on the with of the table, 3 screws across the width would be plenty... remember most of the time these screws are not supporting any weight... they are just stopping the top from sliding around.

If you use a dovetail bit to cut the rebate (this one may have to be deeper) and the edge of the cross pieces, then you can "cross dovetail" the cross pieces onto the underside of the boards. The only boards that need to be glued or screwed into place are the two at each end of the cross pieces.


Jorge