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You are here: Experts > Industry > Construction Industry > Building Homes or Extensions > laying stone for patio and knee wall
Expert: Bruce E. Johnson
Date: 8/24/2008
Subject: laying stone for patio and knee wall
Question
QUESTION: Hi Bruce
I have poured a concrete base for a stone patio and built a small retaining wall (Knee wall 18" high) on one side of this concrete base with concrete blocks (CMU). For concrete base a mixture of of Portland Cement Type I-II with mortar sand, gravel and water was used. For concrete blocks, mortar mixture was made from Brixment Type S, mortar sand and water.
The concrete base was poured about 2 months ago and so is cured at this point. The surface of the concrete base is rough and not smooth.
What should be used to adhere flagstone to the concrete patio base? What should be used to adhere natural stone veneer to the vertical surface of the knee wall? What should be used for the grouts?
Should I be using Brixment based mixture or directly a Portland Cement mixture or something else for each of these applications?
The patio and wall in question are outdoors and completely exposed to the weather. I am in Central NC region - lots of sun and rain; not much snow but do get some during winter.
Thanks
Tom
ANSWER: Hi Tom, just regular mortar mix works the best for all of the above applications. I believe Brixment is just another brand of pre-mixed mortar, just add water. Type S is a good all around mix for flagstone and veneer. Grout is usually just mortar mix thinned down a touch to move smoothly through a grout bag or stiff sponged into place similarly to grouting tile. When grouting try to keep the stone as clean as possible as you go, it makes final cleaning so much easier. I hope this information helps, feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
---------- FOLLOW-UP ----------
QUESTION: Thanks Bruce, this was helpful. I have another question related to the same project:
I am doing an outdoor patio project. The patio will have a concrete base 4" with natural flagstone (irregular top surface) laid on top. This is the Central NC area.
Question 1:
I need to build deck stairs from my deck which is 5 ft off the ground to this patio. One side of the stairs will be against the house wall (Hardi-cement siding) and the other side will have a wood railing with pickets. The concrete here has been already poured.
A contractor suggested to lay the flagstone (irregular surface on top) over the concrete, then put a 2x6 PT lumber flat (not screwed / anchored to concrete or stone), and then screw the post and stair stringers to this 2x6 lumber. This contractor said that if concrete anchors were used to hold the 4x4 post to the concrete base, then water would collect around the post bottom and seep into concrete causing problems in the long run.
Another contractor said exactly the opposite. He said just laying the PT 2x6 lumber on the stone and screwing the stringers & posts to this lumber would cause instability problems. Instead he said concrete anchors should be used to fasten a post bracket to the concrete and then the post should be screwed into this post bracket.
How do I connect the 4x4 post for the bottom of the stairs to the concrete - should one of the above approaches be taken or a different one?
Question 2:
Also one of these contractors said, flashing should be used on the side of the stairs butting against the house wall and the other said there is no need for this. Should there be flashing installed where the deck stairs butt the house?
Question 3:
In this same project, I am building a small hut on the same patio to serve as a bar counter. For this I had planned on putting four 8' PT round lumber posts(3-4" diameter) 1-1.5' deep in a concrete footing. I have these holes in the concrete patio already created - just need to put the post and pour concrete around it in the holes. The plan is to use these four posts as support for the framing and bamboo panels that would be attached to form the hut.
But now I am wondering if that is a good idea or if this would result in water seepage/collection and lead to post wood rot and the concrete patio base to crack over time? And if instead, I should use the same approach as for the deck stairs above?
I would appreciate your advice on this.
Thanks
Tom
Answer
Hi again Tom, a post base attached to the concrete with a redhead or sleeve anchor is the best way to attach your posts. No flashing is needed along the wall unless you cut into the wall for some reason. If you attach the side stringer directly to the wall trying to get a little silicone on the penetration wouldnt hurt but any water getting behind the stringer will run past it down to the concrete so water penetration into the wall shouldn't be a big concern. Or you can attach a cleat to the wall and seal the cleat real well. Then attach the stringer to the cleat with deck screws.I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
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